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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.
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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).
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
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
.—. . .—.—\\\\\
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