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HIGH ENE-R_GYNE~TR-ONS IN A COT,T,TMA mm Qm A w

~. LOS ALAMO$ FAST PLUTONTTTM nwAe~nn I

LEGAL NOTICE

This report was prepared as an account of Govern-ment sponsored work. Neither the United States, nor theCommission, nor any person acting on behalf of the Com-mission:

A. Makes any warranty or representation, expressor implied, with respect to the accuracy, completeness,or usefulness of the information contained in this report,or that the use of any information, apparatus, method, orprocess disclosed in this report may not infringe privatelyowned rights; or

B. Assumes any liabilities with respect to the useof, or for damages resulting from the use of any infor-mation, apparatus, method, or process disclosed in thisreport.

AS used in the above, “person acting on behalf of theCommission “ includes any employee or contractor of theCommission to the extent that such employee or contrac-tor prepares, handles or distributes, or provides accessto, any information pursuant to his employment or con-tract with the Commission.

LA-2185PHYSICS AND MATHEMATICS(TID-4500, 14th edition)

Il)S ALAMOSSCIENTIFIC LABORA~RYOFTHEUNIVERSITYOFCALIFORNIALOSALAMOS NEWMEXICOREPORT WRXTTEN: February 1951

REPORT DISTRIBUTED: February16, 1959

HIGH ENERGY NEUTRONS IN A COLLIMATED BEAM

FROM THE LOS ALAMOS FAST PLUTONIUM REACTOR

by

Bob E. Watt

TMS report expresses the opinions of the author orauthorsand does not necessarily reflect the opinionsor views of the Los Alamos Scientific Laboratory.

ContractW-7405 -ENG. 36 wit-b the U. S. Atomic Energy Commission

Estimatesof

that it would be

energyspectrum.

fissionspectrum

ABSTRACT

the excitationenergies

usefulto searchfor an

The work reportedhere

as far as was permitted

of fissionfragmentssuggested

upperlimitto the neutron

was undertakento extendthe

by the beam intensityavailable

fromthe LosAlamosFast PlutoniumReactor. A secondaryobjectivewas

to improvethe accuracyof the spectrumreportedin IosAlamosScientific

LaboratoryReportIA-718. The spectrumreportedtherewas determined

fromfissionsindncedby thermalneutrons.

Neutronswere observedin the energyrangeof 24 to 25 Mev, implying

fragmentexcitationenergiesgreaterthan 30Mev. In the spectrumno

changeof slopeis evident,suggestingthat fragmentsof stillhigher

excitationare reasonablyabundant.

3

The objectivesof the experimentwere: (1)to determinewith a

precisionof 10 per cent or betterthe numberof neutronsemittedfrom

a thicksourceat 14

(2)to searchfor an

Mev relativeto the numberat some lowerenergyand

upper limitto fissionneutronenergies.

Apparatus

A besm of fastneutronswas obtainedfromthe 5W port of the Lx

AlamosFast PlutoniumReactor(Clementine)*and allowedto pass through

the neutronspectrcxneterdescribedin LosAlamosScientificLaboratory

ReportIX-718(December1948).~Figurelshowsthe5Wport and Fig. 2

showsthe spectrometer.

Severalmodificationsto the spectraneterwere made,primarilyto

reducethe backgroundcountand permita searchfor very high energy

neutronsin the beam.

Fivemil (28.4n@cm2) aluminumabsorberswere insertedin the

groundplanesbetweencounters1 and 2 and counters2 and 3 in orderto

reducethe numberof reactionand knock-onparticleswhich couldpene-

trateall threecounters. The lastabsorber

countin&at the lowerenergies,resultingin

givencountingtime.

x

alsopermitteddifferential

betterstatisticsfor a

“A generaldescriptionof the reactorhas been givenby E. J. Jurneyet al.,ReportTID-100~ (1A-1679)(May1954). Detailsof the 5Wporthavebeen reportedly NorrisNeresonjLosAlamosScientifickboratoryReportIA-I-234(February1951). Neresonused techniquesotherthan theone reportedhere to studythebeam.**Also B. E. Watt,Phys.Rev. 87, 1037—

5

(1952)(selecteddatafrom IA-718).

..-——

—---

—.

.

