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UNCLASSIFIED

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ill J

May30, 2945

SPATIAL AND ENERGY DISTRU3UTION 0S?NEUTRONS IN

$i?fjMETAL CRITICAL ASN3MBLIES

WORK DONE W t

T/s Jo B istline

h!e Cmfd

Jo Hughes

PvtoJe Van ~O~ll@

REPORT WRITTEN W :

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The distribution of noutrona WRS studied in critical &ss@.ib3ieB

@on&isting of’25 tamped with W and F@~ with WC alone. and with TUG An indiaa.

tion of the energy distribution of tho natron~ is obtained by the use of varioua

fission chambers and detecting f.oilao ..—....-.......-

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SPATZAL AND ENERGY D3STRIBUT2XM4OF NEUTRONS IN

.-.

.

25 METAL CK.ITXCALASSEMBLIES

.—

MS e, cheak on tho

distribution of neutrons in

detectors were activated in

detectors were investigateds

20 25 fis6i05 WRS

chamber of the “spiral” type

ASSIFIEI)

stud.iedboth by use of a mall ioni~ation [!~fission”)

and by measuring in a Geiger counter the gamma

activity resulting from i’ixsionin a foil of 25 m6Aua10 The chamber was kindly

supplied to us by tho members af group R.~g who alma assisted in its operation

em wdl as in

20

measnroment0

30

the interpretation of the results obtained therefrom

28 fissions Aspiral chamber and foils were both used for this

Fission in 28 has an effective energy threshold d’ 41.4 Mev.

37 fission was studied by the use of a opiral chamboro Fiaeion

of 37 has an energy threshold of about ok Maro

.!+.Au autivity ind.med by the (nay) reaction was measured by the

U6a of gold

F

activity

3s 6uppoaed

foills. These were activated in the assembly. and the resulting

measured in a Geiger counterg The cross seotion for this reaction

to vary approximatdy as l\E in the 100-WO Mev region. and hence

this detector

8p&WI than

so

dependence of

60

the crevsasection for this reaction is not known.

k(n~y) was investl.gatedc The crobs sectimn for this reawtion

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was presumed to vary with neutron mlocity as I/w. though it has never been

measured for the neutron enerEies”involved5 The experiment indicates tkat the

diameter and #8t~ long. These

were corfipletolysurrounded*

to their poaitionomo as to

made by the chamboro

in the critical amembly~ by the mwterialu appropriate

minimize the perturbation introduomi by the hole

The foils waro l/2n $quaro@ and pLRCed@ flat, in the central p3ane of

thfjori~ical W!$aemblya Thus$ the activity of a foil represents an average over

a squttrel/2~’on a side~ the ponition of a foil ma taken ae the position of

the oanter of *ho foil (the smno was true for the ohambers)o This places

a limit on the resolution of these measurernents~

The irradiations were made by stroking m slightly super-critical

assembly and allowing the power to rise to a level sufficient to activate the

detectors the desired amount; the aesembly was then run exactly critical for

five minutes. after which the assembly -s made sub-critical by the removal of

sufficient tamper materialfi The foils wore stacked into tho assembly$ and

afterwards removed and ‘~ountedr’in Geiger

.

be compared received the same irradiatkno

A remmmble oha~nel was provided

@ount@raa Thusp all the foiB8 to

for the chamber measurements, ints

wh~ch the chamber$ completely “surroundedby the appropriate materi.alseoould

be intredueed at any desired positione The ratio d’ oounting rate of the

daxber to the oounting rate of

the chashor meaaurememts may be

Spatial distributions

a fixed %.onitor” -s alwaylltaken. 86 that all

said to have boon made at the came powere

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we of’

mg Q of’

the 25

the spiral chwrbars<. Th@ reault~ of these meammenents, in oounlxspep

.,

at the omtar of the 25 corec A rough calibration of the foil activities in

termu of f’is8ionaper mg can be obtained by using tho chamber meammovmta.

Wwevero since the ~~rrcction for the hole wade by the chamber i8 somewhat W.

certain and prtobablydkfforent for the two materials@ $t would be dmirablo

to obta%x an absolute calibration for the foils:, This has not yot beem provided.

thou~h it is intended to do 60 in the near futuro by irradiating both a 25 and

1 ~t the centero To compare direutly the average crosrnseations of 25 and Au$

tvm such foils were irradiated for five minutes in the oamo thwmd neutron

flux in the wwter boiler thermal Columne and their activities coxnparedo From

center of this critical assembly~ thfivaluo 906~0

were irradiated The &3tiViti08 tareahawn

.

the center are again normalized to 10 Using

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-6

● ● ✝

_ —_

.= ‘.=

the abova disoussed intercallbrationothe value of =(25) K(Au) at

.,

oenter is found &o be 10.4.

the

In the thermal-neutron interoalibration of the 25 and Au foilse the

Tho 25 folla wcwe not thin . having a lXCWMJmiaBiOn8for a thermml rioutroxibeam

falling pwpendkulwly upon them. of about 72 per cent. The gold foils

had a corroaponding trancimieaionof $??pm oont, We have as~umed the aotivitiea

per mg to be 75 per cent and 93 per sent reapcwtivdy of the activities per mg

of infinite~hally thin foi16 irradiated at the Earneplaoe in the graphite

thermal column of the water boil@.

IQ inches from graphite. The result$ngy activity was counted in the samo

geometry and with the same Geiger coranteras the foile irradiated in the

critical assemblies.

25 foils was tho 6AmQ at the

and the Tu tampe~g The 28/2cj

center of the oore divided by ‘

the 28/2cjretio for fiss~on neutronsO[i,e. -A !3, where (nw)A refork

=-..

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-n

✎ ●☛☛

-- m,-: “?0

.=**. .“-

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● =a* ,=

-7”

-A= = 09200 Sime the general

25 und 28D these two values are dependat on

fh8iOXi decay is

the counting 9ot

q minute

the same

26*b per

not tho same for

upe For a

from 10 to 120 mixmlxm after activation

Tha Crapha of the 25 fiaaion distribution are not corrocted for the

of the core$ but drops off rapidly in the tarnpero The 28 foils contain about

I part in 3000 d’ 25 and no correction is necessary,

Sinoe it

d91ay* we ‘havecot

fa~% neutrons from

irIfelt that tho~e results should be made available without

included the results of contemplated intercalibrationaby

the W long tank and of re2ative activation of the

assemblies. which will be reported in ad?eada tothis report as soon as they

become available

Tho compmrinon of our acperirnentalldi~tributions with thooretioal

calcul,atiouis being c~rriad out by momber6 of Serbor5s and Wo$s$kopf”s groups

in the theoretical aeotiono

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