Ultra short duration flash radiographs


Material Information

Ultra short duration flash radiographs
Series Title:
United States. Atomic Energy Commission. MDDC ;
Physical Description:
5 p. : ill. ; 27 cm.
Clark, J. C
Los Alamos Scientific Laboratory
U.S. Atomic Energy Commission
Atomic Energy Commission
Place of Publication:
Oak Ridge, Tenn
Publication Date:


Subjects / Keywords:
Radiography   ( lcsh )
X-rays   ( lcsh )
federal government publication   ( marcgt )
bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )


Includes bibliographical references.
"Document declassified: 3/24/47."
Statement of Responsibility:
by J.C. Clark.
General Note:
Manhattan District Declassified Code.
General Note:
"LADC 392."

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 005024367
oclc - 288607867
System ID:

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LADC 392



H;i ,by

J. C. Clark

Los Alamos Scientific Laboratory

Published for use within the Atomic Energy Commission. Inquir-
ies for additional copies and any questions regarding reproduction
by recipients of this document may be referred to the Documents
Distribution Subsection, Publication Section, Technical Information
Branch, Atomic Energy Commission, P. 0. Box E, Oak Ridge,

Inasmuch as a declassified document may differ materially from
the original classified document by reason of deletions necessary
; to accomplish declassification, this copy does not constitute au-
.thority for declassification of classified copies of a similar docu-
i:i: ment which may bear the same title and authors.

SDocument Declassified: 3/24/47

This document consists of 5 pages.

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Ultra Short duration Flash Radiographs


C. Clark

Los Alamos Scientific Laboratory

The duration and nature of the bursts of x-rays produced by the
high speed flash radiographic equipment described by Slack and Ehrkel
Shave been studied by radiographing a very thin strip of lead cemented
to a piece of explosive of high detonation velocity as the detonation
proceeds. Such radiographs show that bursts of x-rays varying in
duration between 1 and 3 microseconds are produced and that they are,
in general, multiple in character, indicating an interrupted or oscil-
latory type of discharge through the x-ray tube.

For many purposes a single burst of x-rays of much shorter
duration is desirable. By studying the method of initiation of a cold
cathode, Slack type tube, a circuit has been assembled which operates
satisfactorily at relatively low voltages (60-100 KV), producing a single
x-ray burst of duration 10-1 sec., as measured by the same timing

'Slack and Ehrke, J.A.P., 12, 165, 1941




J. C. Clark

Los Alamos Scientific Laboratory

The Westinghouse flash radiographic equipment is described in
principle in a report by C. M. Slack and L. F. Ehrkel. This equipment
utilizes a high vacuum cold cathode field emission type x-ray tube and
a surge generator of the conventional circuit design for producing tube
potentials of the order of 350 KV. The tube also contains a third elec-
trode,called the initiating electrode, which is part of the cathode assem-
bly To initiate the discharge through the x-ray tube a high potential is
established between the initiating electrode and the tube cathode. Pre-
sumably a metallic arc is thus established and from this arc electrons
are drawn to the target of the tube. The entire circuit is arranged so
that the discharge which results through the x-ray tube exists only as
long as any charge remains on the capacitors in the surge generator
circuit. With this equipment, flash radiographs have been made and
widely published of such things as machinery in motion, rifle bullets in
the barrel and in free flight, and the fragmentation patterns resulting
from high explosive loaded shells during detonation.

It had earlier been determined by measurements of the radiography
of such objects as rifle bullets in flight that the duration of the x-ray dis-
charge produced by Slack's equipment was of the order of one micro-
second, but little Information could be obtained from these radiographs
about the nature of the x-ray burst. Extremely fast-response type ion-
ization chambers and associated electronic circuits which could be used ,
for analyzing the characteristics of the x-ray burst of the kind produced
by this equipment are in themselves a major research project, and a
much simpler and more direct method has been adopted. This consists
in radiographing a verythin foil indicator strip cemented along the top
of a small piece of explosive of high detonation velocity as the detonation
proceeds along the piece of explosive. The radiograph thus obtained
furnished direct evidence of all of the characteristics of the x-ray burst
except the effective wave length of the x-rays, and such a determination
can be made with the well known step wedge filter technique.

Figure 1 is a flash radiograph made by using the above technique
of a 2 mil thick, 3/32" wide lead foil, securely cemented with Duco

1 Slack and Ebrke, J. A. P., 12 165.1941

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MDDC 768


!W d~ment to the explosive, with the detonation proceeding from left to right
S at approximately 7.5 mm per microsecond. It will be seen that the x-ray
discharge in this case consists of several bursts of variable intensity,
S.variable duration and lasting over two microseconds. This intermittent
S type of discharge is characteristic of the Westinghouse microflash equip-
ment. It is seen that the electrical energy stored in the capacitor of the
l circuit is not being utilized to the best advantage for producing a single
x: -ray burst, preferably of duration much less than the average of two
microseconds. It appears that insufficient energy in the initiation of the
discharge does not make it possible for all of the stored electrical energy
in the power circuit to dissipate itself through the tube in a single flash,
but that it acts only to start the discharge phenomena.

A circuit has been assembled for use with the same type of x-ray
tube which, in principle, produces sufficient electrons in the initiation
. process to allow the main capacitor of the circuit to discharge completely
in a single burst, as well as in a time lasting no more than 1/10 micro-
second. Consider the circuit shown in Figure 2.


FIG. 2

i:i f C is made .04 mfd, and the charging potential 80 KV, then the average
current which must flow through the tube to discharge completely the
capacitor in 10-7 sec. is 32000 amperes, and this means that 2 x 1016
S electrons must be provided for this discharge regardless of the method
of initiation. When the output of the pulsing circuit is applied directly to
'the initiator electrode of the x-ray tube, an x-ray burst similar to that
S show n in Figure I results. However, by using the additional energy made
available by an auxiliary capacitor C2 any (reasonable) amount of energy

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can be applied for the initiation, provided, of course, the discharge of -
this initiation circuit is made to take place within the time limit of 1/10
microsecond. The potential which must be applied to the initiator elect-
rodes depends upon the spacing of the electrodes In the cathode assembly.
For high vacuum breakdown between sharp-edged electrodes this is rela-
tively high, even for small distances of separation. Actually. each tube
which is used must be tested for minimum breakdown potential between
the initiation electrodes, and at least this potential applied to the capacitor
C2. For several standard flash radiographic tubes manufactured by West-
inghouse, minimum initiation potentials of 20-25 KV must be used.

With the arrangement shown in Figure 2 and with care being given
to the arrangement of the circuit to minimize the impedances of the vari-
ous components, flash radiographs similar to that shown in Figure 3 are
obtained, wherein the single x-ray burst had a duration of no longer than
10-7 seconds.


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