6

cor-i

IG%-lF-4

TableI givesthe thicknessesof the polytheneradiators,and

TableII the thicknessesof the variousabsorbers. The additionof the

5 mil absorbersin the countersrequiredrecalculationof the proton

energiesacceptedwith particularabsorbercanbinations.The range-

energycurvesin IA-718were used and the resultsare givenin TableIII.

The factorf was obtainedfromFig. 12 of IA-7182and mp was computed

usingthe averagerate of energylossreportedby J. H. Smith [Fhys.Rev.

74, 32(194.7]. Thiswas also the sourceof the range-energycurve.

The eqyationfor neutronfluxas relatedto protoncant was derived

inLA-718andis

&-/P(tp)Nn(E)= 5.06x 104 up ~

s P(1)

neutronfluxthroughthe polythene,M is theP

per minuteat the two energiesforprotons,

intervalin Mev, U9 the total (rip)scattering

whereNn(E)is the total

differencein the counts

AEp is the protonenergy

crosssection(isotropywas assumedin the derivationof Eq. 1) in barns,

P is the reactorpower in kw, and (tp)is the thicknessof the polythene

radiator.

Theunitsof Nn(E)are

The circuitsusedwere

pulsesand spuriouscoincidencesdue to a largepulsein one or more

counters. Model502 amplifierswith a 2 p.secdelaylineclippingunit

weremodifiedslightlyto improvethe overloadcharacteristics.Three

neutrons/Mevkw sec.

new and permittedrejectionof both Hne

8

TABLEI

POLYTHXNERADIATORTHICKNESSES

Polythene Thickness(tp),Foil mg/cm2

2 2.063 5.704 21.011 71.88

RangeEquivalentin Aluminum,mg/cm2

2.887.9629.35100.4

TABLEII

ALUMINUMABSORBERTHICKNESSES

RangeEquivalentAbsorber in Aluminum,mg/cm2

CountergasCounterwindowCounterabsorber1234

27

7.1 * 17.42 * 0.157.0? 0.26.39 L 0.0313.04t 0.0527.0 t 0.142.2 f 0.211o.4t 0.4215.8? 0.9216.0t 2

9

10

were connectedto the countersand a fourthwas connectedto a dummy

chamber(a capacitance),whichwas connectedto the spectrometer.Line

pulseswere effectivelycanceledwith this setup.

The counterconstructionwas suchthata smallcouplingcapacitance

existedbetweenall counters,with the resultthat a specificcounter

wouldregistera spuriouspulse if a pulse70 timeslargerthan the

detectionthresholdoccurredin anothercounter. Spuriouscoincidences

fromthis sourcewere avoidedby usingthe coincidencecircuitdescribed

in IASLdrawing4Y-26121and by operatingin ‘tChannel”coincidence.

The dualoutput,five inputunit describedin the drawingpermits

independentselectionof the operationof all inputs,and two independent

coincidencecircuitsare operatedfromthe ssmeinputsto permitsimul-

taneous observationswith

Each inputcanbe:

eitherthe same or differentinputcombinations.

1.

2.

3.

4.

Disconnected.

Operatedin coincidence.A pulse largerthan the commondetectionthresholdis requiredfor an outputpulse.

Operatedin channel. A pulse largerthan the commondetectionthresholdbut smallerthan the individuallysetanticoincidencelevelis requiredfor an outputpulse.

Operatedin anticoincidence.Any pulse largerthan theindividuallyset anticoincidence-levelwill-preventanoutputpulse.

The Model502 amplifierscoulddeliver33 to 35 volt pulsesat the

100 lineterminationsin the coincidencecircuit,and the lowestdetec-

tion leveldesirablewas 2 volts. The maximumusefulchannelwidth for

11

any particularinputwas then 15:1,whichwas used for counters1 and 2.

The acceptablerangeof pulseheightsfrcm counter3 had to be consider-

ably largerthan the rangefrom 1 and 2 becausethe protonmight stop

in the chamberor mightpass throughwith considerableenergy. In

addition,all pulseswere not smplifiedthe ssmeamountin the gas, re-

sultingina pulseheightspreadof about30:1 frcm counter3. To

providechanneloperationwith a ratioof 40:1,it was necessaryto

connecta fifthamplifierto the outputof the preamplifierfor counter

3 and operateit in anticoincidence.

