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Subject Category: PHYSICS
UNITED STATES ATOMIC ENERGY COMMISSION
THE TRANSMISSION OF NEUTRONS AND
GAMMA-RAYS THROUGH AIR SLOTS.
PART I. THE MECHANICAL CONSTRUCTION
OF THE SLOTS AND REPRESENTATIVE
Robert D. Schamberger
Ferdinand J. Shore
Harvey P. Sleeper, Jr.
M y I ,3,s i
September 1, 1954,
Brookhaven National Laboratory
Upton, New York
Technical Information Service, Oak Ridge, Tennessee
Date Declassified: November 21, 1955.
This report has been reproduced directly from the best
Issuance of this document does not constitute authority
for declassification of classified material of the same or
similar content and title by the same authors.
Printed in USA, Price 20 cents. Available from the
Office of Technical Services, Department of Commerce, Wash-
ington 25, D. C.
This report was prepared asa scientific account of Govern-
ment-sponsored work. Neither the United States, nor the Com-
mission, nor any person acting on behalf of the Commission
makes any warranty or representation, express or implied, with
respect to the accuracy, completeness, or usefulness of the in-
formation contained in this report, or that the use of any infor-
mation, apparatus, method, or process disclosed in this report
maynot infringe privatelyowned rights. The Commission assumes
no liability with respect to the use of,or from damages resulting
from the use of, any information, apparatus, method, or process
disclosed in this report.
THE TRANSMISSION OF NEUTRONS AND GAMMA-RAYS THROUGH AIR SLOTS
The Mechanical Construction of the Slots and Representative Background Data
Robert D. Schamberger
Ferdinand J. Shore
Harvey P. Sleeper, Jr.
1 September 1954
The work to be reported is, in every sense, a group effort
which required diligence, patience, and cooperation on the part
of many people. We are especially indebted to Mr. Anthony N.
Fasano for the careful control which has been exercised in the
assembly of the air slots. We also wish to acknowledge the
cooperation and conscientious efforts of Anna Potter, Robert D.
Schmidt, George R. Hardman, Daniel C. Vance, Robert R. Hambley,
and William Ware, Sr.
BROOKHAVEN NATIONAL LABORATORY
Associated Universities, Inc.
under contract with the
United States Atomic Energy Commission
Work performed under Contract No. AT-30-2-Gen-16
THE TRA! SHISSION OF NEUTRONS AND CAMTA-RAYS THROUGH AIR SLOTS
The Mechanical Construction of the Slots and Reoresentative Background Data
The data which we have obtained on the transmission of neutrons and
gamma-rays through rectangular air slots are being reported in a series of ten
preliminary reports. This, the first of the series, describes the apparatus
which was used in the investigation. It also contains representative background
data which we found useful in evaluating the measurements. The other nine reports,
all of which bear the same principal title, The Transmission of Neutrons and
Gamma-rays Through Air Slots, are:
Part II The Transmission of Neutrons Through Straight Slots in Water;
Part III The Transmission of Gamma-rays Through Straight Air Slots in Water;
Part IV The Effect of an Offset on the Transmission of Neutrons Through an
Air Slot in W'ater;
Part V The Effect of the Vertical Position of a Single Offset on the Neutron
Transmission of an Air Slot;
Part VI The Effect of Multiple Offsets on the Neutron Transmission of an Air
Part VII The Effect on the Neutron Transmission of Changing the Wall Material
of an Air Slot;
Part VIII The Effect of the Source Size on the Neutron Transmission of an Air
Part IX Thermal and Erithermal Neutron Distributions around an Air Slot in
Part X The Angular Distribution of the Neutrons Emerging from an Air Slot
These preliminary reports contain some simple analyses of the data.
It is felt that the material warrants a more careful interpretation than has yet
been attempted. He expect, therefore, to prepare a single document presenting
the conclusions which may be drawn from the data presented in this series of
The Construction of the Air Slot
Tehcn the measurements were being planned, the following criteria were
used to determine how the slots were to be fashioned.
1. The system should be flexible; i.e., a large variety of sizes and shapes
should be available. It was felt that building a separate and complete
slot for each measurement would be unnecessarily expensive and time-consuming.
2. The apparatus defining the slot should not become too radioactive during a
measurement. The apparatus should be "cool" enough to be handled within two
hours after the removal of the apparatus from the high flux region.
3. The effect of the material, which defines the slot and keeps it dry, on the
neutrons and gamma-rays reaching the top of the apparatus should be small
compared to the transmission of the slots.
4. The task of changing from one slot size to another should not be too laborious.
5. The size and position of the slot should be reproducible.
Figures 1 and 2 are vertical sections of the slot holder as it was
located in the shielding tank. Figure 1 also indicates the location of the
natural uranium source plate. The apparatus consists essentially of a series
of metal boxes, open at the top and bottom, which can be bolted together, as
desired, with flat rubber gaskets between the several boxes to produce a water-
tight seal. The top and the bottom were closed with flat aluminum plates.
