• TABLE OF CONTENTS
HIDE
 Title Page
 Acknowledgement
 Table of Contents
 List of Tables
 List of Figures
 Abstract
 Introduction
 Backscatter mine detection and...
 Equipment and materials
 Radiation transport
 X-ray source
 Detector response
 Mine detection mechanisms
 Application to imaging
 Conclusions
 Characteristics of landmines
 Historical examples of mine...
 Other mine detection and neutr...
 X-ray transmission measurement...
 Gadolinium oxysulfide detector
 X-ray spectra used in measurem...
 Computer codes
 Monte carlo techniques
 Reference
 Biographical sketch
 Copyright














Title: Landmine detection by scatter radiation radiography
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Permanent Link: http://ufdc.ufl.edu/UF00089972/00001
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Title: Landmine detection by scatter radiation radiography
Series Title: Landmine detection by scatter radiation radiography
Physical Description: Book
Creator: Campbell, John G.,
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Bibliographic ID: UF00089972
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Table of Contents
    Title Page
        Page i
    Acknowledgement
        Page ii
        Page iii
        Page iv
    Table of Contents
        Page v
        Page vi
        Page vii
        Page viii
        Page ix
        Page x
    List of Tables
        Page xi
        Page xii
        Page xiii
        Page xiv
    List of Figures
        Page xv
        Page xvi
        Page xvii
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        Page xix
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        Page xxvii
        Page xxviii
        Page xxix
        Page xxx
    Abstract
        Page xxxi
        Page xxxii
    Introduction
        Page 1
        Page 2
        Page 3
    Backscatter mine detection and imaging
        Page 4
        Page 5
        Page 6
        Page 7
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    Equipment and materials
        Page 20
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    Radiation transport
        Page 52
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    X-ray source
        Page 105
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    Detector response
        Page 139
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    Mine detection mechanisms
        Page 169
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    Application to imaging
        Page 248
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    Conclusions
        Page 363
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    Characteristics of landmines
        Page 368
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    Historical examples of mine warfare
        Page 378
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    Other mine detection and neutralization
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    X-ray transmission measurements
        Page 397
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    Gadolinium oxysulfide detector
        Page 414
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    X-ray spectra used in measurements
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    Computer codes
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    Monte carlo techniques
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    Reference
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    Biographical sketch
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    Copyright
        Copyright
Full Text














LANDMINE DETECTION BY
SCATTER RADIATION RADIOGRAPHY







by

JOHN G. CAMPBELL


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN
PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY




UNIVERSITY OF FLORIDA


1987

















ACKNOWLEDGMENTS


A number of individuals and organizations have played

an important role in my research. First and foremost, I

thank my wife, Becky, for her support, understanding, and

patience.

Dr. Alan Jacobs, who was my research advisor, was

always willing to help, whether the assistance required

discussion of new ideas or manual labor. The basic concept

of the project, using imaging techniques for mine detection,

was his. The generous amount of time he took from a very

busy schedule is greatly appreciated. I thank the other

members of my committee, Dr. Edward Carroll, Dr. Edward

Dugan, Dr. John Staudhammer, and Dr. Mark Yang, for their

time and guidance.

Two graduate students, who worked on other aspects of

the research problem, also contributed to my efforts.

Captain Dale Moss designed the soil box positioning system

and its computer control. Linda Hipp was an equal partner

in the assembly of the positioning system and spent many

long hours performing measurements.

I thank Bill Coughlin of the Radiation Control

Department for the loan of an ionization chamber for the

exposure rate transmission experiments, and Harvey Norton,










of the same organization, for the use of a calibration set

of radionuclide sources. Dr. William Ellis provided the

filter sets used in the measurements.

The electronics skills and untiring efforts of Ken

Fawcett were solely responsible for keeping an old x-ray

machine in operation for the measurements. His expertise

was crucial to this research.

I thank Lois Carroll, who typed this manuscript, for

her professional and always cheerful assistance.

Gary Melocik of Bicron Corporation provided details on

the composition and geometry of the NaI(Tl) detector used in

the experiment. Dr. William Frank, 3M Corporation, provided

information on the composition of the Trimax 12 rare earth

intensifying screens. Without their assistance, the detec-

tor response calculations could not have been performed.

Andrew Lickly of Applied Reasoning Corporation, and

David Hampton of Seattle Telecom and Data, Inc., ran bench-

mark versions of the Monte Carlo code on their accelerator

boards. Two graduate students of the Nuclear Engineering

Sciences Department also helped test the code. Samer Kahook

ran a benchmark on the IBM PC/AT. Kiratadas Kutikkad ran a

mine detection problem on the Cray X-MP/48 using the MCNP

code.

I thank the United States Army for allowing me the op-

portunity to continue my education and for financial support

during the research effort. The measurements portion of

this work was supported by the U.S. Army Belvoir Research


iii










and Development Center under contract, DAAK 70-86-K-0016.

thank those individuals associated with the administration

of the contract for their active interest in the research.

Of those individuals, I particularly thank Edward Ostrosky

for his participation in the early measurements, and Dr.

Robert Moler for his reviews of the progress of the work.
















TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS .. . . . . . . . ii

LIST OF TABLES. .. . . . . . . .. xi

LIST OF FIGURES . . . . . . ... xv

ABSTRACT. . . . . . ... . . . xxxi

CHAPTERS

I INTRODUCTION . . . . . . . 1

II BACKSCATTER MINE DETECTION AND IMAGING . 4

Previous Uses of Scattered Radiation . 5
Backscattered Photon Mine Detection. . 8
Fluorescence Emission . . . .. 8
Rayleigh Scattering . . . . .. 9
Compton Scattering .... .. . 10
Backscatter Radiation Radiography. . . 13
Genesis of Current Research Effort. . 13
Improvements on Previous X-Ray
Backscatter Efforts . . . . .. 17
Research Goals. . . . . . . 17

III EQUIPMENT AND MATERIALS. . . . . 20

Equipment. . . . . . . . . 20
X-Ray Source. . . . . . . 20
Soil Box and Positioning System . . 27
Detector and Related Electronics. . 30
Computer Control System . . . . 36
Materials. . . . . . . . . 38
Soils . . . . . . . . 38
Nonmetallic Antitank Mine Model . . 39
Materials Tests . . . . . . 46

IV RADIATION TRANSPORT. . . . . . 52

Photon Interactions. . . ... . .. 52
Coherent Scattering . . . . . 52
Incoherent Scattering . . . . 59
Photoelectric Effect. . . . . 67
Mass Interaction Coefficients . . 71










TABLE OF CONTENTS continued


CHAPTERS Page

Single Scatter Model . . . . . 73
Computation Scheme. . . . . . 73
Interaction Modeling. . . . . 78
Monte Carlo Model. . .. . . . .. 78
Problem Parameters and Data . . . 80
Random Number Generators. . . . 81
Computation Scheme. . . . . . 83
Modelling Scattering Interactions . 87
Russian Roulette. . . . . . 89
Code Output . . . . . . . 89
Validation of the Monte Carlo Codes. . 90
Number and Energy Albedo Calculations 91
Energy Spectra Comparisons. . . . 91
Comparison with Buried Mine
Calculations. . . . . . . 91
Testing the Scattering Routines . . 99

V X-RAY SOURCE . . . . . . . 105

Kramers' Formula Method. . . . . 105
Kramers' Formula. . . . . . 106
Time Dependent Accelerating Potential 107
Characteristic X-Ray Production. . 108
Attenuation by Materials in the Beam
Path. .... . . . . 109
Anode Self-Attenuation. .. . . . 111
Effects Neglected in Model. . . . 116
General Features of the Calculated
Spectra . . . . . . . . 117
Testing the Modified Kramers' Formula
Model. . . . . . . . . 119
Exposure Rate Transmission
Measurements . . . . .. 119
Comparisons with Published Spectra. . 122
Other Methods to Determine X-Ray Spectra 126
Measurement . . . . . . 126
Monte Carlo Calculation . . . 130
Laplace Transform Pair Method ... 130

VI DETECTOR RESPONSE. . . . . . . 139

Plane Detector Code. . . . . . 141
Assumptions in the Plane Detector
Response Calculation . . . . . 144
Energy Deposition. . . . . . . 146
Case of Zero Degree Incidence . . 146
Case of Large Angle Incidence . . 149
Counts Per Incident Photon . . . . 151










TABLE OF CONTENTS continued


CHAPTERS


Discriminator Setting Corresponding
to 0 MeV. . . . . .. . .
Discriminator Setting Corresponding
to Energies Greater Than 0 MeV. . .
Validation of the Plane Detector Response
Calculations . . . . . . .
Iodine Escape Ratio . . . .
Measured Energy Spectra . . . .
Shield and Edge Effects. . . . .
Calculation of the Correction Factor.
Results of the Correction Factor
Calculation . . . . . .

MINE DETECTION MECHANISMS. . . . .

Backscattered Photon Signal Differences.


Fluence . . . . . .
Energy Fluence. . . .
Spatial Distribution. . .
Angular Distribution. . .
Energy Spectra. . . . .
Edge Effects. . . . .
Conclusions Based on Signal
Differences . . .
Irradiation Geometry and Optimum
Height of Detector. . . .
Angle of Incidence. . . .
Raster Gap Size . . . .
Detector Collimator Length .
Detector Panel Dimensions .
Segmented Detector Geometry
Source Beam Collimation . .
Source Energy Optimization. .
Depth of Burial . . . .
Polyenergetic Sources . .


Conclusions Based on Optimizat


Page


151

153

157
157
163
163
163

166

169

169
170
176
182
186
194
198


. . . 204
Energy. 208
. . . 208


. . .
. . .
. . .
. . .





ions. .


VIII APPLICATION TO IMAGING . . . .

Comparisons with Measurements. . .
Spatial Distribution of Detector
Response . . . . .
Detector Response with Mine Present
Edge Effects. . . . . . .
Energy Window Detector. . . .
Environmental Parameters . . . .
Height Sensitivity. . . .
Soil Density Variation. . . .
Soil Moisture Content . . .
Inhomogeneities . . . . .


209
216
222
223
227
229
232
241
244
246

248

249

249
251
258
262
266
266
272
277
283


vii


VII


















TABLE OF CONTENTS continued


CHAPTERS


Imaging. . . . . . . . .
Monte Carlo Generated Images . .
Measured Images . . . . .
Dual Energy Subtraction Technique .
Power Requirements . . . . .
Variables . . . . .
Fraction of Source Photons Reaching


the Detector. . . . .
Source Flux . . . .
Pixel Dwell Time. . . .
Calculation Technique . .
Power Calculations. . .


IX CONCLUSIONS. . . . . .


APPENDICES


CHARACTERISTICS OF LANDMINES .

Mine Classification. . . .
Metallic or Nonmetallic .
Antitank or Antipersonnel .
Conventional or Scatterable
Surface or Buried . . .
Fuzing Type . . . .
Employment of Landmines . .


HISTORICAL EXAMPLES OF MINE WARFARE .

Mine Development . . . . . .
Forerunners of Modern Mines . .
Mines of World War II . . . .
Countermine Warfare in World War II .
Mine Employment. . . . . . .
North Africa. . . . . . .
Eastern Front . . . . . .
Korea and Vietnam . . . . .

OTHER MINE DETECTION AND NEUTRALIZATION
METHODS. . . . . . . . .


Detector Technololgies . . .
General Considerations. . .
Microwaves. . . . . .
Neutrons. . . . .
Magnetic Resonance Techniques
Trace Gases . . . .
Animals . . . . .
Biochemical Methods . . .
Infrared Methods. . . .


viii


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287
287
296
342
347
348

350
350
353
353
357

363


368

368
371
372
372
373
375
376

378

378
378
379
380
380
380
382
383


385

385
385
386
388
390
391
392
392
393










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. . . .

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*


.*



















TABLE OF CONTENTS continued

APPENDICES

Neutralization . . .. . . .
Mechanical Systems. . . . . .
Explosive Methods . . . .
Magnetic Signature Duplication. . .

D X-RAY TRANSMISSION MEASUREMENTS. . . .

E GADOLINIUM OXYSULFIDE DETECTOR . . .

Detector Description . . . . . .
Detector Design . . . . . .
Screen Composition. . . . . .
Detector Response Matrix Calculation . .
Calculation Technique . . . . .
General Results of Calculations . .
Description of the Detector Response
Matrix . . . . .. . . .
Perpendicular Incidence . . . .
45 Degree Incidence . ... . .
75 Degree Incidence . . . . .
Comparison with Published Results . .
Response Matrix Relationship to
Detector Electronics . . . . .
Shortcomings of the Detector . . .
Sensitivity . . . . .
Fluorescence Decay Constant . . .
Corrective Actions . . ......
Comparison of Measured and Calculated
Responses . . . . . . .
Calculation Technique . . . .
Measurements. . . . . . . .

F X-RAY SPECTRA USED IN MEASUREMENTS . .


COMPUTER CODES . . . . .