It was observedthat the pulsesfrom cmnter 2 were about25 per cent

largerthan thosefrcxncounter1, in part due to the protons’greater

specificionization.The gainswere set to give abouteqyaloutput

pulsesand a detectionlevelin the coincidencecircuitcorrespondingto

aboutO.lmv from the counters. The gain for counter3was set tobe the

sameas for counter1, as no directevidenceof pulseheightsfor rela-

tivelyhigh energyprotonswas available.

TableIV showsthe outputvoltagefrom each of the threecanters

whichwas requiredfor a coincidenceor whichwouldpreventan output

by anticoincidence.

1.2

TABLE Iv

COUNTERPULSEHEIGHTS

Output VoltageforCoincidence, Anticoincidence,

Counter mv mv ChannelWidth

1 0.09 1.3 15:12 0.11 15:13 0.09 ;:l 40:1

With the settingsused,any pulselargeenoughto give a coincidence

by capacitycouplingto the othercounters was also above its own anti-

coincidencelevel.

With the operatingvoltageson the counters,but no besm, the back-

groundratewas lessthan 0.01cpm. With the fullpowerbeam through

the apparatusthe lowestcountingratereachedwas 0.03 cpm,at least

50 per cent of whichis believedto be due to protonscapableof pene-

tratingall the absorbersavailable.

CounterCharacteristics

As a guidein estimatingthe gas simplificationand channelwidths

necessary,the energyloss in each of the threechannelswas computedfor

a 6 Mev protonand a 12 Mev protonleavingthe insideof the 1 mil window.

The valuescanputedare givenin TableV.

13

TABLEv

EIWIRGYLOSS IN THE CHANNELS

I?rotonEnergy, EnergyLoss,MevMev Counter1 Counter2 Counter3

6 0.14 0.2 0.8512 0.085 0.09

For the inputcircuitused

AE/V = 8

where

A = gas

E = the

amplification

ionizationenergyloss in Mev

v = pulsevoltageoutputin mv

As it provedconvenientto set the detectionlevelat about0.1 mv,

AE= 0.8 Mev. As shownin TableV, the minimumgas amplificationhad

to be at least10 to observethe more energeticprotonsand so get a

reasonablyaccurateintegralcount.

Frcxnthe relation

R= 4.16x 10-3E 1“73 (2)

it is computedthat an electroncapableof penetratingau threecounters

(R = 64 mg/cm2= 43 cm of air) has an energylossrate

dE 23.6— = 23.6R-0”42= ~ = 4.9 kev/cmdR .

14

(3)

For 6 Mev protons

dE~ = 77 kev/cm

whichis 16 timesthe calculatedrate for the electrons.

From the curvesfor countingratevs voltage,Fig. 3, and the pulse

heightdistributionat the electroncountthreshold(1.9kv),Fig. 4,

it seemsthatthe ratioof maximumprotonpulseheightto maximum

electronpulseheight (threshold)is more nearly7.5:1than the 16:1

expected. ~is is regardedas acceptableagreementin view of the

errorsassociatedwith Eq. 2 and the possibilityof longerelectron

pathsin at leastthe firstcounter.

The maximumenergylossby the electronsis then observedto be

about19 kev, and the energylostby a X2 Mev protonis 85 kev, only

4.5 timesthe valuefor the electrons. The pulseheightdistribution

seems to implya rather large spread (of the order 5:1) for a given

energy1Oss,whichresultsin lessthanunityefficiencyfor the

counters.

The efficiencyof counter2 was checkedat severalvoltagesby

cmparing 1.,3 coincidenceswith 1, 2, 3 coincidencesand was found

be 97 per centefficientfor the protonsgivinga 1, 3 coincidence.

Becauseamplifiergainsof counters1 and 2 were set to deliverthe

ssmerangeof pulseheights,counter1 is expectedto have the same

to

efficiencyas counter2. Unfortunately,a directcomparisonof 1, 2, 3

coincidenceswith 2, 3 coincidencescan onlyset a lowerbound,observed

15

I0:

I0’

I03

CPM

I0:

10

@@@ NOabsorbersScale at left I Y I @\/

[

Sc~/e @ 1,2,3 c MRight @ l,2ch;3c

/rl

Fig. 3 -

KV ON COUNTERS

Countingratevs countervoltage.

