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Report written:November 20, l$)S2

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LA-1493

This document consists of*$ -11 pages

Series A

T(YIALNEUTRON CROSS SECTIONS FOR U235, NOW ~m~, PU239

Work done by

L. CranbergR. B. DayR. L. HenkelG. A. JarvisR. NoblesJ. E. Perry

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ABOUT THIS REPORT

This official electronic version was created by scanning the best available paper or microfiche copy of the original report at a 300 dpi resolution. Original color illustrations appear as black and white images. For additional information or comments, contact: Library Without Walls Project Los Alamos National Laboratory Research Library Los Alamos, NM 87544 Phone: (505)667-4448 E-mail: [email protected]

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Distribution: ~~c t a 1952

Los Alemos Report LibraryJ R. OppenheimerArgonne ?!~.tional LaboratoryArmed Forces Special ~Veapons Project (Sandia)Armed Forces Special Weapons Pm ject, WashingtonAtomic Energy Commission, WashingtonBrookhaven National LaboratoryBureau of ShipsCaltiornia Research and Development CompanyCarbide and Carbon Chemicals Company (K-25 Plant)Carbide a(ldCarbon Chemicals Company (ORhZ)Carbide and C.a.rbonchemicals Company (Y-12 Plant)Chicago Patent GroupChief of Naval ResearchColumbia University (Havens)duPont CompanyGeneral Electric Com~a~ (ANPP)General Electric Company, Richlan3Hanford Operations OfficeIdaho Operations OfficeIowa State CollegeKnolls Atomic Power Laboratorylkmsachusetts Institute of Technology (Kaufmann)L!oundLaboratowNational Adviso-~ Committee for Aeronautics, ClevelNaval Research LaboratoryNew York Operations OfficeNorth American Aviation, Inc.Nuclear Development Associates, Inc.Patent Branch !Vashin~tonRAND CorporationSavannah River Oprations Office, AugustaSavannah River Operations office, ~~ilmingtonUSAF-HeadquartersU. S. Naval RadiolofiicalDefense Labo~.toryUCLA Medical Research Laboratory (Warren)University of California Radiation LaboratoryUniversity of RochesterWalter Kidde Nuclear Laboratories, Inc.Westinghouse Electric CorporationWriyht Air Develo~cnt CenterTechnical Information Service, Oak Ridge

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ABSTRACT

The total neutron cross sections for normsl uranium, U235, and

Pu239 =’e reported as a continuous function of energy for neutron

energies”between 40 and 7500 kev. Three additional measurements were

made between 17 and 20 Mev. The data were obtained from a good geom-

etry transmission experiment in which 1 in. diameter cylindrical sam-

ples were interposed midway between a neutron source and a neutron

counter ‘spacedat about 20 in. separation. In general, the statistical

errors were within *[

agreement with those1

cross section curves

0.2 barn. Measurements on normal uranium were in

made at Wisconsin in 1950 (LA-106O). The total

are almost identical for the three materials and

very similsr to those of other heavy elements. This suggests that the

total neutron cross section for other heavy elements canbe predicted

with some accuracy.

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INTRODUCTION

Since the total neutron cross sections for

been measured for fast neutrons, it was decided

~239 ~ &3~ hd never

to measure these cross

sections as a continuous function of neutron energy at the large Van de

Graaff accelerator. In addition, since the five different experiments

previously carried out to measure the total neutron cross section for

1 2 3 4 ~ showed such disagreement, normal uranium wasnormal uranium ~ ~ ~ ~

again included in the present work in an attempt to verify some of the

previous work. By taking advantage of the low neutron background in

the accelerator building at P-9 and using improved geometry to reduce

the errors due to unscattered neutrons, cross sections were obtained

whose values are more reliable than those of the previous work.

7‘J. H.

2 H. H.

3 W. H.

L D. H.

5 H. H.

Williams, CF-507 (1943).

Barschall, V. F. Weisskopf, LA-169 (1944).

Zinn, CP-2638 (19!45).

Frisch, LA-256 (1945).

Barschall, LA-106O (1950).

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PROCEDURE

Data for measuring total neutron cross sections were obtained from

a good geometry transmission experiment in which samples were placed mid-

way between a neutron source and counter 20 in. apart. The samples of

material which were used in this simple transmission experiment were 1 in.

diameter cylinders of different lengths. The length of sample used at a

given energy was governed by the fact that the greatest accuracy in cross

section value

is about 60%.

was such that

measured in a given time is obtained when the transmission

At neutron energies between 120 and 7500 kev the geometry

a neutron which was scattered through an angle greater than

b“ would not enter the counter. For lower energies, where closer geometry

was used, neutrons scattered more than 10° would not be detected. For data

taken between 17 and 20 Mev the spacing between counter end detector was

1S0 cm, in which geometry neutrons scattered more than 2° were not counted.