Computer Hardware. . . . .
Personal Computers and Monte
Carlo Calculations. . . .
Computer Selection. . . .
Comparison with Cray X-MP/48
Supercomputer . . . .
Computer Software. . . . .
Computer Languages. . . .
Radiation Transport Codes .
X-Ray Spectra Codes . . .
Detector Response Codes . .
Imaging Codes . . . .
Utility Codes . . . .
Photon Interaction Data Files
Commercial Software . . .


Page


394
394
395
396

397

414

414
414
417
422
422
425

428
429
436
444
449

450
455
456
460
463

464
464
465

470

484

484

484
485

487
489
489
489
491
492
493
494
496
497




























TABLE OF CONTENTS continued


APPENDICES Page

H MONTE CARLO TECHNIQUES . . . . . 501

Angular Scattering Distributions . . 501
Momentum Transfer Variable. . . . 501
Incoherent Scattering . . .. .. 502
Sampling the Klein-Nishina
Distribution. . . ... . . .. 504
Coherent Scattering . . . . 506
Random Number Generators . . .. . 509
MCPHOT.PAS Generator . . .. 509
MCPHOT.P Generator . . . . 509
Fluorescent Emission . . . . 510
Application to Polyenergetic Sources . 512
Available Methods ........ 512
Application of the Fit Method .. .. 513

LIST OF REFERENCES. . . . . . . . . 516

BIOGRAPHICAL SKETCH . . . . . . . . 527
















LIST OF TABLES


TABLES Title Page

III.1 Geometry of the Sodium Iodide Detector
and Shield. . . . . . . . 32

111.2 Sources Used in Determining Lower Level
Discriminator Setting . . . .. 35

111.3 Composition of Soil Types. . . . ... 40

111.4 Characteristics of Common Warsaw Pact
Nonmetallic Antitank Mines. . . . 42

111.5 Ratios of the Linear Interaction
Coefficients of Sucrose to TNT. . .. 47

IV.1 Energy Mesh Structure for Mass
Interaction Coefficients. . . . 74

IV.2 Energy at Which Photoelectric and
Incoherent Scattering Mass Interaction
Coefficients Are Equal. . . . . 76

IV.3 Comparison of Number and Energy Albedo
Calculations for Iron . . . ... 92

IV.4 Comparison of Number Albedo
Calculations for Concrete . . . 93

IV.5 Comparison of Energy Albedo
Calculations for Concrete . . . 94

IV.6 Comparison of Number Albedo
Calculations for FTB Soil and
Buried DNB Mines. . . . . . 101

V.1 Energies of Tungsten K Characteristic
X Rays. . . ...... . . 110

V.2 Comparison of Exit Path Lengths from
Tungsten Anodes . . .. . . 115

VI.1 Energies of Iodine Fluorescent Emission
X Rays Used in the Detector Response
Calculations . . . . . 143










LIST OF TABLES continued


TABLES Page

VII.1 Comparison of the Linear Relationship
Between the Ratio of Number to Energy
Albedo and Source Energy at
Perpendicular Incidence . . . . 180

VII.2 Mine to Soil Response Ratios at Selected
Beam Angles of Incidence. . . . 211

VII.3 Front to Back Panel Fluence Ratios of
the Collimated Detector for 100 keV
Photon Beams Incident at 60 Degrees . 214

VII.4 Results of Calculations for the Geometric
Parameters of the Collimated Fluence
Detector. . . . . . . . 220

VII.5 Mine to Soil Fluence Ratio Dependence on
Panel Width and Raster Gap Size for an
Uncollimated Detector .. . . . 226

VII.6 Optimum Source Energies for the Uncolli-
mated Fluence Detector. . . . . 235

VII.7 Comparison of the Segmented and Unseg-
mented Uncollimated Fluence Detectors 238

VII.8 Mine to Soil Fluence Ratio at Selected
Depths of Burial of the TST Mine. . 242

VII.9 Mine to Soil Fluence Ratios Versus Depth
of Mine Burial for the Energy Window
Detector with Source Energy of 100 keV. 245

VIII.1 Parameters for Spatial Distribution
Comparison. . . . . . . . 250

VIII.2 Comparison of Calculated and Measured Mine
to Soil Detector Response/Ratio with the
TST Mine at 0.0 cm . . . . . 257

VIII.3 Energy Window Measurements for the TST
Mine at 2.54 cm Depth of Burial . . 265

VIII.4 Mine to Soil Fluence Ratio from the
Collimated Detector with Recently
Buried Mines. . . . . . . 276

VIII.5 Mine to Soil Fluence Ratio from the
Energy Window Detector with Recently
Buried Mines. . . . . . . 278


xii









LIST OF TABLES continued


TABLES

VIII.6 Mine to Soil Fluence Ratio of the
Uncollimated Fluence Detector for
Three Water Contents of HTL Soil
with the TST Mine at 2.5 cm Depth
of Burial . . . . . . . .

VIII.7 Mine to Soil Fluence Ratio of the
Collimated Fluence Detector for
Two Water Contents of HTL Soil
with the TST Mine at 2.5 cm Depth
of Burial . . . . . . . .


Page


280





281


VIII.8 Mine. to Soil Fluence Ratio of the
Energy Window Detector for Three
Water Contents of HTL Soil with
the TST Mine at 2.5 cm Depth of
Burial. . . . . .. . . 282

VIII.9 Object to Soil Fluence Ratio from
the Collimated Detector for
Selected Inhomogeneities. . . . 286

VIII.10 Object to Soil Fluence Ratio from
the Energy Window Detector for
Selected Inhomogeneities. . . . 288

VIII.11 Operational Requirements for a Vehicle-
Mounted Antitank Mine Detection
System. . . . . . . . ... 349

VIII.12 Photon Output of the GE Maxitron 300
X-Ray Therapy Unit. . . . . . 352

VIII.13 Imaging Quantities Necessary to Fulfill
Operational Requirements. . . . 354

VIII.14 Power and Signal to Noise Ratio Require-
ments for Imaging and Mine Detection
with the Uncollimated Detector . 358

VIII.15 Power and Signal to Noise Ratio Require-
ments for Imaging and Mine Detection
with the Collimated Detector. . . 362


E.1 Gadolinium Oxysulfide Screen Model . .


E.2


423


Energies of Fluorescent Photons Used
in the DETECT.PAS Code. . . . .


426


466


E.3 Calculated Ratios of Radiation Field
Quantities, 33Ba to 137Cs. . . .


xiii









LIST OF TABLES continued


TABLES Page

G.1 Benchmarks for Monte Carlo Transport
Codes. . . . . . . . . 488

G.2 Photon Interaction Data Files. . . . 498

H.1 Fluorescent Emission Probabilities . . 511


xiv

















LIST OF FIGURES

Title


FIGURES


Page


II.1 Conceptual large area backscatter detector
system. . . . ... . . . . 18

III.1 X-ray source, soil box and positioning
system and detector . . . ... 22

111.2 Detector electronics, computer and x-ray
source console. . . . . . 24

111.3 Lead shield for tube head and detector . 29

111.4 Geometry of sodium iodide detector and
shield. . . . . . . . 31

III.5 Components of the counting system. . . 37

III.6 Soil mass attenuation coefficients . . 41

III.7 TST mine used in measurements. . . . 45

111.8 Transmission comparison for TNT and
substitute. . . . . . . .. 49

111.9 Transmission comparison for NSL and
local soil. . . . . . . . 51

IV.1 Atomic form factor versus momentum
transfer variable. . . . . . 55

IV.2 Solid angle differential coherent
scattering cross section versus
scattering angle. . . . . . 57

IV.3 Coherent cross section versus photon
energy. . . . .. . . . ...... 58

IV.4 Fractional energy of Compton scattered
photons versus incident photon energy 60

IV.5 Solid angle differential Klein-Nishina
cross section versus scattering angle 62










LIST OF FIGURES continued


FIGURES Page

IV.6 Incoherent scattering function versus
momentum transfer variable. . . . 64

IV.7 Comparison of the solid angle differential
Klein-Nishina and incoherent scattering
cross sections. . . . .. .. 65

IV.8 Incoherent scattering cross section versus
photon energy . . . . . . 66

IV.9 Photoelectric interaction cross section
versus photon energy. . . . . 69

IV.10 Probability of K shell fluorescence
versus atomic number. ... . . .. 70

IV.11 Energies of K fluorescent photons versus
atomic number . . . . . . 72

IV.12 Mass interaction coefficients of aluminum
versus photon energy. . . . . 75

IV.13 Boundaries and materials of Monte Carlo
calculations. . . . . . . 84

IV.14 Number albedo versus energy for concrete 95

IV.15 Backscattered energy spectrum, 0.200 MeV
on aluminum . . . . . ... 96

IV.16 Backscattered energy spectrum, 0.6616 MeV
on aluminum . . . . . . . 97

IV.17 Backscattered energy spectrum, 0.6616 MeV
on iron . . . . . . . . 98

IV.18 Comparison of calculations of the solid
angle differential coherent cross
section . . ... . . .. 101

IV.19 Comparison of calculations of the solid
angle differential incoherent cross
section . . . . . . . . 102

V.1 Transmission curve without anode self-
attenuation . . . . . . . 112

V.2 Typical x-ray spectrum calculation . . 118


xvi










LIST OF FIGURES continued


FIGURES


VI.3 Plane detector response, discrimination
less than 0.03317 MeV . . . ..

VI.4 Plane detector response, discrimination
greater than 0.03317 MeV .. . ..

VI.5 Iodine escape peak ratio versus energy .

VI.6 Measured and calculated NaI(Tl) spectra.

VI.7 Plane detector response . . . .


V.3 Heel effect displayed by spectra . .

V.4 Heel effect displayed by half value
thickness . . . . . . .

V.5 Typical transmission curve comparison.

V.6 Spectrum comparison with Epp and Weiss
at 80 kVp . . . . . .

V.7 Spectrum comparison with Epp and Weiss
at 105 kVp. . . . . . .

V.8 Spectrum comparison with Fewell and
Shuping at 70 kVp . . . .

V.9 Spectrum comparison with Fewell and
Shuping at 80 kVp . . . .

V.10 Spectrum comparison with Fewell and
Shuping at 90 kVp . . . .

V.11 Archer-Wagner method fit to measured
transmission data . . . .

V.12 Comparison of modified Kramers' method
and the Archer-Wagner method at
80 kVp. . . . . . . .

V.13 Comparison of modified Kramers' method
and the Archer-Wagner method at
150 kVp . . . . . .

VI.1 Fraction of incident energy absorbed
perpendicular incidence . . .

VI.2 Fraction of incident energy absorbed
75 degree incidence . . . .


S 152


S 156

S 158

S 164

S 167


xvii


Page

. 120


. 121

. 123


. 124


. 125


. 127


. 128


. 129


. 134



. 136



. 137


. 147


. 150










LIST OF FIGURES continued


FIGURES

VI.8 Detector response with edge and shield
correction. . . . . . . .

VII.1 Number albedos versus energy for HTL soil
and two TST mine cases. . . . .

VII.2 Number albedo ratios versus energy for the
TST mine at 0.0 cm in three soils . .

VII.3 Number albedo ratios versus energy for the
TST mine at 2.5 cm in three soils . .


VII.4 Energy albedos versus energy for HTL soil
and two TST mine cases. . . . .

VII.5 Multiple scatter fraction versus energy
for HTL soil and two TST mine cases

VII.6 Ratio of number to energy albedo for HTL
soil and two TST mine cases . . .

VII.7 Spatial distribution of backscattered
fluence from 100 keV photons per-
pendicularly incident on HTL soil .

VII.8 Spatial distribution of backscattered
fluence from 100 keV photons per-
pendicularly on the center of the TST
mine at 0.0 cm. . . . . . .

VII.9 Spatial distribution of mine to soil
ratio of backscattered fluence from
perpendicularly incident 100 keV
photons . . . . . . . .


Page


168


172


174


175


S 177


S 179


S 181



183




184




185


VII.10 Spatial distribution of the single
scattered mine to soil ratio from
perpendicularly incident 100 keV
photons . . . . . . .

VII.11 Angular distribution of backscattered
fluence from 100 keV photons perpen-
dicularly incident on HTL soil and
two TST mine cases. . . . . .