16

I

Ii

A/B

A/B

I.0‘1I

.8 ; r 1 *3 SINGLESRATE 2500CPmI :111

.6 ; M 1 0 #lA.l~h;2,3cI II I

A+2 A ,[,3r.; 2~h.4

II O ‘3A =I,2c; 3 ch

II

.2 I B =I,2,3cI II I I

o I ANT ICOINCI DENCEO 4 8 12 16 20 24 28 32 36 40 LEVEL (DIVISIONS)

6 I I I 1 I I I I23456789 (. /l ,; A E (gas amp X Mev IOSS)

1.0 IA

F II I 1.90 KV 1I II -#I I *3 SINGLES RATE 3500 cDm.8

1.1I Im I I I I I, I II I 1“.6 I 1r 1

I I ~ #t I A , f ~h; 2,3CI ~ # 2 A , l,3c ; 2ch

.4 ; II

I I.2 I I

I I

o 1 I ANTI COINCIDENCEO 4 8 12 16 20 24 28 32 36 40 LEVEL (DIVISIONS)

1 1 1 I 1 1 1 1 1 1 1 1 10 .1 .2 .3 .4 .5 .6 .7 .8 .9 1.0 1.1 1.2 1.31.41.5 ‘v INPUT 1‘3

1 9

I & 1 I 1 1 1 1 1 I 8 1 1 1 1 1 I t0.1 .2 .3.4.5.6.7.8.9 I.O 1.11.21.31.4151.61.71.8 MV INPUT 2

,Detection Level for Saturation Level for# all curves *“ all curves

1.011’I 1’I 1.95 KvF*3 S[NGLES RATE 5500 cPm/a/ I

A/6

#

.-1

.8II ;111

.6 -I iO *I A= I ch; 2,3cI

}I A ‘2 A= 13c; 2ch

II f

iI

.2 I I

III

o ANTI COlfkCIDENCEO ‘4 8 12 16 20 24 28 321 36 40 LEVEL (DIVISIONS)

Fig. 4 - Pulseheightdistribution.

17

to be 90 per cent,owingto the uncertaintythat the 2, 3 coincidence

was causedby a protonwhichtraversedcounter1. Sincethe energy

distributionof the protonspenetratingthe countersis nearlyindepend-

ent of the precedingabsorbercombination,it is thoughtthatall such

efficiencyfactorsappearonly in the constantdeterminingthe absolute

neutronflux,leavingthe relativeaccuracyof variouspointsuntouched.

The intensitycomputedfor the integraland differentialmethods

will,however,be differentif the 1, 2, 3 coincidenceefficiencyis

differentfrcmthe 1, 2 coincidenceefficiency.

The countingefficiencyis a fimctionof both the spatialdistribution

and energyof the protons. The spatialdistributionis independentof

the absorbercombinationsprecedingthe counters,so definefi(Ec)as

the spaceaveragedcountingefficiency,a functiononly of the proton

energyon enteringthe counters(Ec).

Assumethat the protondistributionis exponentialin the rangeof

interest,becauseassumptionsnearerto the conditionsof this experiment

leadto seriesexpressionsfor the integrals. The protoncountcan then

be written

Jw

c =K f (E - EU) e-E/QdEpio

whereEU is the energylostby a protonof initialenergyE in the ab-

sorbercombinationprecedingthe counters. It is also a functionof E,

but a relativelyslow one,beinggivenreasonablywellby the relation

18

‘Li : ~EU - k(E - ~EH “-Eo)

whereoEIi

is the lossby a protonenteringthe counterswith the

minimumdetectionenergyEo, and k is a constantdependingon the value

of (Eo+ OEU): k = 0.08 for (E.+ OEM) = 6Mev andk . 0.04 for

(Eo+ OEU)= I-2 Mev.

c =KJi

m (1 + ki) (E - oELi) 1-kiEo e-E/Q~pi

DefiningF, by the ratio

Jm

Ki 1(1 + ki) (E - OEU - kiEo e-E/QdE

(E.+ oEfi)Fi .