Flat gaskets, again, insured a water-tight seal. The entire apparatus was lifted
from the bottom by demountable rods, shown in figure 1. As the length; i.e., the
vertical dimension, of the slot holder was changed, sections of the rods were
also added or removed as required. Inside the boxes, lucite slabs, cut to the
proper size, filled all of the space not allotted to the air slot. By adding
or removing boxes, the length of the slot under investigation could be varied.
Moving the lucite slabs within the boxes permitted the location of the slot
anywhere within the six inch interval defined by the container.
Fabrication of the slots in this manner made it possible to obtain
a large variety of slots within the framework of the apparatus. In addition to
the many sizes available, it was possible to vary the material which made up the
walls of the air slot simply by inserting slabs of the desired material. Also,
a simple displacement of one section of a slot with respect to another resulted
in the formation of an offset slot. The system, then, fulfills the first of the
The choice of materials was made in an attempt to satisfy criteria 2
and 3. A preliminary measurement with a 3 x 36 x 48 inchl block of lucite in-
dicated that lucite and water were indistinguishable as far as their neutron
attenuation properties were concerned. The higher density, of course, makes the
lucite a better gamma-ray shield than water. The thermal neutron traverse ob-
tained above the lucite, mentioned above, is presented in figure 3, with a simi-
lar traverse obtained in pure water. The figure also contains a sketch showing
the location of the lucite in the tank. On the basis of these measurements and
the fact that there was a readily available supply, it was decided to define the
walls of the air slot with lucite.
It was originally decided to fabricate the boxes from 0.75 inch 61ST
aluminum to satisfy the requirement that the activation be small and still give
the required mechanical strength. However, a measurement of the neutron trans-
mission of a 0.75 x 30.5 x 40.5 inch sheet of aluminum, presented in figure 4,
indicated that the streaming through the walls of the container would not be
negligible if the walls were made entirely of aluminum. Accordingly, some boxes
with 0.5 inch, 304 stainless steel walls were incorporated in the container to
reduce the neutron streaming. The bottom box was always aluminum to keep the
activation low. On the few occasions when a single apparatus was activated for
the last dimension is, unless otherwise indicated, the vertical dimension.
several days, the lower box and base plate required approximately s day to cool
to the point where they could be dismounted. The introduction of the steel
further reduced the gamma-ray transmission of the apparatus. As a result of
these decisions, the slot-holder, as it finally evolved, satisfied the criteria
where neutrons were concerned, but did not satisfy criterion 3 for g-an.-r-ay-.
The fourth criterion was moderately well satisfied. The task of
disassembling and rebuilding a typical four-foot long slot normally required
about five man-hours.
With reference to the fifth criterion, the thickness of the slot was
determined by one-inch wide lucite spacers at the ends of the slot. These spacers
normally bridged the junction between sections so that the line-up problem was
somewhat simplified. In addition, small aluminum spacers were used near the cen-
ter of the slot to maintain the proper slot thickness throughout the width of
the slot. These spacers, indicated in figure 2, were pieces of thin-walled
aluminum tubing cut to the proper length. We have been unable to detect the
effect of the aluminum spacers on the transmission of the slots.
Uith this method of fixing the thickness, it was possible to arrange
the lucite so that the deviation of the slot thickness from its nominal value
was less than 0.03 inch throughout its entire length and width. Adjacent sections
were lined up so that the displacement from one section to the next was never
greater than 0.03 inch. The displacement was usually less than 0.01 inch. These
errors should not have materially affected the measurements on slots whose thick-
ness was at least one inch. Whon the slot thickness was reduced to 0.125 inch,
however, the errors became quite serious. Even when unusually great care was
taken to assemble long 0.125 inch slots, the results were not reproducible. A
further difficulty was encou-nt:red with the thin slots in keeping them plane.
The deviation from planarity was normally less than 0.015 inch. In addition,
for the short 0.125 inch slots; i.e., for one or two foot lengths, the neutron
transmission of the slot was not large compared with the background. The sub-
traction of the large backgrounds introduced a large error in the measurement.
No measurements with the 0.125 inch slots are, therefore, considered reliable.
The width of the slots was as large as would conveniently fit into
our facility. ~ith the thirty-six inch inside dimension of the boxes and the two
one inch lucite spacers, the width of the slots was thirty-four inches. This
dimension was held fixed throughout the measurements.
To be certain that the slot was filled with air and not with water,
air was continuously forced Ihr-cugh the apparatus. The air inlet was located
in the top plate and the outlet in the bottom plate, as indicated in figure 1.
Several neutron detectors have boon used during this investigation.
Thermal neutrons have been measured with a natural uranium fission chamber and
with one-inch diameter, six-inch long BF counters. Both enriched (BlOF ) and
slightly depleted (B F3) counters have oon employed. !hen it was feqssble,
fast neutron measurements wore made with a Hurst type nondirectional fast neutron
dosimeter. Gamma-ray measurements were made with 20 cc aluminum-walled ionization
Labeling the Data
The curves included in this series of reports will be designated by
the symbols which were used when taking the original data. This designation con-
sists of the following parts:
1. A Roman numeral indicating the basic tank structure in which the measurement
2. An indication of the box or boxes involved. Table I lists the boxes which
have been used, their sizes and composition, and a few comments about their
use. This item may also include an indication of the length of the slot
section being described.