VII.12 Angular distribution of the multiple
scattered fluence from 100 keV photons
perpendicularly incident on HTL soil
and two TST mine cases. . . . .


xviii


187




188




189










LIST OF FIGURES continued


FIGURES Page

VII.13 Mine to soil fluence ratio versus
collimator acceptance angle for 100
keV photons perpendicularly incident
on the TST mine at 0.0 cm in HTL soil 191

VII.14 Mine to soil fluence ratio versus
collimator acceptance angle for 100
keV photons perpendicularly incident
on the TST mine at 2.5 cm in HTL soil 192

VII.15 Multiple scatter fraction versus colli-
mator acceptance angle for 100 keV
photons perpendicularly incident on
the TST mine at 0.0 cm in HTL soil. . 193

VII.16 Differential energy spectra for 100 keV
photons perpendicularly incident on HTL
soil and two TST mine cases . . . 195

VII.17 Ratios of mine and soil integral energy
spectra for two TST mine cases in HTL
soil. . . . . ... .. . . .. 197

VII.18 Edge effect geometries . . . . . 199

VII.19 Spatial distribution of the single scat-
tered fluence from a 100 keV photon
beam perpendicularly incident on the
inside edge of the TST mine . . . 201

VII.20 Spatial distribution of the single scat-
tered mine to soil fluence response
ratio for a 100 keV photon beam perpen-
dicularly incident on the inside edge
of the TST mine . . . . . . 202

VII.21 Spatial distribution of the single scat-
tered mine to soil fluence response
ratio for a 100 keV photon beam per-
pendicularly incident on the outside
edge of the TST mine . . . . 203

VII.22 NaI(Tl) detector response and fluence
response versus source beam energy. . 206

VII.23 Ratio of NaI(Tl) detector response to
fluence response as a function of
source energy . . . . . . 207


xix









LIST OF FIGURES continued


FIGURES Page

VII.24 Ratios of integral energy spectra for
100 keV photons incident on the TST
mine at 2.5 cm in NSL soil for the
cases of 0 to 60 degree incidence . 212

VII.25 Spatial distribution of the fluence
response from a 100 keV beam inci-
dent at 60 degrees on the TST mine
at 2.5 cm in NSL soil . . . . 215

VII.26 Fluence response versus distance from
beam axis for 100 keV photons perpen-
dicularly incident on the TST mine at
2.5 cm in NSL soil. . . . . . 217

VII.27 Relationship between the raster gap size,
the length of the collimator, and the
spacing of the first collimator element
required to exclude single scattered
Photons from the detector . . . 224

VII.28 Geometry of the segmented fluence
detector. . . . . . . . 228

VII.29 Fluence response ratio matrices for the
segmented detector for perpendicularly
incident 150 keV photon beams on the
TST mine at 2.5 cm in HTL soil. . . 231

VII.30 Source energy optimization curve for the
uncollimated fluence detector with mine
depth of burial of 5 cm in NSL soil . 234

VII.31 Source energy optimization curve for the
segmented fluence detector with mine
depth of burial of 2.5 cm in NSL soil 237

VII.32 Source energy optimization curve for the
energy window detector with mine
depth of burial of 5 cm in NSL soil . 240

VIII.1 Calculated and measured spatial distribu-
tion of detector response from back-
scatter from sandy soil at 100 kVp. . 252

VIII.2 Calculated and measured spatial distribu-
tion of detector response from back-
scatter from sandy soil at 150 kVp. . 253










LIST OF FIGURES continued


Page


FIGURES


VIII.3 Calculated and measured spatial distribu-
tion of detector response from back-
scatter from sandy soil at 200 kVp..

VIII.4 Comparison of the number albedos of
sucrose and TNT . . . . . .

VIII.5 Three dimensional image diagram of
measured detector response for the
lucite annulus experiment . . .

VIII.6 Two dimensional image diagram of
measured detector response for the
lucite annulus experiment . . .

VIII.7 Three dimensional image diagram of
measured detector response for the
steel annulus experiment. . . .

VIII.8 Two dimensional image diagram of
measured detector response for the
steel annulus experiment. . . .

VIII.9 Fluence response as a function of
height above the soil surface for
selected panel widths of the
uncollimated detector . . . .

VIII.10 Fluence response as a function of
height above the soil surface for
selected acceptance angles of the
collimated detector . . . . .

VIII.11 Fluence response as a function of
height above the soil surface for
the energy window detector. . . .

VIII.12 Ratio of fluence responses for two
densities of HTL soil with the TST
mine at selected depths of burial
as a function of source energy for
the uncollimated detector . . .


S 254


S 256



S 260



S 261



S 263



S 264


268




270



273





274


VIII.13 Object to soil fluence response ratio
for selected materials as a function
of source energy for the uncolli-
mated detector. . . . . . .


284


xxi









LIST OF FIGURES continued


FIGURES

VIII.14 Monte Carlo generated image for the TST
mine buried flush to an NSL soil
surface for the uncollimated fluence
detector. . . . . . . .

VIII.15 Monte Carlo generated image for the TST
mine buried flush to an HTL soil
surface for the uncollimated fluence
detector . . . . .

VIII.16 Monte Carlo generated image for the TST
mine buried flush to an MCL soil
surface for the uncollimated fluence
detector. . . . . . . .


Page




S 290




S 291


292


VIII.17 Monte Carlo generated image for the TST
at 2.5 cm depth of burial in NSL soil
for the uncollimated fluence detector .

VIII.18 Low pass filtered Monte Carlo image for
the TST mine at 2.5 cm depth of burial
in NSL soil for the uncollimated
fluence detector. . . ... .

VIII.19 Monte Carlo generated image for the TST
mine at 5.0 cm depth of burial in NSL
soil for the uncollimated fluence
detector. . . . . . .. ..

VIII.20 Monte Carlo generated image for a simu-
lated water puddle on HTL soil with
20% water content by weight for the
uncollimated fluence detector . .

VIII.21 Monte Carlo generated image for an iron
disk buried flush to the surface of
NSL soil for the uncollimated fluence
detector. . . . .. . . .

VIII.22 Three dimensional image diagram of the
measured uncollimated detector response
to a 100 kVp source beam filtered by
Pb for the TST mine buried flush to
the soil surface. . . . . . .

VIII.23 Two dimensional image diagram of the
measured uncollimated detector response
to a 100 kVp source beam filtered by
Pb for the TST mine buried flush to
the soil surface. . . . . . .


xxii


294




295




297




298


299


301





302










LIST OF FIGURES continued


FIGURES Page

VIII.24 Three dimensional image diagram of the
measured uncollimated detector response
to a 200 kVp source beam filtered by
Pb for the TST mine buried flush to
the soil surface. . . . . . 303

VIII.25 Three dimensional image diagram of the
measured uncollimated detector response
to a 100 kVp source beam filtered by
Pb for the TST mine at a depth of
burial of 2.54 cm . . . . . 304

VIII.26 Low pass filtered image diagram of the
measured uncollimated detector response
to a 100 kVp source beam filtered by
Pb for the TST mine at a depth of
burial of 2.54 cm . . . . . 306

VIII.27 Three dimensional image diagram of the
measured uncollimated detector response
to a 200 kVp source beam filtered by
Pb for the TST mine at a depth of
burial of 2.54 cm . . . . . 307

VIII.28 Low pass filtered image diagram of the
measured uncollimated detector response
to a 200 kVp source beam filtered by
Pb for the TST mine at a depth of
burial of 2.54 cm . . . . . 308

VIII.29 Three dimensional image diagram of the
measured uncollimated detector response
to a 100 kVp source beam filtered by
Pb for the TST mine laid on the soil
surface . . . . . . . . 309

VIII.30 Two dimensional image diagram of the
measured uncollimated detector response
to a 100 kVp source beam filtered by
Pb for the TST mine laid on the soil
surface . . . . . . . . 310

VIII.31 Three dimensional image diagram of the
measured collimated detector response
to a 200 kVp source beam filtered by
Al for the TST mine at a depth of
burial of 2.54 cm . . . . . 311


xxiii










LIST OF FIGURES continued


FIGURES

VIII.32 Two dimensional image diagram of the
measured collimated detector response
to a 200 kVp source beam filtered by
Al for the TST mine at a depth of
burial of 2.54 cm . . . . .

VIII.33 Three dimensional image diagram of the
measured collimated detector response
to a 200 kVp source beam filtered by
Al for the TST mine at a depth of
burial of 7.62 cm . . . . . .

VIII.34 Three dimensional image diagram of the
measured collimated detector response
to a 200 kVp source beam filtered by
Al for the TST mine laid on the soil
surface . . . . . . . .

VIII.35 Two dimensional image diagram of the
measured collimated detector response
to a 200 kVp source beam filtered by
Al for the TST mine laid on the soil
surface . . . . . . . .

VIII.36 Three dimensional image diagram of the
measured uncollimated detector response
to a 100 kVp source beam filtered by
Al for the TST mine at a depth of
burial of 2.54 cm with overlying rock
array . . . . . . . . .

VIII.37 Three dimensional image diagram of the
measured uncollimated detector response
to a 150 kVp source beam filtered by
Al for the TST mine at a depth of
burial of 2.54 cm with overlying rock
array . . . . . . . .

VIII.38 Three dimensional image diagram of the
measured uncollimated detector response
to a 150 kVp source beam filtered by
Sn for the TST mine at a depth of
burial of 2.54 cm with overlying rock
array . . . . . . . . .

VIII.39 Three dimensional image diagram of the
measured uncollimated detector response
to a 200 kVp source beam filtered by
Sn for the TST mine at a depth of
burial of 2.54 cm with overlying rock
array . . . . . . . .


xxiv


Page


312





314





315


316


318






319






321






322










LIST OF FIGURES continued


FIGURES

VIII.40 Three dimensional image diagram of the
measured collimated detector response
to a 150 kVp source beam filtered by
Al for the TST mine at a depth of
burial of 2.54 cm with overlying rock
array . . . . . . . . .

VIII.41 Three dimensional image diagram of the
measured collimated detector response
to a 200 kVp source beam filtered by
Al for the TST mine at a depth of
burial of 2.54 cm with overlying rock
array . . . . . . . . .

VIII.42 Irregular soil surface used in measure-
ments . . . . . . .

VIII.43 Three dimensional image diagram of the
measured uncollimated detector response
to a 200 kVp source beam filtered by
Pb for the TST mine at a depth of
burial of 2.54 cm with irregular soil
surface . . . . . . . .

VIII.44 Two dimensional image diagram of the
measured uncollimated detector response
to a 200 kVp source beam filtered by
Pb for the TST mine at a depth of
burial of 2.54 cm with irregular soil
surface . . . . . . . .


VIII.45 Three dimensional image diagram of the
measured collimated detector response
to a 100 kVp source beam filtered by
Al for the TST mine at a depth of
burial of 2.54 cm with irregular soil
surface . . . . . . . .

VIII.46 Three dimensional image diagram of the
*measured collimated detector response
to a 150 kVp source beam filtered by
Al for the TST mine at a depth of
burial of 2.54 cm with irregular soil
surface . . . . . . . .

VIII.47 Three dimensional image diagram of the
measured collimated detector response
to a 200 kVp source beam filtered by
Al for the TST mine at a depth of
burial of 2.54 cm with irregular soil
surface . . . . . . . .


XXV


Page


323






324


326


327






328


330






331






332










LIST OF FIGURES continued


FIGURES Page

VIII.48 Two dimensional image diagram of the
measured collimated detector response
to a 200 kVp source beam filtered by
Al for the TST mine at a depth of
burial of 2.54 cm with irregular soil
surface . . . . . . . . 333

VIII.49 Three dimensional image diagram of the
measured uncollimated detector response
to a 100 kVp source beam filtered by
Pb for a wood disk buried flush to the
soil surface. . . . . . . 334

VIII.50 Three dimensional image diagram of the
measured collimated detector response
to a 200 kVp source beam filtered by
Al for a wood disk buried flush to the
soil surface. . . . . . . 335

VIII.51 Three dimensional image diagram of the
measured uncollimated detector response
to a 100 kVp source beam filtered by
Pb for a steel disk buried flush to the
soil surface. . . . . . . 336

VIII.52 Three dimensional image diagram of the
measured collimated detector response
to a 200 kVp source beam filtered by
Al for a steel disk buried flush to the
soil surface. . . . . . . 337

VIII.53 Three dimensional image diagram of the
measured uncollimated detector response
to a 100 kVp source beam filtered by
Pb for water contained in a thin plastic
container buried flush to the soil
surface . . . . . . . . 339

VIII.54 Three dimensional image diagram of the
measured collimated detector response
to a 200 kVp source beam filtered by
Al for water contained in a thin
plastic container buried flush to the
soil surface. . . . . . . 340

VIII.55 Three dimensional image diagram of the
measured collimated detector response
to a 200 kVp source beam filtered by
Al for a hole filled with loose soil. 341


xxvi










LIST OF FIGURES continued


FIGURES Page

VIII.56 Failure of the dual energy subtraction
technique . . . . ... . 345

VIII.57 Two dimensional image diagram of the
measured uncollimated detector
response to a 100 kVp source beam
filtered by Pb for the TST mine at
a depth of burial of 2.54 cm with
irregular soil surface. . . . . 346

A.1 Typical antitank mine. . . . . . 370

D.1 X-ray fluence spectrum, 80 kVp,
2.00 mm Al. . . . . . . . 398

D.2 Measured and calculated transmission of
exposure rate, 80 kVp, 2.00 mm Al.. .. 399

D.3 X-ray fluence spectrum, 80 kVp,
2.24 mm Al. . . . . . . . 400

D.4 Measured and calculated transmission of
exposure rate, 80 kVp, 2.24 mm Al . 401

D.5 X-ray fluence spectrum, 100 kVp,
2.00 mm Al. . . . . . . .. 402

D.6 Measured and calculated transmission
of exposure rate, 100 kVp, 2.00 mm Al 403

D.7 X-ray fluence spectrum, 100 kVp,
2.24 mm Al. . . . . . . . 404

D.8 Measured and calculated transmission of
exposure rate, 100 kVp, 2.24 mm Al. . 405

D.9 X-ray fluence spectrum, 150 kVp,
3.00 mm Al. . . . . . . .. 406

D.10 Measured and calculated transmission of
exposure rate, 150 kVp, 3.00 mm Al. . 407

D.11 X-ray fluence spectrum, 150 kVp,
3.34 mm Al. . . . . . . . 408

D.12 Measured and calculated transmission of
exposure rate, 150 kVp, 3.34 mm Al. . 409

D.13 X-ray fluence spectrum, 200 kVp,
3.00 mm Al. . . . . . . . 410


xxvii










LIST OF FIGURES continued


FIGURES


D.14 Measured and calculated transmission of
exposure rate, 200 kVp, 3.00 mm Al.