J

wK -E/Q~

(E.+ oEU)e

we expect~i to vary onlyvery slowlywith variousabsorbercombinations

(oELi),as ki is smalland variesslowly. If frises to a constant

valuein a smallenergyinterval,the variationwith energyis unimportant.

Then

19

The differencebetweentwo integralcountsis then

cc{

= K ~ e-(Eo+ oE=)/Q - ~ e-(Eo+ oEM)/Q~b a b 1

. KQ~ e-(Eo + ~Eh)/Q[

-(EO + oE#Q-e 1

if~a=~..

b

Let 22 be the averageefficiencyfor 1, 2 coincidence,~3 the

averageefficiencyfor 1, 2, 3 coincidence,and writethe energies

(E.+ EM) as Ei.

Then

{LY2p(integral)=K e-E1/Q- e-

1E~Q F3

and

{&p(differential)=K ~2 e-E~Q

1- F= e-EdQ

Then

&p( differential)Fe -EdQ - ~= e-E~Q

~p( integral)=1.4= 2

p7=e -E= Q 1-e-Ez/Q

{1.4 ? e-EJQ - e-EJQ + ?3 e-E21Q.72 e-EJQ

1

20

i72=1. 1 - e-(E2- 1El)/Q + ~-(Ea- E,)/Q

F3 = 1.4 - 0.4 e-(JI2- %.)/Q

For (E= - El) = 1 Mev and Q = 1.4 Mev,we have

0.83. The ratioFz/2sis surprisinglylarge.

intensityat variousenergiesis thoughtto be

statisticalerrors.

The observations,

TablesVI throughXI.

Unfortunately,it

extendedtime,during

showedsignsof drift.

with the calculationsof

F~?3 = 1.2, or 7~F2 =

However,the relative

correctwithinthe

N(E),

was necesssxyto take the data

are presented

overa rather

in

whichthe powermonitoringsystemson the reactor

!lhepowerobtainedby a heatbalancewas thought

tobe the most reliableso it was enteredin the powercolumn. The

in TablesX and XI were takenin a singleday and

be thebest for determiningre~tive intensities.

indicateddeviationsfroma smoothcurvewhichis

on a semi-logplot.

henceare thought

None of we runs

nearlya straight

data

to

line

Figure5 is a plot of all the data,exceptthe differentialmethod,

with the photographicplatedataobtainedby NorrisNereson.* Figure6

is a plot of selecteddatafrom the presentexperiment.

*SeeReportIA-1234.

21

22

23

.

+-l-ii+””;,

I

24

I

.—. . .—.—\\\\\

4.3X106Sinh (2E)~ e<E in Mev

L \

——

\\

o 5W beam spectrumphotographic plate data (1

I IProton recail counterdata[P

——

sent experimer

+

\\

\ \\

\ \

14

----

K~\!

\,

——

234)—.-

\+\

\\

T—.—...

.—

\\\\

. .

—

—.—

—..

i

i

—.—

—

—-

-.

-.

I---

28

Compositeplot of beam spectrum.

,.6

[

\

105

Poly

TT$ Differential Method

——\ I

T/4/’1/

Sv

I\\ ~

, Slove: 3.50 Mev/decade

I;Paly 4

Slope: 3,46 Mev/decode

dIultoneously01 method —

‘“KIntegrol Me!Oato tokenwith differe

Integrol MethodOther runs

13 lbEn,Mev

Fig. 6 - Comparisonof the neutronspectraobservedwith differentialand integralprotoncounting. Notethat the slopesare thesame. The differencein absolutevaluesresultsfrompoorprotoncountingefficiency,givingan errorin absolutevaluesbut not in shape.

29

The appearanceof 25 Mev neutronsseemsto implythata reasonably

largefractionof the fissionfragmentsare formedwith at least30 Mev

excitation,considerablyabovethe 20 Mev averagederivedfromthe

liquid-dropmodeland roughlycheckedby experimentalindications.

If the rangeof totalfragmentkineticenergyobservedby Brunton

and Hanna[CanadianJournalof Research &A, 190 (1950)}for partic-

ularmass ratioscanbe interpretedas the rangeof excitationenergy

in the fragments,it is easy to believethat excitationslargerthan4.0

Mev can existwith fairprobability.

30