The cross sections were calculated in the usual wayby assuming that

the transmission varied exponentiall.ywith the thickness of the sample.

A correction for the neutron background caused by neutrons scattered from

the floor, etc., was obtained by placing a paraffin or polyethylene shad-

ow cone between the source and counter. The correction was only 1 to 3$

above 400 kev. When the T(d,n) reaction was used, an additional run was

made with hydrogen in the targef to correct for extraneous neutron back-

ground caused by deuterons striking places other than the gaseous target.

This amounted to less than a 3% correction in the total cross section in

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each case. Small angle scattering from the sample into the detector was

not taken into account since so little is known about this correction and

since the favorable geometry used in the experiment would make the cor-

rection a small one.

Monoenergetic neutrons were produced from three sources for ,thepres-

ent work. For the neutron energy interval between .40and 200 kev, the

Li(p,n)Be7 wource was used with evaporated lithium targets of about 10 kev

stopping power. Between 16o and 35~ kev, neutrons were produced by the

T(p,n)He~ reaction with a tritium gas target having 30 to 10 kev stopping

power. The source of neutrons for the range of 4 to 7.5 Mev was the re-

action D(d,n)He3, which was produced by bombarding a deuterium gas target

about 3CXlto 100 kev thick. Neutrons emitted in the forward direction

were used from these reactions, except for the energy interval of bO to

120 kev where data were obtained at llSO from the proton beam direction.

Neutron energies were calculated from the known thresholds and dy-

namics of the source reactions, taking into account the stopping power

of the target and of the nickel or aluminum foil window used on the gas

targets. They were checked occasionally by measuring the energy at which

the sharp 2.080 Mev scattering resonance occurred in carbon.

Three types of neutron detectors were employed. For neutron energies

above 250 kev, recoils in a gas ionization chamber were counted. The gas

counter was filled with hydrogen at pressures up to 500 psi or helium at

700 psi. The helium-filled chamber proved to be more useful.because it

could be used from @O to 7@0 kev without changing gas pressure, while

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the hydrogen counters required pressure reduction at low neutron energies.

For energies below 200 kev, a boron trifluoride proportional counter im-

bedded in paraffin was used. A stilbene scintillator using a S819 photo-

multiplier tube served as the neutron counter above 17 Mev.

For the @O to 3500 kev energy interval, data were taken every 30 kev

with a tritium target 30 kev thick to make certain that, at least to this

resolution, the cross section varied smoothly as a function of energy.

With the thicker targets used at higher

In general, enough counts were obtained

the cross sections were less than &O.2

100 kev had large mean deviations which

fluctuationswhile operating on a steep

energies, larger steps were tsken.

so that the statistical errors in

barn. The low energy points below

were probably due to proton energy

portion of the Li(p,n) yield curve.

RESULTS

The data are presented in curves of totsl cross section in barns

plotted against neutron energy in Mev. Although 30 kev steps were also

taken between /@O and b Mev, these data are not shown since within the

statistical errors they were in agreement with the smooth curves shown.

The statistical error for most of the points is about ~0.2 barn. The

points plotted at lb Mev were taken from the work of Coon, et sl. (LA-1446).

On the normal uranium curve, the triangles are averaged data taken from the

Wisconsin work published in LA-106O.



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DISCUSSION

The agreement between the normal uranium curve ad the data from

LA-1C6C is quite good. It seems to verify the Wisconsin measurements5

1S2~3~b The poorer geometry used in LA-1C60instead of the earlier work.

led to a 3%inscattering correction assuming isotropic scattering. The

agreement with the present data indicates that this isotropic correction

seems to be a reasonable correction at low energies.

The similarity of the curves for the three materials is striking.

PU239 is about 0.1 barn higher than normal uranium and 0.2 barn higher

than U235 over most of the energy range.

The additional similarity with nuclei of atomic weight greater than

about 180 suggests that the tot~ neutron cross sections for other very

heavy nuclei can be predicted with some accuracy on the basis of known

measurements. This similarity of total neutron cross sections for neigh-

boring nuclei has been demonstrated by Barscha116 at the lower energies

for nuclei of lower atomic weights.

6H. H. Barschall, Phys. Rev. 86, 431 (1952).

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