D.15 X-ray fluence spectrum, 200 kVp,
3.34 mm Al. . . . . . .

D.16 Measured and calculated transmission of
exposure rate, 200 kVp, 3.34 mm Al.


E.1 Gadolinium oxysulfide based detector .

E.2 Active region of the detector. . . .

E.3 Spectrum and transmission curve at
115 kVp . . . . . . .

E.4 Fraction of incident energy absorbed,
perpendicular incidence . . . .

E.5 Fraction of incident energy absorbed
in each screen, perpendicular
incidence . . . .. ..

E.6 Fraction of incident energy reflected,
perpendicular incidence . . . .

E.7 Fraction of incident energy transmitted
perpendicular incidence . . . .

E.8 Fraction of incident energy absorbed,
45 degree incidence . . . .

E.9 Fraction of incident energy absorbed
in each screen, 45 degree incidence .

E.10 Fraction of incident energy reflected,
45 degree incidence . . . . .

E.11 Fraction of incident energy transmitted,
45 degree incidence . . . . .

E.12 Fraction of incident energy absorbed,
75 degree incidence . . . . .

E.13 Fraction of incident energy absorbed in
each screen, 75 degree incidence. .

E.14 Fraction of incident energy reflected,
75 degree incidence . . . . .


*


.*


.*


xxviii


Page


S 411


S 412


S 413

416

S 418


S 421


S 430



S 431


S 432


433


S 437


438


S 440


S 441


445


S 446


447










LIST OF FIGURES continued


FIGURES

E.15


Fraction of incident energy transmitted
75 degree incidence . . . .


E.16 Emission spectrum of gadolinium
oxysulfide with 0.3 atom % terbium.

E.17 Emission spectrum of 3M Trimax
12 screens. . . . . . .

E.18 Average number of visible photons
produced per incident x-ray photon.

E.19 Dark pulse count rate versus time. .

E.20 Measured pulse height spectra . .

E.21 Response versus distance for 137Cs .
E.22 Response versus distance for 133Ba .


F.1 X-ray fluence spectrum, 100 kVp,
1.01 mm Al. . . . . .

F.2 X-ray fluence spectrum, 150 kVp,
1.01 mm Al. . . . . . .

F.3 X-ray fluence spectrum, 200 kVp,
2.67 mm Al. . . . . . .

F.4 X-ray fluence spectrum, 100 kVp,
9.52 mm Al. . . . . .

F.5 X-ray fluence spectrum, 150 kVp,
9.52 mm Al. . . . . . .

F.6 X-ray fluence spectrum, 150 kVp,
1.85 mm Sn. . . . . . .

F.7 X-ray fluence spectrum, 200 kVp,
1.85 mm Sn. . . . .

F.8 X-ray fluence spectrum, 100 kVp,
0.25 mm Al, 0.24 mm Pb. . . .

F.9 X-ray fluence spectrum, 100 kVp,
0.75 mm Pb. . . . . . .

F.10 X-ray fluence spectrum, 150 kVp,
0.25 mm Al, 0.75 mm Pb . . .


Page


. 448


. 452


. 453


. 454

. 459

. 462

. 468

. 469


. 471


. 472


. 473


. 474


. 475


. 476


. 477


. 478


. 479


. 480


xxix










LIST OF FIGURES continued


FIGURES Page

F.11 X-ray fluence spectrum, 200 kVp,
0.75 mm Pb. . . . . . . 481

F.12 X-ray fluence spectrum, 200 kVp,
0.25 mm Al, 0.75 mm Pb. . . . . 482

F.13 X-ray fluence spectrum, 200 kVp,
0.25 mm Al, 1.35 mm Pb. . . . . 483

H.1 The fit technique. . . . . .. .. 514


xxx















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


LANDMINE DETECTION BY SCATTER
RADIATION RADIOGRAPHY

By

John G. Campbell

August 1987

Chairman: Alan M. Jacobs
Major Department: Nuclear Engineering Sciences

The application of scatter radiation radiography to

the detection of buried nonmetallic antitank landmines is

examined. A combination of calculations and measurements

is used to address the problem. The primary calculation

tool is a Monte Carlo photon transport code. Measurements

are made with an x-ray source, sodium iodide detector, and

soil box positioning system. The soil box containing a

model of a nonmetallic antitank mine is moved beneath the

x-ray source to simulate both the forward motion of a

vehicle transporting the detection system and raster of the

beam to search a path of sufficient width to allow safe

passage. Calculations are used to suggest mine detection

mechanisms and to optimize geometric parameters and x-ray

beam quality. Measurements are used to validate the

calculation results for a small detector and produce images

of buried mines. The calculations are extended to large


xxxi










area detectors which are required to provide path searches

of approximately three meter widths. Environmental para-

meters, such as height sensitivity, soil density and mois-

ture content, and inhomogeneities are examined in both

calculations and measurements. Power requirements are also

addressed.

A system based upon detector collimation to emphasize

differences in the multiple scattered components, character-

istic of soil and the explosive found in mines, is found to

be capable of mine detection at depths of burial of at least

7.5 cm at power levels compatible with portability, and at

speeds, path widths, detection probabilities and false alarm

probabilities consistent with operational requirements. De-

tection at greater depths is possible in soil recently dis-

turbed by mine burial.

Images of holes refilled with loose soil can be dis-

tinguished from those of buried mines by their character-

istic features. However, the refilled hole images bear some

resemblance to those of mines laid on the soil surface. A

compound detector, consisting of both collimated and un-

collimated regions, can be used to overcome this problem and

increase the probability of detection of mines buried at

shallow depths.


xxxii
















CHAPTER I

INTRODUCTION


This research studies the use of backscattered x rays

to detect and image buried nonmetallic antitank mines. A

source of x-ray photons is directed at the soil surface.

After interacting with soil or a buried object, backscat-

tered photons strike a detector located above the soil

surface producing a response. Detection of a buried object

depends upon differences between the photon interaction

characteristics of soil and the object. The x-ray source is

rastered over the soil surface producing an array of respon-

ses, each of which carries information related to the mater-

ials through which photons passed before reaching the detec-

tor. This array of detector responses is manipulated to

produce an image characteristic of those materials. Calcu-

lations are performed to optimize the detection and imaging

process. A variety of detector geometries and types are

examined by these calculations. Measurements are made with

a small sodium iodide scintillation detector to examine the

predictions of the calculations and to produce images of

buried objects.

Chapter II provides a summary of the use of scattered

x-ray and gamma-ray photons to provide information about










materials they irradiate. The general concepts for the mine

detection and imaging system are also introduced in this

chapter. Three related appendices (A, B and C) provide

background on the characteristics of landmines, a short

history of landmine warfare, and a description of other

technologies which have been applied to mine detection.

Chapter III describes the equipment and materials used

in the research. Included in this chapter are descriptions

of soil and mine materials used in the calculations and

measurements.

Chapter IV describes the photon interaction character-

istics important to the mine detection problem. The single

scatter and Monte Carlo photon transport codes used in the

calculations are also described. Validation of the Monte

Carlo calculation method is presented.

Chapter V describes the method used to produce calcula-

tions of the x-ray source spectrum and the validation of the

technique. Other source calculation methods are discussed.

A related appendix (D) provides a graphical display of one

of the validation methods. Appendix F provides graphs of

spectra used in experiments.

Chapter VI describes the method used to calculate the

response function of the sodium iodide scintillation crystal

used in the experiments. Validation of the calculated re-

sponse function is provided. Response calculation for a

detector based on terbium activated gadolinium oxysulfide is

described in a related appendix (E). Detectors similar to









this could prove useful for covering the large areas neces-

sary to find vehicle paths through minefields.

In Chapter VII, results of the application of the Monte

Carlo transport code to the physics and geometry of mine

detection employing backscattered radiation are provided.

Based on these calculations, several detector types are

selected for further investigation. Optimization of the

geometry and source energy is made for each type of detector

selected.

Chapter VIII applies the results of the previous chap-

ter to producing images of mines. Calculated and measured

images are examined. The effects of environmental para-

meters on images are discussed, and power requirements are

estimated.

Chapter IX presents conclusions derived from this re-

search effort along with recommendations for directions for

future work.
















CHAPTER II

BACKSCATTER MINE DETECTION AND IMAGING


Conventional radiography uses the transmission of

photons through an irradiated object to produce an image.

The image depends upon the photon attenuation properties of

the internal structure of the object. Conventional radi-

ography cannot be used to examine objects buried in soil,

such as mines, because of the obvious inability to locate

the detector below the object. Backscatter radiography,

which depends upon differences in the photon scattering

properties of irradiated objects to produce an image, is

suited to the geometry of mine detection. Photons can

originate and be detected above the soil surface. Scattered

radiation has been used in medical and engineering applica-

tions to determine properties and form images of irradiated

objects. Nonmetallic mine detectors using backscattered

radiation have been constructed and tested, but have not

been considered useful enough for actual field use. The

detection and imaging principles investigated in this re-

search are designed to overcome problems inherent in the

previous work.










Previous Uses of Scattered Radiation

The first suggested use of Compton (incoherent) scat-

tering to determine characteristics of a material was by

Odeblad and Norhagen (1956). They showed that the intensity

of the scattered radiation for a fixed source energy and

scattering angle depends on the electron density of the

scattering medium. In a small volume of uniform composi-

tion, the electron density is proportional to the material

density. Using a collimated 6Co gamma-ray source and a

collimated scintillation detector, they were able to measure

the relative electron densities of materials in the small

volume defined by the intersection of the fields of view of

the detector and source collimators.
192
Lale (1959) used a collimated 92r source and a col-

limated detector positioned to receive forward scattered

Compton photons to measure electron density within trans-

verse cross sections in rabbits and guinea pigs. The sub-

jects were moved with respect to the beam to produce an

image of density variation. The process was very slow, and

subject to considerable quantum noise and attenuation of the

incident and scattered beams, but demonstrated that air in

organs would provide a large change in measured electron

density in images. In an extension of this work, Lale

(1968) used 5.6 MV x rays to reduce attenuation losses.

A patient platform was lowered through the beam. Forward

scattered photons were detected with a liquid scintillator.










Kondic and Hahn (1970) suggested the use of Compton

scattering to measure density variations in two-phase flow.

They examined collimated sources used with both collimated

and uncollimated detectors. With the uncollimated detector,

energy discrimination was used to determine the path taken

by a scattered photon. The relationship between energy and

angle in a Compton single scattering event determines the

position along the source beam from which the photon is

scattered, and the intensity (corrected for attenuation) at

that energy determines the electron density of the material

at that point. Farmer and Collins (1971) independently used

the same uncollimated detector technique in a medical appli-

cation. They used a collimated 1Cs source and an uncolli-

mated Ge(Li) detector to examine cross sectional structure.

Rather than move the patient or scan the beam, the energy

discrimination technique was used to determine origin of the

scattered photons. Problems with this method are attenua-

tion of both the primary and scattered photons, and resolu-

tion reduction caused by detection of multiply scattered

photons. Extensions of this method (Farmer and Collins,

1974) using two higher resolution Ge(Li) detectors, above

and below the patient, and focused to the plane of interest,

also suffered from attenuation and multiple scatter. Reiss

and Shuster (1972) and Dohring et al. (1974) used collimated
137
Cs sources and collimated detectors with patient motion

to determine lung function and measure lung density.

Problems with multiple scattering were again noted.









Clarke and Van Dyke (1973) and Garnett et al. (1973)

developed a two-source method to determine bone density.

The two source technique is used to eliminate the problem of

attenuation by tissue above the bone. The second source is

of the same energy as the single scattered beam of the first

for a selected scattering angle. Measurements of both

transmitted and scattered beams are made in two orientations

to allow correction for attenuation.

Battista et al. (1977) examined the physics of scatter

imaging and described the two major limitations to be atten-

uation of the single scattered photon fluence and contamina-

tion by multiply scattered photons. They provide methods

for obtaining a correction for the multiple scattering prob-

lem. Battista and Bronskill (1978) extended this investiga-

tion and concluded that multiple scatter is an inherent

limitation whose effect can be reduced, but not eliminated,

by improving the energy resolution of the detector. They

also showed that the use of forward scattered components

both reduces dose to patients and the effects of multiple

scatter. Anghaie (1982) showed that predictions of the

multiple scattered component could be used to improve image

resolution by subtracting it from the total signal.

