The transmission of neutrons and gamma-rays through air slots

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Material Information

Title:
The transmission of neutrons and gamma-rays through air slots
Series Title:
BNL ;
Physical Description:
v. : ill. ; 27 cm.
Language:
English
Creator:
Schamberger, Robert D
Shore, Ferdinand J
Sleeper, Harvey P
Brookhaven National Laboratory
U.S. Atomic Energy Commission
Publisher:
United States Atomic Energy Commission, Technical Information Service
Place of Publication:
Oak Ridge Tenn
Publication Date:

Subjects

Subjects / Keywords:
Nuclear physics   ( lcsh )
Neutrons -- Diffraction   ( lcsh )
Gamma rays   ( lcsh )
Genre:
federal government publication   ( marcgt )
non-fiction   ( marcgt )

Notes

Statement of Responsibility:
by Robert D. Schamberger, Ferdinand J. Shore, Harvey P. Sleeper, Jr.
General Note:
Cover title.
General Note:
Originally published 1954.
General Note:
"September 1, 1954."
General Note:
"Subject category: Physics."
General Note:
"Brookhaven National Laboratory, Upton, New York."
General Note:
"Date Declassified: November 21, 1955."--P. 2 of cover.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 004703692
oclc - 432319550
System ID:
AA00009214:00010

Full Text
-_. -' -% 03


.-201 ASSIFIED


UNCLASSIFIED


BNL-2028

Subject Category: PHYSICS



UNITED STATES ATOMIC ENERGY COMMISSION



THE TRANSMISSION OF NEUTRONS AND
GAMMA-RAYS THROUGH AIR SLOTS. PART
X. THE ANGULAR DISTRIBUTION OF THE
NEUTRONS EMERGING FROM AN AIR SLOT

By
Robert D. Schamberger
Ferdinand J. Shore
Harvey P. Sleeper, Jr.







September 1, 1954

Brookhaven National Laboratory
Upton, New York


Technical Information Service, Oak Ridge, Tennessee







('f


Date Declassified: November 21, 1955.


This report has been reproduced directly from the best
available copy.

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.






Page i


BNL-2028 "





THE TRANSMISSION OF NEUTRONS AND GAMMA-RAYS THROUGH AIR SLOTS


Part X

The Angular Distribution of the-Neutrons Emerging from an Air Slot
0


Robert D. Schamberger
Ferdinand J. Shore
Harvey P. Sleeper, Jr.




1 September 1954












REACTOR DEPARTMENT

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




































Digitized by t[e Internet Archive
in 2012 with funding from
University of Florida. George A. Smathers Libraries w'th support from LY'RASIS and the Sloan Foundation


http://archive.org/details/transmissionofne9842broo







Page 3


The angular distribution and the spectrum of neutrons emerging
from the end of an air slot in water are important in interpreting a
number of different experiments involving one or more air slots. For
example, the attenuation in water beyond an air slot and the effect of a
step in an air slot are functions of both the angular distribution and
the spectrum of the radiation.

Several experiments have been performed to study the angular
distribution in a qualitative manner. These studies have included an in-
vestigation of the effect of inserting scattering material near the middle
of a 0.5 inch thick by 48 inch long air slot, as well as experiments to
measure the attenuation of fast and slow neutrons in water and in air
above the top of a long, thin air slot.

The Effect of a 1.8 inch lucite Scatterer on neutron Transmission Through
a 0.5 x /8 inch Air Slot

Measurements were made in water above an air slot, 0.5 x 34 x
inches, in the standard configuration with a 6 inch long 1 inch diameter
BF counter. These measurements were compared with those for the same
experimental arrangement with the addition of a lucite bar, 0.5 x 34 x 1.8
inches, located in the air slot twenty-four inches above the bottom,

The results are given in Figure 1. Vertical traverses over
the end of the slot and horizontal traverses, X, one and one-half inches
above the end of the slot are compared for two conditions; one, with the
scatterer in and the other, with the scatterer out. The data show that
there was a large reduction of the slow flux at the end of the slot with
the scatterer present, about a factor of thirty, and a small reduction in
the fast flux as measured far out in the water, about a factor of 2.3. The
factor of 2.3 is approximately what one wou3d expect for fast neutron
attenuation in 1.8 inches of lucite on the assumption that each interaction
is equivalent to an absorption. The large attenuation of the slow flux
may also be interpreted by assuming that much of the slow flux was highly
collimated; that it was scattered by the lucite plug and effectively lost
in the surrounding material.