3. A number indicating the slot thickness in inches.
4. A number indicating the slot position in the box. This number is "0" when
the north side of the slot was 1,00 inch from the north side of the box
and increased as the slot was moved away from the north wall.
5. An indication of the slot wall material if it was not lucite. This item
also included any other information which seemed pertinent.
h]en items three, four, and five were the same for at least two con-
secutive boxes, item two usually listed the appropriate boxes in order from the
bottom to the top. Thus, III 1, AB,3 0.25 0 0.5 inch steel indicates
a measurement in basic structure III in which boxes 1, A, B, and 3 (see Table 1)
were used to contain a 0.25 x 34 x 48 inch straight slot with 0.5 inch steel
walls. The designation III 1, A 0.50 0
B, 3 0.50 1 refers to a four foot lucite-walled
slot consisting of two sections; each 0.50 x 34 x 24 inches where the upper sec-
tion was displaced to provide a one inch offset.
4hen background data were obtained with the container filled with
water rather than lucite, items 3, 4, and 5 of the designation were replaced
with the word, "water" (or H20).
In addition to these indications of the characteristics of the test
piece, the designations usually include information on the specific measurement
which was made. These may include the run number, the detector employed, and
position of the detector in the tank. The run number was usually inserted
either immediately before or immediately after the Roman numeral indicating the
basic structure. The other items customarily appear at the end of the designation.
2 A description of the basic structures is given in BNL-265, "The Streaming of
Neutrons in Carbon Steel", Shore, Schamberger, and Sleeper, Jr.
Structure III, which was used almost exclusively in this investigation, is
illustrated in figures 1 and 2. It consists of the angle iron frame in the
The position of the detector or test piece in the water tank is
normally specified in terms of a cartesian coordinate system whose origin is
at the center of the tank bottom with the Z axis vertical, the positive X axis
being to the north and the positive Y axis extending to the west side of the
Figures 5 through 8 present horizontal traverses close to the top
of the slot holder with a thermal neutron detector. These curves represent
several types of background data. The dashed curve represents data obtained
with the boxes filled with water; for the dotted curves, the data were obtained
with the boxes filled with lucite. The solid curves represent the flux in the
water with the entire apparatus removed.
Figures 5 and 6 indicate the effect of neutron streaming in the
aluminum walls of the boxes. They illustrate the need for inclusion in the
walls of a good scatterer for intermediate energy neutrons.
The data presented in Figure 7 illustrate the variation in the ther-
mal neutron background which resulted from changing the location of the stain-
less steel in the assembly. The background is lower with the stainless steel
at the top of the container than when it was in the middle. The data also
indicate that the background with the lucite-filled apparatus, with, of course,
some small cracks and crevasses where the lucite did not fit exactly, is no
more than slightly higher than the background with the water-filled container.
For the neutron data, therefore, it is probably permissible to use the water-
filled backgrounds whenever desirable.
The data shown in Figure 8, for the six-foot lucite-filled assembly,
are appreciably lower than the pure water data because at this height in the
tank (78.5 inches), many of the background neutrons are produced by the photo-
disintegration of deuterium in the surrounding water. The reduction in the
neutron background, therefore, corresponds to a reduction in the gamma-ray flux
by the apparatus.
Figures 9 and 10 contain gamma-ray traverses immediately above two
and four foot assemblies. The curves in Figure 10 indicate that replacing box
No, 2 with boxes A and B, which added two steel flanges to the container, re-
sulted in a reduction in the gamma-ray background. 4hile not indicated in the
figures, the water-filled backgrounds, when the length was greater than two
feet, were not particularly useful because of the difference in the gamma-ray
attenuation properties of lucite and water. A further complication was intro-
duced by the fact that, for the thin slots which have been investigated, the
gamma-ray background was never negligible and was usually of the same order as
the dose measured with the slot.
In conclusion, when the container consisted of both steel and
aluminum boxes, the neutron background was reasonably flat and not particularly
sensitive to the distribution of the material in the walls. For many of the
slots which have been investigated, the variation in the background caused by
the rearrangement of the boxes, by filling the container with water rather
than lucite, or even by using pure water data, was small enough to be neglected.
For the one and two foot lengths, however, where the container usually had only
aluminum walls, the backgrounds were not small compared with the slot trans-
mission, and the proper background was required.
The gamma-ray backgrounds were more seriously affected by the dis-
tribution of the wall materials. In the evaluation of the data, care must be
taken to obtain the proper background to be subtracted from the dose measured
above the slot.
Boxes Included in the Slot Holder
Inside Dimensions of Box
Box Length Width Thickness Box
Designation (inches) (inches) (inches) Material Comments
1 12 36 6 aluminum
2 24 36 6 steel Box No. 2 was slightly
sprung when welded, the
thickness being less in
the middle than at the ends.
3 12 36 6 aluminum
4 24 36 6 aluminum The large faces of Box No. 4
were badly bowed during the
welding. The thickness was
about 6.5 inches at the
center instead of the 6
inches which was maintained
at the flanges. The side
bulges were filled with wax
to provide the straight
A 12 36 6 steel
36 6 steel
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