Hanson et al. (1983) successfully used heavily filtered

x-ray beams in the two source densitometry method, taking

advantages of the high intensities and well-defined beams

available from x-ray machines. Errors from contamination by

multiple scattering were found to exceed those due to the

polychromatic nature of the source.









Jacobs et al. (1979) proposed an imaging scheme much

different than those discussed thus far. A collimated

scanning x-ray source with an uncollimated detector was used

to view large angle backscattered photons. Energy modula-

tion of the source was used to produce two images. The

image at the lower source energy is characteristic of the

overlying materials. When subtracted from the higher energy

image (after multiplication by an appropriate factor), the

result is an image characteristic of deeper layers within

the irradiated object. The technique was found to be sensi-

tive to regions of air within the object. This dual energy

approach was shown to allow irregular surface features to be

removed from the final image.

Backscattered Photon Mine Detection

A number of attempts have been made to use backscat-

tered photons to detect buried nonmetallic mines. With only

one exception, the published descriptions are from efforts

sponsored by the United States military. Detailed accounts

of these mine detection systems are contained in classified

documents. The descriptions provided here are from unclas-

sified summaries (Roder, 1975; Nolan et al., 1980). The

wide range of other technologies which have been examined as

possible mine detection methods are described in Appendix C.

Fluorescence Emission

Although fluorescent emission is not a backscatter

technique, attempts to use it as a mine detection mechanism

are similar. Between 1954 and 1957, the Armour Research









Foundation used a 150 kVp x-ray source to attempt to produce

fluorescent emission from the lead or mercury contained in

mine fuzes. These elements are found in approximately one

gram quantities of lead azide, lead styphnate or mercury

fulminate (U.S. Department of the Army, 1986). High resolu-

tion detectors were used to look for the energies of the

characteristic x rays of lead or mercury. The Compton scat-

tered fluence was found to completely obscure any fluores-

cent emission signal which might be present. Additional

efforts using more modern detectors produced this same re-

sult, and the technique was deemed infeasible.

Rayleigh Scattering

From 1958 to 1961, Tracerlab, Inc. attempted to use

Rayleigh (coherent) scattering from these same high atomic

number components of the mine fuze as a detection mechanism.

Because the probability of Rayleigh scattering decreases and

the direction of scattering becomes more forward peaked as

incident energy increases, the technique is limited to shal-

low depths of burial. A 120 kVp x-ray source was employed,

in an attempt to produce Rayleigh scattered photons when the

beam struck the high atomic number materials in the fuze.

Because the incident beam was polyenergetic, the Rayleigh

scattered photons were also produced in a spectrum of ener-

gies. Compton scattering was again the dominant source of

the detected signal. Two rastering detectors were tightly

collimated to focus on a very small volume which might con-

tain the mine fuze. Detection was signaled by a small shift









in the backscattered spectrum to higher energies. The very

small sampled volume, which led to scans on the order of

hours per square foot, was the cause for termination of this

effort. Unfortunately, this detection system was not tested

with a real mine. Had it been employed in such a fashion, a

large difference between cases of irradiation of soil only

and of soil containing a buried mine would have resulted.

The mine detection mechanism would have been the difference

between the photoelectric cross sections of soil and mine.

The higher photoelectric absorption of photons striking soil

results in fewer photons capable of being backscattered than

in the mine present case.

Compton Scattering

The first attempt to examine the contrast mechanism

afforded by the difference photoelectric cross sections of

soil and mine materials was made by the Naval Ordnance

Laboratory in 1960 to 1961. Unfortunately, 2 and 10 MV

bremsstrahlung sources were selected for the experiments.

At these energies the photoelectric cross sections of soil

and mine materials are both very small. The dominant inter-

action is Compton scattering, and the respective material

cross sections for this interaction at these energies are

nearly the same. Additionally, the material densities of

soil and explosive are similar. As a result no contrast

mechanism existed and only negative results were obtained.









Beginning in 1967 and extending until 1973, Texas

Nuclear Corporation conducted experiments to produce non-

metallic mine detection systems using backscatter of gamma

or x rays. These experiments culminated in a nonmetallic

antitank mine detector. A less successful nonmetallic anti-

personnel mine detector was also produced. Both systems

used vertically collimated sources and vertically collimated

detectors. Low energy x-ray sources were used to enhance

the photoelectric contrast between mine and soil. The

antitank mine detector used a 130 kVp x-ray source mounted

on the front of a 1/4-ton truck and four CsI(Tl) scintilla-

tion detectors. It was capable of operation at several

miles per hour. Field tests were conducted at a variety of

military installations with U.S. and Soviet mines filled

with dinitrobenzene (a nonexplosive substitute for trini-

trotoluene). Although depths of detection of up to 10 cm

were achieved, a number of conditions of the test were

optimized to enhance the mine detection process. The tests

were conducted in areas free of buried organic material,

such as tree roots, whose response was known to produce

false alarms in the detector. The areas of the tests were

also fairly level, minimizing sensitivity to irregular

surfaces. Deep ruts or depressions comparable to the size

of a mine were also capable of producing false alarms. The

test areas were free of vegetation, which if present would

have lowered the contrast between soil and soil with mine,

and if nonuniform, could have produced false alarms. The










extent of coverage was also a problem; mines located midway

between two detectors were missed unless they were on the

surface. A final objection to the system involved soil

density. It was found that the detector was sensitive to

density changes whether a mine was present or not. Concern

was expressed that dummy minefields could be produced by

simply digging and refilling holes. The act of emplacing a

mine significantly alters the soil density, reducing it, on

the average, to 75% of the undistrubed value (Roder, 1975).

After weathering had returned the soil to its normal

density, detection was no longer possible at 10 cm, and the

response difference at 7.5 cm was much reduced. Further

work on this detection system was terminated primarily as a

result of the apparent superiority of a competing techno-

logy, and secondarily as a result of the inadequacies de-

scribed (Nolan et al., 1980).

Coleman (1971) performed Monte Carlo calculations for

several cases in support of this effort. They are discussed

in Chapter IV. These calculations and the majority of Texas

Nuclear experiments were conducted with solid blocks of di-

nitrobenzene. The mines used in the field tests were filled

with dinitrobenzene, but it is unclear whether any air

space, characteristic of real mines (described in Appendix

A), was provided.

Preiss and Livnat (1973), working in Israel, provide

the only non-U.S. military publication of research on the

detection of nonmetallic mines by the backscatter of










ionizing radiation. The system consisted of an uncollimated

7Se source with a NaI(Tl) detector collimated to view a

10 cm diameter circle at the soil surface at a fixed detec-

tor height. The system was placed on a cart and pushed by

the operator over dirt road surfaces. The electronic com-

ponents of the detector were carried by backpack. This

research effort was the first to consider the effect of the

air space which exists at the top of mines. Experiments

with solid explosive filling the mine and with an actual

mine, containing an air space, revealed completely different

effects. In the case of the solid mine, the ratio of the

detector response with mine present to mine absent (soil

only) was found to be greater than 1.00. In the case of the

mine with air space, the ratio was less than 1.00. Mine

detection was accomplished by selecting an energy range in

the backscattered spectrum which enhanced the reduction in

response with the mine present.

Backscatter Radiation Radiography

Genesis of Current Research Effort

A high priority has been placed on research into the

detection of landmines (West et al., 1985; U.S. Department

of the Army, 1986). The increased interest in this research

is based upon a combination of factors, driven by the imple-

mentation of a new dynamic operational concept by the U.S.

Army. This new concept, termed the AirLand Battle, is

oriented primarily to the threat of a Warsaw Pact attack

into Western Europe. The superseded defensive concept,









termed the Active Defense, concerned itself with attrition

of numerically superior attacking forces by use of defensive

positions prepared in depth on the battlefield (DePuy, 1984;

Holder, 1985). Simulations indicated that this concept

would be successful against the first echelon of a Warsaw

Pact attack, but without extensive, rapid reinforcement

would be of doubtful utility against the following echelons.

The AirLand battle concept emphasizes the use of aggressive

engagement of the attacking force using both fire and man-

euver at varying depths on the battlefield. Counterattacks

into the flanks of the attacking force and into rear supply

and transport areas are encouraged to disrupt the rigid

plans and time tables characteristic of Soviet military

operations. These maneuvers are also designed to enhance

one of the few perceived advantages of Western military

forces in conventional combat with Soviet forces: the

capability of Western leaders of virtually any size unit to

use their initiative in fluid situations compared to the

discouragement (at least until recently) of any deviation

from detailed plans, regardless of tactical situation,

applied to Soviet leaders, especially of smaller sized units

(Suvorov, 1984; Walker, 1986; Baxter, 1986). Another key

factor in the development of the AirLand Battle concept is

the advent of a series of technological advances and equip-

ment modernizations which make rapid maneuver feasible

(RisCassi, 1986).










The landmine represents a serious challenge to rapid

maneuver. The employment of mines by the attacking force to

protect its flanks and rear areas could do much to neutral-

ize the new operational concepts. The effectiveness of

landmines is high. More than 20% of Allied tank casualties

in World War II were caused by mines. United Nations tank

casualties in the Korean conflict were as high as 70% in

offensive operations. In Vietnam (through 1970) 70% of all

U.S. vehicle losses were due to mines (U.S. Department of

the Army, 1973). The keystone manual of the U.S. Army, FM

100-5, Operations (U.S. Department of the Army, 1982),

emphasizes synchronized execution. Clearly, the capability

of mines to produce delays and disruptions is inconsistent

with the new maneuver oriented operational concepts.

Adding to this concern is the mine warfare capability

and experience of the Soviet Army, which is unsurpassed by

any army in history (Honeywell, 1981). Appendix B provides

examples of the Soviet experience with mines. The primary

mission assigned to Soviet engineer units is to insure the

momentum of maneuver mobility by rapidly overcoming natural

and manmade obstacles, while at the same time hindering

enemy force movement (Sidorenko, 1973). The second portion

of this mission, directly affecting the new U.S. Army oper-

ational concepts, is accomplished by Mobile Obstacle Detach-

ments, which provide countermobility support by laying mine-

fields and establishing other expedient obstacles along

enemy avenues of approach (Plyaskin et al., 1973; Uli,










1986). In short, the Soviet Army is aware of its vulner-

abilities on its flanks and in its rear areas, and is

organized to address the threat, in part, by employing

mines. Soviet doctrine has long included the rapid

emplacement of mines on the surface without burial (U.S.

Department of the Army, 1979a). In the 1970's, mechanical

minelayers and mine dispensing chutes for vehicles and

helicopters were fielded to allow rapid minefield emplace-

ment. More recently fielded scatterable mine systems fur-

ther enhance the capability to respond to the new U.S. op-

erational concepts (West et al., 1985).

Aside from manual probing, a hand-held nonmetallic mine

detector of questionable capability (the hand-held metallic

detector works well), and actual mine detonation in an

adverse encounter, the U.S. Army has no method for detecting

buried nonmetallic mines (U.S. Department of the Army,

1986). These slow or adverse detection mechanisms are

incompatible with the advent of new operational concepts

which rely upon maneuver mobility. Accordingly, reviews of

all previous detection technologies have been conducted by

the U.S. Army in an attempt to find systems which might be

made to work. One such review (Moler, 1985) examined the

range of nuclear techniques (x-ray backscatter is included

within this category, even though it is actually an atomic

technique). This review recommended imaging using x-ray

backscatter as the highest priority nuclear technique for

additional research.










Improvements on Previous X-Ray Backscatter Efforts

The shortcomings of the Texas Nuclear Corporation re-

search effort, described above, provide the basis for

improvements in the x-ray backscatter technique. The

concepts investigated in this dissertation differ from the

previous efforts in a number of areas. The major difference

is the examination of the formation of images of buried

objects, rather than detection based upon a single differ-

ence between soil and soil with buried object. Creating an

image requires capabilities that were unavailable in the

past. X-ray sources capable of long linear scans and the

image processing technology to allow real time analysis of

data have been developed since the Texas Nuclear Corporation

efforts. An image provides the important capability to

discriminate between buried mines and other buried objects

which have photon interaction characteristics similar to

mine materials. Coupling the scanning x-ray beam with a

detector large enough to assure coverage of width of the

largest vehicle which must traverse a mined area eliminates

another shortcoming of the previous effort. A diagram of a

conceptual detector is shown in Figure II.1.

Research Goals

The goals of this research effort are to optimize

the design parameters of a large area, x-ray backscatter

imaging system and to examine the effect of environmental

parameters on the detection and imaging process. The design

parameters available for optimization are the energies of













Source


direction of
motion of vehicle


raster direction


/ h

I/
soil /







d
s
mine
5--



Figure II.1. Conceptual large area backscatter detector
system. A pencil, incident x-ray beam strikes the soil
surface. The beam is scattered as it penetrates the soil
and mine. Some of the photons scattering within the mine
reach the panels of the detector after single or multiple
scatters. Distances indicated on the diagram are the height
of the detector above the soil, h; the depth of burial of
the mine, d; the size of the gap between the two panels, d ;
and the width of a panel, d.









the x-ray beams, beam angle of incidence, beam size, beam

collimation, detector geometry, and detector collimation.