The Neutrons Emerging from a Long Thin Air Slot

Two types of traverses have been made above a .125 x 34 x 96
inch air slot, In most cases, measurements were made with both the BF3
counter, mentioned above, and a Hurst-type nondirectional fast-neutron
dosimeter.

Type I: Vertical and horizontal traverses were made in water above
the slot.

Type II: The water level in the tank was lowered, and vertical and
horizontal traverses were made in the air above the water as a
function of the depth of the water above the slot. Two series






Page 4


of horizontal traverses were taken.

In the first, the distance between the top of the slot and
the plane in which the detector was moved was held fixed as the
water level was altered. Two separation distances, ten inches
and thirty-eight inches, have been employed. In the second
series, the counter was kept one inch above the water as the
water level was changed. The measurements made in air suffered
somewhat in that the general radiation background was appreciable.
No satisfactory background measurements were made, but the resultE
can be used to indicate general trends.

The data in Figure 2 give the thermal flux in air ten inches
above the air slot measured as a function of the depth of water over
the slot. The most striking feature of the results is the well collimated
beam of slow neutrons emerging from the end of the air slot. The curve
marked D=l" shows the dramatic effect of a single inch of waferon this
collimated beam, with a reduction in peak intensity by a factor of thirty.
The same process seems to be involved in this instance as in the experiment
with the lucite bar, mentioned above. The addition of more water reduces
the overall intensity of the transmitted slow flux but does not greatly
affect the shape of the distribution.

Data in Figure 3 show the thermal neutron traverses in air one
inch above the water level as a function of the depth of the water above
the .125 inch air slot. These data show that the distribution near the
surface of the water is still peaked when an inch of water is above the
slot. The horizontal traverses remain peaked above the slot as the water
level is raised. Unfortunately, the details of these cures are masked
due to the high radiation background in the air. The data seem to indicate
however, that the peak in the distribution becomes slightly broader as
the depth of the water is increased.

From the data presented in Figure 3, it is apparent that the
effective width of the source of thermal neutrons emerging from the
water is of the order of a few inches. If the detector is so placed
that this dimension is small compared to the separation distance between
detector and water, the angular distribution may be inferred from the
detector response when a horizontal traverse is made. The measurements
shown in Figure 2, made ten inches above the top of the slot, indicate
that the angular distribution was not changed very much when the amount
of water over the slot was increased beyond one inch.

Fast neutron dose data obtained above the .125 inch slot are
presented in Figures 4 through 8. Figure 4 shows the usual horizontal
and vertical traverses in water above the slot. The horizontal traverse
was made with the detector one inch above the top of the slot. Figure 5
compares the fast dose in water with the fast dose in air as a function
of the depth of water above the air slot. The slow decrease in the dose
measured in air indicates the highly collimated nature of the fast neutrors.
It should be noted that in one direction the beam of fast neutrons is







Page 5


considerably narrower than the detector. This makes the rate at which
the dose falls off as Z is increased slower than would otherwise be the
case. Figure 6 compares the X traverses in air one inch and thirty-eight
inches above the air slot with no water over the slot. These data show
that the fast dose is rather highly collimated as it emerges from the slot.
The data obtained one inch above the slot indicate the presence of an
uncollimated component but do not justify a calculation of its magnitude.
It may be observed that the full width of the dose distribution at half
maximum is quite small considering the size of the detector and the thick-
ness of the slot,

Figures 7 and 8 give the horizontal dose traverses in air as a
function of the depth of water over the slot, with the detector located
one inch above the water level and thirty eight inches above the slot
respeqkivoly. The results show that the shapes of the-traverses and
consequently the angular distribution of the emergent radiation, over
the limited angular range for which the data are useful, are not greatly
affected by the presence of small amounts of water.


Summary

The results of this series of measurements may be summarized
in a few short sentences.

1. There is a large, highly-collimated component in both the high and
low energy neutron fluxes emerging from an air slot.
2. The effect, on the fast neutrons at least, of an obstruction far from
either end of an otherwise straight slot can be estimated under the
assumption that the total neutron cross section is an effective absorpti
cross section.
3. The fast neutron dose distribution above an air slot remains sharply
collimated after traversing several inches of water.
4. The low energy component is rapidly decollimated in the water.
5. After the rapid initial decollimation, the angular distribution of
the low energy component is not greatly altered by the addition of
more water.
6. The spatial resolution of the fast neutron dosimeter appears to be
anomalously small.





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