Environmental parameters are soil type, soil density, soil

moisture content, inhomogeneities with the soil, surface

irregularities, mine geometry, and mine depth of burial.

The method for examining parameters is a combination of

calculations and measurements. The primary calculation tool

is a Monte Carlo photon transport code written specifically

for the mine detection problem. Measurements are made with

a small NaI(Tl) detector to validate the Monte Carlo pre-

dictions, allowing extension of the code to large area

detector configurations.
















CHAPTER III

EQUIPMENT AND MATERIALS


The apparatus used to perform measurements is designed

to simulate the raster of the x-ray beam across a soil sur-

face which may contain buried objects. This raster simula-

tion is accomplished by moving a soil box under a fixed

x-ray beam. The complete simulation system consists of the

x-ray machine, the soil box and its positioning system, the

detector and its related electronics, and the computer con-

trol devices. Figure III.1 shows the x-ray source, soil box

and positioning system, and detector. Figure III.2 shows

the detector electronics, computer control and x-ray source

control systems. Materials used for soil and buried objects

are selected to simulate those items found under field con-

ditions.

Equipment

X-Ray Source

An x-ray machine is selected as the source of the pho-

tons for backscatter imaging applications because of its

capability to produce intense photon beams which can be

rastered. Extremely high activity radionuclide sources

would be required to produce similar intensities in the

collimated beams necessary for the imaging process. Such























Figure III.1 X-ray source, soil box and positioning system, and detector. The GE
Maxitron 300 x-ray generator (top center) is held in a fixed position while the soil
box (center) is moved in raster mode by drive screws powered by DC motors with
controlled clutch/brakes. The positioning interface to the controlling computer is
on the right side of the photograph.










L "-






















Figure III.2. Detector electronics, computer and x-ray source console. This
photograph shows, from left to right, the detector high voltage supply, scaler and
timer, amplifier and single channel analyzer, count rate meter, the IBM PC computer,
and the GE Maxitron 300 control console with remote TV picture of exposure room.
















ii 2


; ,,,,,,,,,,,,,, I n I









sources require heavy shielding at all times and pose a

constant radiological safety concern. An x-ray machine

poses the same hazard only when in operation. Since the

mine detection problem requires a minimum path width equal

to the widths of following vehicles (on the order of 3

meters), rastering of the beam is required. Mechanical

systems are not practical for rastering a radionuclide

source at the speeds required for the imaging problem (on

the order of 103 m/s), or alternatively, moving a collimator

along a line source at those speeds. The electron beam of

an x-ray machine can be scanned along an extended anode at

very high speeds to provide the raster required. An addi-

tional advantage of an x-ray machine is the capability to

alter intensities by varying beam current and to alter beam

quality by varying tube voltage or filtration. Separate

radionuclide sources would be required to accomplish such

alterations.

The source of the photons used in the backscatter imag-

ing experiments is a General Electric Maxitron 300 X-Ray

Therapy Unit (General Electric, 1962). The unit is capable

of producing continuous beams of 70 to 300 kVp at beam cur-

rents between 5 and 20 mA. The primary voltage waveform

accelerating the electrons to the anode is single phase,

self-rectified at approximately 1200 Hz. The accelerated

electrons strike a 45 degree angle tungsten anode. If the

electron energy exceeds that of the K shell binding energy

of tungsten, K characteristic x rays are produced in









addition to the continuous x rays produced at all energies.

All beams pass through a 4.75 mm thick beryllium window.

Additional filtration can be provided both within and out-

side the head of the unit. Adjustable internal, rectangular

collimators are employed to shape the beam. When an ex-

ternal filter is used, an additional external collimator is

employed to prevent the majority of scattered or fluorescent

photons produced within the external filter from reaching

the soil plane.

The shielding of the head of the x-ray unit is supple-

mented by a 0.16 cm (1/16 inch) thick layer of lead. This

additional shielding was found to be required when measure-

ments were performed using an uncollimated detector with a

heavily filtered beam at higher accelerating potentials.

The higher accelerating potentials produce photons more

likely to penetrate the standard shielding of the unit.

This fact, combined with low intensity fields produced by

heavily filtered beams, results in a significant fraction of

the detector response being caused by head leakage scatter.

The lead shielding reduces the probability of head leakage

photons reaching the soil and subsequently scattering into

the detector. The shielding employed does not entirely

eliminate the problem, requiring two sets of measurements

to be made at high energies when an uncollimated detector is

used. The first image scan is made with the desired beam

filtration. A second scan is then made with a very thick

lead external filter which prevents beam photons from









reaching the soil. This second image scan is, therefore,

the result of the head leakage scatter. Subtraction of the

second scan from the first corrects the imaging data for the

head leakage scatter. Structural constraints caused by the

weight of the shield prevent thicker layers from being used.

Figure III.3 shows lead shielding covering the head of the

x-ray machine.

Soil Box Positioning System

The soil box positioning system was constructed accord-

ing to a design by Moss (1986). The control system was con-

structed by Moss. The soil box is positioned in the x-y

plane (the plane parallel to the floor of the exposure room)

beneath the source by a two level linear bearing system

driven by ball screws which are powered by DC motors with

controlled clutch/brakes. The scan motion is boustrophe-

donic. Both local and remote control of the positioning

system are available. Local control is used to provide the

initial beam-soil intercept position prior to irradiation.

Remote control of the soil box motion is through an RS-232

serial interface bus. It is used in the imaging process to

move the soil box through the array of measurement posi-

tions. Two soil boxes of dimensions of 66 cm by 66 cm by 45

cm deep and 122 cm by 91 cm by 45 cm deep are used. The

larger box is required for measurements with a collimated

detector. Both are filled with locally obtained sandy soil

typical of North Central Florida.






















Figure III.3. Lead shield for tube head and detector. The detector within its
shield and the shielding of the head of the x-ray generator are viewed from below.
The shielding is required to attenuate x-ray leakage from the generator head in
directions other than that of the beam. The filter holder with filter and external
collimator is also shown.







29



























































'I




ra.










Detector and Related Electronics

Two types of detectors have been used in the imaging

measurements. The x-ray sensing portion of the first de-

tector is based on terbium activated gadolinium oxysulfide

rare earth intensifying screens. This device was construct-

ed to provide an inexpensive, sensitive, large area detec-

tor. For reasons detailed in Appendix E, this detector is

found to be unsuitable for the detection and imaging tasks.

It is replaced by Bicron Model .5M.390/.5L-X, sodium iodide

detectors. This detector type is used in all imaging

measurements. The geometry of this detector is shown in

Figure III.4. Also included in this diagram is a composite

shield designed to allow the detector, when operated in an

uncollimated mode, to simulate small regions of a large area

plane detector by permitting photons to enter only through

the exposed face. Several regions of the detector (labeled

3, 4, 5 and 6 in Figure III.4) are not identified in the

diagram. Bicron Corporation, the manufacturer of the detec-

tor, provided the compositions and densities for these

materials with the understanding that they would not be

published due to their proprietary nature (Melocik, 1986).

They are included in the Monte Carlo calculations performed

to determine the detector response function (described in

Chapter VI). Table III.1 provides the dimensions of the

materials shown in Figure III.4.

Because a large area, plane detector is a possible can-

didate for an actual fielded system (Chapter II), it is




















10 9 10 9 9

81


9 9 10 9
- i


Figure III.4. Geometry of the sodium iodide detector and shield. A cross section of
the Bicron Model .5M.39Q/.5L-X NaI(Tl) detector (Melocik, 1986), and locally fabri-
cated shield is shown (not to scale). Numbers in detector and shield regions
correspond to materials and dimensions provided in Table III.1.


10


6=


I










TABLE III.1

Geometry of the Sodium Iodide
Detector and Shield


# Material Diameter or Thickness
Width (cm)

1 NaI(Tl) crystal 1.2700 0.9906

2 Quartz light pipe 1.2700 1.2700

3 Bicron proprietary 0.1588 a

4 Bicron proprietary 1.5875 a

5 Bicron proprietary 1.5875 a

6 Bicron proprietary 1.5875 a

7 Aluminum housing (face) 1.6383 0.0254

8 inner Air space 0.04 1.0643

9 inner Tin 0.07 1.0643

8 mid Air space 0.06 1.0643

9 mid Tin 0.07 1.0643

10 inner Lead 0.1588 15.0343

8 outer Air space 0.08 2.3343

9 outer Tin 0.07 2.3343

10 outer Lead 0.3175 2.3343

Dimensions of the Bicron Model .5M.390/.5L-X NaI(Tl) detec-
tor and locally fabricated shield used in measurements and
calculations. Numbers (#) in the table are keyed to Figure
III.4.

aMaterials and thicknesses are proprietary information of
Bicron Corporation.









desirable to retain as much similarity to such a configura-

tion as possible. The shield is employed to assist in re-

taining this similarity in the small sodium iodide detector

by preventing large numbers of photons from striking the

sides of the crystal. The responses of a small detector,

taken at a number of positions, can then be used to simulate

a large detector. Additionally, considerably greater detail

is available with a small detector than with a large detec-

tor which averages detailed response information over its

greater area. The purpose of the tin inner layer of the

shield is to prevent K fluorescent x rays produced in the

lead of the shield from entering the sides of the detector.

If this layer were not present and a lead layer was adjacent

to the detector, lead K fluorescent x rays from the lip of

the layer would enter through the side of the detector. The

high photoelectric cross section of tin at these energies

(72.794 to 87.343 keV) makes it an attractive material for

shielding lead x rays. The lower level discriminator of the

counting system is set high enough to preclude counting of

tin K fluorescent x rays (25.042 to 29.106 keV) (Storm and

Israel, 1970). The face of the NaI(Tl) crystal and the

bottom of the shield are at the same level to preclude

collimation of the detector. Collimators are attached to

the detector shield when such a configuration is desired. A

detailed description of the modeling of the detector

response function, including correction for edge effects and

the shield, is provided in Chapter VI.









The usual purpose of the lower level discriminator

setting of the counting system is to preclude pulse height

events corresponding to electronic noise. As described

above, an additional purpose in this detector system is to

prevent tin K fluorescent x rays, which could enter through

the sides of the detector, from being counted. A set of

radioactive sources is used to determine the relationship

between photon energy and lower level discriminator setting

(in combination with a fixed detector high voltage supply,

and amplifier and preamplifier settings). Sources and ener-

gies used for this calibration are given in Table III.2. A

discriminator setting corresponding to 35 keV was selected

to prevent counting of spillover of the tin K ray peak as a

result of the resolution of the detector. Based upon the

Monte Carlo spectral and number albedo calculations (Chapter

IV provides examples), this setting results in only a small

reduction of the total detector response compared to the

case when no discrimination is used. The fluence spectral

calculations show that only when the source energy is small

is there any significant contribution below 35 keV. The

number albedo (the fraction of incident photons which are

reflected from a surface) calculations show that low energy

source photons produce significantly less backscatter than

high energy photons (this is true up to about 300 keV).

Additionally, results of the detector response calculation,

provided and described in Chapter VI show that low energy

photons produce a much lower response than all others except






















TABLE III.2

Sources Used in Determining Lower
Level Discriminator Setting


Enerav (keV)a


(Ag Kal x ray)
(Cs Kl1 x ray)
(Ba K.l x ray)
(gamma)
(gamma from 109mAg)
(gamma)


22.162
30.970
32.191
80.999
88.037
122.06135


aphoton energy data are from Lederer and Shirley (1978).


Source


109Cd
133Bs
137Cs
133Ba
Ba
109Cd
57Co










very high energy photons (which pass through the detector

without significant interaction). The 35 keV value also

provides some safeguard for the lower level discriminator

setting determination from non-linearities observed in the

low energy response of NaI(Tl) (Aitken et al., 1967). The

light output and hence pulse height is not proportional to

the amount of energy deposited in the NaI(Tl) crystal for

low photon energies. Figure III.3 shows the detector and

shield. The slotted wooden structure supporting the

detector allows the distance between the beam axis and the

detector to be varied.

The detector is operated in a pulse counting mode. The

detector high voltage is supplied at -900 volts. Figure

III.5 provides a diagram of the components of the counting

system. Remote control of the counting system is by an

IEEE-488 General Purpose Interface Bus (GPIB).

Computer Control System

An IBM PC personal computer controls both the RS-232

serial interface bus, which operates the soil box position-

ing system, and the IEEE-488 GPIB, which operates the

counting system. Software for these two functions was

provided by Moss (1986). The RS-232 serial interface bus

transmits the direction, distance and axis of motion to the

motor controllers. The GPIB controls the counting channel

and time through the counter/timer. The two systems are

integrated by the computer to allow complete automation of

the scanning and counting tasks required to produce an












Bicron
.5M.39Q/.5L-X
NaI(Tl) Detector


ORTEC 556
High Voltage
Power Supply


ORTEC 113
Preamplifier




ORTEC 590A
Amplifier and Single
Channel Analyzer


ORTEC 974
Timer and Quad
Counter




IEEE 488
General Purpose
Interface Bus



IBM PC XT
Personal Computer


ORTEC 449
Log/Linear
Rate Meter


Figure III.5. Components of the counting system.


I r


I


H









image. Independent operation of the positioning and count-

ing systems is also possible.

Simple graphical display programs, written in Turbo

Pascal (Borland, 1985), are used to rapidly analyze the

image data. These programs accept the data files produced

by the counting system control program.

Materials

Soils

Three soil types are selected for calculations to

represent a range of soil properties. Norfolk sandy loam

(Jaeger, 1975) has a high silicon dioxide content and is

similar to the North Central Florida sandy soil used in the

measurements. Hagerstown loam (Bear, 1955) is close to the

average of all soil types examined in elemental composition.

Malatula clay loam is a lateritic soil with high iron con-

tent. Lateritic soils are produced under conditions of high

rainfall and high temperatures. These conditions, over geo-

logic periods of time, lead to the decomposition of organic

materials and selected minerals. The result is a soil low

in silicon dioxide and high in hydrated oxides of iron and

aluminum (Bear, 1955). A global average soil constructed

from the average elemental composition of the crust of the

earth is also used in some calculations (Jaeger, 1975).

This global average soil is very similar in its photon in-

teraction properties to Hagerstown loam. Hereafter, these

soils will be referred to as NSL (Norfolk sandy loam), HTL

(Hagerstown loam), MCL (Malatula clay loam) and GAD (global









average). The elemental compositions, densities and weight

percentages of water of these soils are given in Table

III.3. A comparison of the mass attenuation coefficients of

the NSL, HTL and MCL soils is given in Figure III.6. The

coefficients are calculated from Hubbell's data (1982).

Nonmetallic Antitank Mine Model

Nonmetallic antitank mines of the Warsaw Pact are the

subject of the mine detection effort. Nonmetallic mines are

important subjects for study because of the difficult prob-

lem they present to all current mine detector types and

because their implications to changes in U.S. operational

doctrine. Metallic mines are not considered since other

techniques are more applicable to their detection. While

nonmetallic antipersonnel mines are also very difficult to

detect, they are a secondary concern for mounted armor

combat operations. Also, while buried, surface laid, and

scatterable mines would be employed in any large scale

conflict in Europe, this study concerns itself primarily

with the buried mine, the more difficult detection problem.

Table III.4 provides characteristics of several common

conventional Warsaw pact nonmetallic landmines. The TST

mine, listed in the table for the purpose of comparison, is

the model used in experiments and calculations. As indi-

cated by the table, it is representative of common Warsaw

pack nonmetallic antitank mines.

The TST model consists of a lucite, right circular

cylinder, with 0.635 cm thick walls and outside diameter of









TABLE III.3

Composition of Soil Types


Element Weight Percentage of Elements in Dry Soilsa
NSL HTL GAD MCL

H 0.070 0.185
C 0.502 1.320 -
O 52.627 49.637 47.330 38.702
Na 0.082 0.629 2.840 0.052
Mg 0.054 0.674 2.110 0.784
Al 1.095 6.236 8.240 18.955
Si 44.142 34.330 28.100 1.730
P 0.026 0.086 0.493
S 0.028 0.162 -
K 0.083 2.327 2.640 0.075
Ca 0.278 0.688 3.650 0.129
Ti 0.425 0.626 8.035
Mn 0.008 0.040 0.504
Fe 0.580 3.061 5.090 30.578


aData for NSL and GAD are from Jaeger (1975). Data for HTL
and MCL are from Bear (1955).


Density and Moisture Ranges

Soil Type Density Range Moisture Range
(g/cm ) (%)
NSL 1.40 1.96 5 25

HTL 0.96 2.17 8 25

GAD 0.96 2.17 10 30

MCL 0.080 1.80 15 30

bData from Hough (1957) and Chilton et al. (1984).







102


10
0


S\ Largest to Smallest:
< MCL, HTL, NSL, TNT

Ca 1





10-1
10 -2 10 -1 1
Photon Energy (MeV)

Figure III.6. Soil mass attenuation coefficients. The mass attenuation coefficients
(cm /g) of the three soils used in the majority of the calculations, Malatula clay
loam (MCL), Hagerstown loam (HTL), and Norfolk sandy loam (NSL) are displayed. The
mass attenuation coefficients for trinitrotoluene (TNT), the explosive contained in
most mines, are also shown for comparison.


















TABLE III.4

Characteristics of Common Warsaw Pact
Nonmetallic Antitank Mines


Mine Country Mass Diameter Height Expl. Expl.
(kg) (cm) (cm) Type Mass (kg)

PM-60 GDR 11.3 32 12 TNT 8.6

TM-60 USSR 11.3 32 11.7 TNT 7.5

TMB-2 USSR 7.0 27.4 15.5 TNT or 5.0
AMATOL

PT-Mi- CZECH 9.9 32.2 10.2 TNT 5.6
Ba-III

TST N/A 10.3 30.2 variable sucrose 7.5


Table adapted from U.S. Department of the Army, TRADOC
Threat Monograph, Comparison of Selected NATO and Warsaw
Pact Engineer Organizations and Equipment (U.S. Army
Training and Doctrine Command, Fort Monroe, VA, 1979b),
p. 88.









30.16 cm. The cylinder is 14.60 cm high and has two 0.635

cm thick covers for the top and bottom. An aluminum cylin-

der with 0.24 cm thick walls, outside diameter of 28.89 cm

and height of 8.57 cm fits inside the lucite cylinder and

holds the explosive substitute material. Only the top 7.50

cm of the aluminum cylinder is filled with explosive mater-

ial. Its lower portion is separated from this material by a

0.24 cm thick base plate. The 0.83 cm high curtain below

the aluminum base plate is drilled with three holes at 120

degree intervals. These holes align with five sets of three

holes in the lucite cylinder and are used to allow variable

setting of the air gap located between the top lucite cover

and the explosive substitute material. The aluminum con-

tainer of the model provides structural support for the

heavy explosive substitute portion of the mine. Addition-

ally, it served as the mold for the molten substitute

material when it was prepared. Aluminum is very similar to

soil in its photon scattering properties, and, as such, is

an acceptable wall material for the backscatter radiation

method of mine detection. Due to its metallic content, it

would be an unacceptable model for many other detection

methods. Figure III.7 shows the TST mine used in the

measurements.

Since actual explosive materials present safety and

administrative problems, a substitute material is required.

Since TNT is the most commonly used explosive in landmine,

it serves as the standard against which substitute materials






















Figure III.7. TST mine used in measurements. The TST mine is designed to simulate
nonmetallic antitank mines. The upper layer of the mine cylinder, whose thickness
can be varied, contains air. The lower portion contains the explosive substitute
material. A detailed description of the geometry and materials of the TST mine is
provided in the text.















~ZI Cr
-;I-~.-
,
~~t^U
~-? `t~!:
r; 3C~-PILL~;
2~~-'' ;z i,.
'' "~'~,~kiS-C;;`-l.-:


- r
._.. r.

"-


17LZ4









are compared. Previous studies made use of dinitrobenzene

as a TNT substitute. Unfortunately, this material is toxic.

Evaluation of a number of common nontoxic materials is made

by comparing linear interaction coefficients with those of

TNT. Sucrose is selected as the substitute. Table III.5

shows the comparison of the interaction coefficients of TNT

and sucrose.

The explosive substitute is solidified KaroTM Light

Corn Syrup. While this material is not sucrose, it has

similar elemental composition and photon interaction

characteristics. Upon heating, a portion of the fructose

contained in the syrup is converted to sucrose. A number of

test batches of the substitute are made by removing water

from the syrup by heating. When the capability to consis-

tently obtain the same material density (1.56 g/cm3) is

achieved, samples are used in the tests described below and

found to be an acceptable substitute for TNT.

Materials Tests

Tests of photon interaction characteristics of the

explosive substitute and soil materials are conducted to

insure that the cross section sets used in the Monte Carlo

photon transport calculations are adequate. As described

above, the TNT substitute is solidified KaroTM Light Corn

Syrup with a density of 1.56 g/cm The soil used in the

experiments is obtained locally. Its high sand content

suggests that it is similar to the Norfolk Sandy Loam (NSL)

soil described above. Samples of each of these materials









TABLE III.5

Ratios of the Linear Interaction
Coefficients of Sucrose to TNT


Energy Interaction Coefficient Ratiosa
(MeV) Coherent Incoherent Photoelectric Total


0.010

0.015

0.020

0.030

0.040

0.050

0.060

0.080

0.100

0.150

0.200

0.300

0.400

0.500

0.600

0.800

1.000


0.9379

0.9346

0.9336

0.9335

0.9334

0.9334

0.9334

0.9334

0.9335

0.9333

0.9338

0.9338

0.9339

0.9339

0.9339

0.9340

0.9339


1.0257

1.0171

1.0117

1.0062

1.0037

1.0024

1.0017

1.0008

1.0004

1.0000

0.9998

0.9997

0.9996

0.9996

0.9996

0.9996

0.9996


0.9505

0.9520

0.9528

0.9540

0.9545

0.9547

0.9559

0.9548

0.9571

0.9571

0.9572

0.9572

0.9572

0.9583

0.9586

0.9581

0.9583


0.9524

0.9581

0.9652

0.9789

0.9873

0.9919

0.9944

0.9967

0.9978

0.9987

0.9991

0.9994

0.9994

0.9995

0.9995

0.9995

0.9996


a Sucrose density: 1.588 g/cm3; TNT density: 1.654 g/cm3
(Weast, 1967). For the purpose of backscatter radiation
effects, the two interaction coefficient ratios of the most
importance in evaluating a substitute material are the
incoherent and total coefficients. Coefficient data are
from Hubbell et al. (1975) and Hubbell (1982).









are placed in the beams of various spectra produced by the

GE Maxitron 300 X-Ray Therapy Unit. Before the materials

tests are conducted, each of the four energy spectra

utilized in the measurements is itself tested using exposure

attenuation by added aluminum filtration as described in

Chapter V. The conditions required for formal half value

layer measurements are observed in these measurements and in

the materials tests (Johns and Cunningham, 1983). The

transmission of exposure rate is also calculated using the

method described in Chapter V for NSL (three sets of data

for NSL at different density and moisture contents) and TNT.

Seven thicknesses of the solidified KaroTM Light Corn

Syrup are each subjected to the four spectra: 80, 100, 150,

and 200 KVp, each filtered by 4.75 mm of beryllium inherent

filtration, 0.25 mm aluminum equivalent monitor chamber,

3.19 mm of aluminum added filtration, and an air path of

67.31 cm. Figure III.8 compares the measured exposure rate

transmissions with those calculated. Perfect agreement

would occur if the ratio for each sample of measurement to

calculation is 1.00 or, in terms of the figure, if the plot-

ted points lie on the line of slope equal to 1.00.

Agreement is very good, and the explosive substitute is

deemed adequate.

For each of the four beam energies listed above, three

sets of five soil samples are prepared (60 samples in to-

tal). Multiple samples are used because of the variability

in composition, density and moisture content characteristic







0.7


0.6
0
0.5

~0.4

0.3

0.2

( 0.1


0 .0 1 1 1 1 1 1
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Calculated TNT Transmission
Figure II.8. Transmission comparison for TNT and substitute. A comparison of the
measured transmission of exposure rates produced by samples of the explosive sub-
stitute material, and the calculated attenuation of exposure rates of TNT for the
same thicknesses as the substitute samples is shown. Calculations were performed by
the XRSPEC.PAS code (described in Chapter V).









of the soil. Two of the sets of samples differed only in

density; the compacted set density is measured to be 1.579

g/cm and the loose soil set, 1.450 g/cm3. Both have an

average moisture content of 3.26%. The third set differs

both in moisture content and density. It is prepared by

heating the soil to remove all moisture. The density of

this soil is 1.62 g/cm The increase in density with loss

of water is a result of combustion of low density organic

matter in the soil during heating. All samples are of the

same thickness. Exposure transmission measurements and

calculations are compared in Figure III.9. Agreement is

very good, indicating that the local soil is, as suspected,

close to NSL soil in its photon interaction properties.









0.4-




0.3
CA)

CO
| 0.2


(

(0.1




0.0
-j



0.0 0.1 0.2 0.3 0.4
Calculated NSL Soil Transmission
Figure III.9. Transmission comparison for NSL and local soil. A comparison of the
measured transmission of exposure rates produced by sets of samples of the locally
available soil, and the calculated attenuation of exposure rates of NSL soil for the
same thicknesses as the local soil samples is shown. Calculations were performed by
the XRSPEC.PAS code (described in Chapter V).















CHAPTER IV

RADIATION TRANSPORT


In the mine detection system, photons, originating from

an x-ray source, travel through air, and strike the soil.

The photons then undergo interactions with the soil and

objects buried within it. Some photons are scattered back

through the soil surface and strike the detector. This

chapter describes the fundamental photon interactions of

importance to the mine detection problem, the radiation

transport models used to simulate those interactions, and

their validation.

Photon Interactions

Photons interact with matter through a variety of

mechanisms. The energy range of interest for mine detection

and imaging (described in Chapter VII) results in only three

photon interactions of importance: coherent scattering,

incoherent scattering and the photoelectric effect. A brief

description of each of these interaction types is provided.

Coherent Scattering

Thomson gave the first description of the interaction

of an electromagnetic wave with a free electron (Jammer,

1966). Applying purely classical physics to the interac-

tion, he showed that the time varying electric field









associated with the electromagnetic wave would cause the

electron to oscillate with the same frequency as the field.

The resulting accelerated charged particle would then radi-

ate an electromagnetic wave of this same frequency. Since

the frequency of the photon is unchanged, there is no change

in photon energy as a result of the coherent scattering

interaction. This elastic scattering process is known as

Thomson scattering. The solid angle differential cross

section (the probability of scatter into a unit solid angle

per electron per unit fluence incident on the electron) for

Thomson scattering is given by

daT re 2
d--- = 2- (1 + cos a ,

da
where dQ is the solid angle differential Thomson

scattering cross section,

re is the classical radius of the electron,

e is the scattering angle.


When the photon energy is such that its associated

wavelength is comparable in size to the atoms in the mater-

ial in which it scatters, the interaction can no longer be

considered to be with a single free electron. The inter-

action is now collectively with all the electrons of an

atom. These atomic electrons oscillate and radiate in

phase. The process is called coherent or Rayleigh scatter-

ing. In this case the solid angle differential cross

section becomes










r (2
dacoh _e 2 2
-d 2e (1 + cos 28)F2(xZ)
d 2'


do
dcoh
where d- is the solid angle differential coherent

scattering cross section,

F(x,Z) is the atomic form factor, which depends

upon the atomic number, Z, of the material,

and the momentum transfer variable, x, given

by


Os
sin-

where is the wavelength of the photon.
where A is the wavelength of the photon.


The integral of the solid angle differential coherent

cross section gives the probability of coherent scattering

per atom per unit incident fluence,


coh = rr2
coh e


(1 + cos2 )sin F2 (x,Z)d ,
(os s s


where acoh is the total coherent scattering cross section

per atom. Coherent scattering cross sections and atomic

form factors are provided in tabular form for all elements

by Hubbell et al. (1975). The square of the atomic form

factor represents the probability that the electrons of an

atom take up the recoil momentum of the interaction without

absorbing any of the incident photon's energy. Figure IV.1

shows a graph of the atomic form factors of aluminum (Z=13)








30.0


\
\

20.0 -
x \ Dashed line: Iron
SSolid line: Aluminum

1

10.0







0.0 1.0 2.0 3.0
x (Reciprocal Angstroms)
Figure IV.1. Atomic form factor versus momentum transfer variable. Atomic form
factors for aluminum and iron are shown as a function of the momentum transfer
variable. Data are from Hubbell et al. (1975).









and iron (Z=26) as a function of x. At large values of x,

the atomic form factor and, hence, the probability of coher-

ent interaction, is small. Large values of x correspond to

small photon wavelengths or high photon energies. The high-

er the atomic number of the material, the larger the atomic

form factor at a given energy and scattering angle. Hence,

at a given energy, coherent scattering is more probable in

high Z materials than in low Z materials. The effect of the

atomic form factor term is to strongly peak the coherent

scattered photons in the forward direction. This forward

peaking is largest in low Z materials and at high energies.

Figure IV.2 displays these effects. Because of the forward

peaking and lack of change in energy, the typical coherently

scattered photon closely resembles the incident photon, and

many calculations ignore this interaction mechanism." In

terms of the mine detection problem, coherent backscatter

will be important only at relatively low energies, and will

have a larger effect in the soils containing the highest

portion of high Z elements Figure IV.3 shows the coherent

cross section of aluminum and iron as a function of photon

energy.

It should be noted that atomic form factors are avail-

able only for individual atoms and a very few compounds.

Since coherent scattering is a cooperative process involving

all the electrons of an atom and the spatial distribution of

the electron density about an atom in a molecule is altered

relative to the free atom, the use of the available atomic








102


-: -1 U
0 10 -.

S" -- 20 keV

(n - -
c 20 keV
0 -

o 10 -


m 10 3 Al 100 keV
Q \^ --------___----



0


0 20 40 60 80 100 120 140 160 180

Scattering Angle (degrees)
Figure IV.2. Solid angle differential coherent scattering cross section versus
scattering angle. The graph shows that for a given material, coherent scattering is
more forward peaked at higher energy, and for a given energy, coherent scattering in
any direction is greatest in the material with the higher atomic number.











10

0 Fe
0 1 -


0
S= Al
U)
10

U 10 -2-


10 -3
10 -2 10 1
Energy (MeV)
Figure IV.3. Coherent cross section versus photon energy. The coherent scattering
cross section of aluminum and iron are shown. The material having the higher atomic
number has the higher coherent scattering cross section at all energies. Data for
the graph are from Hubbell et al. (1975).


10 3









form factors for compounds is only an approximation to

physical reality.

Incoherent Scattering

Compton (1923) first described photon inelastic scat-

tering from a free electron. In his model of this inter-

action, the photon strikes a free, stationary electron pro-

ducing a new, lower energy, scattered photon and a recoil

electron. This free electron case will be approximately

correct if the energy of the incident photon is very large

in comparison with the binding energy of the electron to its

atom. Compton's formula for the dependence of the scattered

photon's energy on the energy of the incident photon and the

scattering angle is

E
E' =
l+a(l-coses)


where E' is the energy of the scattered photon,

E is the energy of the incident photon,

8s is the scattering angle, and

a = E/mec2, where mec2 is the rest mass energy of the

electron (0.511 MeV).

This relationship plays a very important role in the

mine detection problem/ Figure IV.4 shows the fractional

energy (E'/E) in a Compton interaction as a function of in-

cident photon energy for several scattering angles. The

fractional loss is greatest at high energies, and at a fixed

energy, for large scattering angles backscatteringg). Since








1.00

>0.90
,.

i 0.80

0.70
-D

S0.60
4-
0
c 0.50
0

S0.40
LL


0.30 tT
0.00


Scattering Angle
(degrees)
45


90


180


I I I I I I I I I I I I I i i I I I I I I 1 I- I-I I I-
0.05 0.10 0.15 0.20 0.25 0.30
Incident Photon Energy (MeV)


Figure IV.4. Fractional energy of Compton scattered photons versus incident photon
energy. The graph shows that the fraction of energy retained by the scattered photon
is greatest for small scattering angles, and for low incident photon energies.









/igh photon energies are required for deep penetration,

these two factors combine to make backscatter from signifi-

cant depths in the soil difficult./

The Klein-Nishina formula (Evans, 1955) gives the solid

angle differential scattering cross section for the inelas-

tic scattering of an unpolarized photon from a free elec-

tron,


do r2 1+cos28
KN e s
d 2 [l+a(l-cos8s)]2


2 2
a (1-coses) 2
+ -.3
[1+a(l-cos9 )]3


dKN
In this equation d is the solid angle differential
dQ
Klein-Nishina cross section per electron. Figure IV.5 shows

the differential Klein-Nishina cross section as a function

of scattering angle for three energies. At low energies

forward scatter and backscatter are approximately equally

probable. As energy increases, scattering becomes more

forward peaked. This fact increases the difficulty of the

backscatter detection of mines The use of higher energy

photons, which are capable of penetrating to great depths in

soil, will eventually lead to a lower backscattered fluence

due to this forward peaking and the two factors discussed

above with respect to Compton's energy/angle relationship.

In reality, photons are bound, and inelastic events at

energies at which the incident photon energy is not very









0.08

-C
20 keV

0.06
J




C
100 keV
00.04



C)
a 0.02 500 keV
0

0
O 0.00
0 0 .0 0 .i | | i i i l i i i i i i i i i i i i i i i i i i , i I i i i
0 20 40 60 80 100 120 140 160 180
Scattering Angle (degrees)
Figure IV.5. Solid angle differential Klein-Nishina cross section versus scattering
angle. The variation of the cross section with scattering angle is shown for three
incident photon energies. As the incident energy increases, backscattering becomes
less probable.









large compared to the atomic binding energy are not cor-

rectly accounted for by the Klein-Nishina formula. The

Klein-Nishina formula is corrected by multiplication by the

incoherent scattering function, S(x,Z),


inc KN
d d S(x,Z)
dQ d i

do
inc
where dc is the solid angle differential incoherent
dQ
scattering cross section.

The incoherent scattering function represents the

probability that an atomic electron struck by a photon will

absorb energy and be excited or removed from the atom.

Figure IV.6 shows the incoherent scattering function for

aluminum and iron as a function of the momentum transfer

variable. The function has the effect of decreasing the

Klein-Nishina cross section (per electron) with the re-

duction being greatest at low energies and in high Z mater-

ials. Figure IV.7 displays these effects. The incoherent

scattering cross section is given by the integral over solid

angle of the differential cross section


[2 dc
KN
ainc = da S(x,Z)sin9 dsd ,
*0 'O

where inc. is the total incoherent scattering cross section

per atom.

Tabulated values of the incoherent scattering cross

section are provided by Hubbell et al. (1975). Figure IV.8








30.0





20.0 ---

N
x :
C/)

10. -



Dashed line: Iron
Solid line: Aluminum
0.0 i- -- -
0.0 1.0 2.0 3.0
x (Reciprocal Angstroms)
Figure IV.6. Incoherent scattering function versus momentum transfer variable. To
account for incoherent scattering from bound electrons, the Klein-Nishina cross
section is multiple by the incoherent scattering function. Data are from Hubbell et
al. (1975).









c 0.08
L.
o
Lj 0.06

a-


o
0.04
0


&0
0.02
(n


2 0.00
o

S0.08
0

0
EL 0.06


0.04


0
C
0
,-
o
00.02
vC,


0 0.00
0


Klein-Nishina


20 keV


20 40 60 80 100 120 140 160 180
Scattering Angle (degrees)


Klein-Nishina


100 keV


25 50 75 100 125 150 175
Scattering Angle (degrees)


Figure IV.7. Comparison of the solid angle differential
Klein-Nishina and incoherent scattering cross sections. The
solid angle differential Klein-Nishina and incoherent scat-
tering cross sections per electron (in units of barns per
steradian per electron) of aluminum and iron are compared at
20 keV (a) and 100 keV (b).









CO
0.8



S ~ Klein-Nishina

S0.6






0O
Q 0.4 -
0



O-


10 -2 10 -1 1
Energy (MeV)
Figure IV.8 Incoherent scattering cross section versus photon energy. The inco-
herent scattering cross section per electron of aluminum and iron are compared to the
Klein-Nishina cross section. The reduction from the Klein-Nishina cross section is
greatest at low energy and in the material with the higher atomic number. Data are
from Hubbell et al. (1975).









shows the incoherent scattering cross section per electron

of aluminum and iron, and that calculated from the integral

of the unmodified Klein-Nishina formula. /The Klein-Nishina

cross section overestimates the true incoherent cross sec-

tion at low energy. The error in the Klein-Nishina cross

section is larger in high Z materials. Because the effect

of the incoherent scattering function is important only at

low energies, it is often neglected in calculations/ The

same caveat described in the discussion of the atomic form

factor, regarding atomic and molecular electron density

configurations, applies to the incoherent scattering func-

tion.

Photoelectric Effect

In the photoelectric effect, an incident photon strikes

an atomic electron and is completely absorbed. The electron

is emitted from the atom with kinetic energy equal to the

difference in the incident photon energy and the binding

energy of the electron to the atom. If the interaction is

with an inner shell electron, the vacancy remaining after

the interaction will be filled, either producing a fluor-

escent emission photon(s) or Auger electrons. In the energy

region of interest to the mine detection problem, the cross

section per atom for the photoelectric interaction varies

approximately as

Zn/E3

where n varies between 4.0 and 5.0 depending on photon

energy (Anderson, 1984). This approximation indicates the









photoelectric cross section will be large at low energies

and in high atomic number materials. Figure IV.9 shows the

variation of the photoelectric cross section of iodine

(Z=53), gadolinium (Z=64) and lead (Z=82) as a function of

photon energy (each of these materials plays a role in this

research). Superimposed on the variation with atomic number

and energy, discussed above, are edges. These sharp discon-

tinuities in the cross sections are the result of the dis-

crete binding energies of electrons in their atomic shells.

Below an edge energy, the incident photon does not possess

sufficient energy to overcome the binding energy of the

electrons in a particular shell. As photon energy increases

to just above the edge energy, this is no longer the case

and the cross section increases dramatically as a result of

the capability to remove the newly available electrons. As

a result of these edges, a lower atomic number material may

have a higher cross section for the photoelectric inter-

action in an energy range below the edge energy of a higher

atomic number material.

Figure IV.10 shows the probability of K shell fluores-

cent emission following the filling of a vacancy in the

inner atomic shell./In low atomic number materials, this

probability is small; the alternate radiationless emission

of Auger electrons dominates (Evans, 1955). Since soil

and explosive materials contain generally low atomic number

elements, fluorescent emission from these materials is not

very probable/ Even in those few instances in which




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