NEUTRON SPECTRA
OF PLUTONIUM COMPOUNDS'
Spectrometer Measurements
L. W. Brackenbush
L. G. Faust
February 1970 :
AEC RESEARCH &
DEVELOPMENT REPORT
PACIFIC NORTHWEST LABORATORIES
BATTELLE MEMORIAL ~NSTIT
UTE
BATTELLE BOULEVARD,.P
0 . ?OX 999, ,RICHLAND. WASHINGTON W 3 6 1
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LEGAL N O T I C E
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2, plmtenesr, as usefutness of the tnfarmtkn contained in this report, or that ;lh@wa aif any ira'~drma6,
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BNWL-1262
UC-34, P h y s i c s
NEUTRON SPECTRA OF PLUTONIUM COMPOUNDS
P a r t 1:
L.
3 ~ ae n d
W.
6L
i S p e c t r o m e t e r Measurements
Brackenbush and
b.
G.
Faust
R a d i o l o g i c a l Sciences Department
Environmental and ~ i f S
e ciences Division
February 1970
BATTELLE MEMORIAL INSTITUTE
PACIFIC NORTHWEST LABORATORIES
R I C H L A N D , WASHINGTON
99352
P r i n t e d i n t h e U n i t e d S t a t e s o f America
A v a i l a b l e from
C l e a r i n g h o u s e f o r F e d e r a l S c i e n t i f i c and T e c h n i c a l I n f o r m a t i o n
N a t i o n a l Bureau o f S t a n d a r d s , U.S. Department o f Commerce
S p r i n g f i e l d , V i r g i n i a 22151
P r i c e : P r i n t e d Copy $ 3 . 0 0 ; M i c r o f i c h e $ 0 . 6 5
The neutron energy s p e c t r a of compounds of high-exposure plutonium ( 2 3 p e r c e n t 2 4 0 ~ u a) r e p r e s e n t e d .
The n e u t r o n spectrum of
plutonium t e t r a f l u o r i d e shows a peak a t 1 . 3 MeV, which a g r e e s with
p r e v i o u s l y p u b l i s h e d measurements; t h e n e u t r o n spectrum of plutonium
d i o x i d e i s shown t o have a very broad peak a t about 2 MeV.
A
p r e v i o u s l y u n r e p o r t e d spectrum of a plutonium-aluminum a l l o y ( 2 1
p e r c e n t Pu
MeV.
- 79
p e r c e n t ~ li s)p r e s e n t e d w i t h a peak a t about 1 . 6
The average f l u e n c e t o dose conversion f a c t o r s c a l c u l a t e d
from t h e measured s p e c t r a a r e :
Plutonium d i o x i d e
rad/n/cm2, plutonium t e t r a f l u o r i d e
aluminum a l l o y
-
f i s s i o n spectrum)
2.6 x
-
2.6 x
-
2 . 5 x 10"
- 2.7
x
rad/n/cm2, plutonium-
rad/n/cm2, and plutonium m e t a l (spontaneous
rad/n/cm2.
CONTENTS
Page
Introduction
1
2
Theory and @ e r a t i o n of t h e Spectrometers
~ e
Theory of Operation of 3 ~ Ueutron
Energy Spectrometers
C a l i b r a t i o n of t h e 311e Spectrometer
Experimental Procedures f o r Operation and
Data A n a l y s i s f o r t h e 3 ~ Spectrometer
e
Theory and Operation o f 6 ~Neutron
i
Energy Spectrometers
Results
Conclusions
2
-
LIST OF FIGURES
Page
1
Block Diagram o f F a s t Coincidence System f o r 3 ~ i e
Neutron Spectrometer
2
Energy C a l i b r a t i o n Curve f o r 31~eSpectrometers
3
Neutron Cross S e c t i o n s f o r ' ~ e ( n , ~and
) ' ~ i ( n , a ) Reactions
4
S e m i l o e a t i t h m i c P l o t of " ~ r o s s " and "Background" Data f o r
e
Exposed To a PuF4 Source
t h e 3 ~ Spectrometer
5
Neutron Energy Spectrum from PuF,,
6
Neutron Energy Spectrum form a ' ~ Spectrometer
i
Exposed
t o PuF4
7
Neutron Spectrum from PuF,,
8
Neutron Spectrum from Pu02
9
C a l c u l a t e d Neutron Spectrum from Pu02
10
Neutron Spectrum from Pu-A1 Alloy
LIST OF TABUS
1
Neutron Source Data
2
Summary o f Measured Neutron S p e c t r a
3
Summary o f P u b l i s h e d lleutron S p e c t r a
4
Neutron Y i e l d s from Plutonium. I s o t o p e s and Compounds
5
Fluence t o Dose Conversion F a c t o r s
NEUTRON SPECTRA OF PLUTONIUM COMPOUNDS
P a r t 1: 3 ~ and
e 6 ~Spectrometer
i
Measurements
-
INTRODUCTION
To determine neutron dose r a t e s and s h i e l d i n g requirements from
plutonium compounds used i n t h e manufacture and processing of r e a c t o r
f u e l elements, it i s d e s i r a b l e t o know t h e neutron energy s p e c t r a .
Knowing t h e neutron spectrum and t h e neutron f l u x , it i s p o s s i b l e t o
c a l c u l a t e neutron dose r a t e s .
Pure plutonium metal has a r e l a t i v e l y
low neutron emission r a t e a r i s i n g from spontaneous f i s s i o n .
However,
when plutonium comes i n t o c o n t a c t w i t h m a t e r i a l of low atomic number,
t h e neutron emission may be i n c r e a s e d by o r d e r s of magnitude.
The
neutron s p e c t r a from t h e s e ( a , n ) r e a c t i o n s w i t h elements of low atomic
number i s q u i t e d i f f e r e n t from t h e spontaneous f i s s i o n spectrum.
This
paper p r e s e n t s t h e neutron energy s p e c t r a from P u - ~ ( a , n ) , P U - O (,~n ) ,
and P u - ~ l ( a , n ) r e a c t i o n s along with a l i s t of f a c t o r s t o convert from
neutron f l u t o neutron dose r a t e .
Accurate neutron s p e c t r a from plutonium compounds a r e q u i t e
d i f f i c u l t t o o b t a i n because of t h e r e l a t i v e l y low neutron y i e l d s of
small amounts of plutonium compounds and because most neutron spect r o m e t e r s a r e very i n s e n s i t i v e .
Helium-3 and lithium-6 semiconductor
spectrometers were used f o r t h e s e measurements because (1)t h e y have
very good r e s o l u t i o n , ( 2 ) t h e y a r e moderately s e n s i t i v e ( t h e y have a
s e n s i t i v i t y of
to
events p e r neutron/cm2), ( 3 ) t h e e l e c -
t r o n i c equipment r e q u i r e d i s f a i r l y simple, and ( 4 ) t h e d a t a do not
r e q u i r e an e l a b o r a t e unfolding technique a s do proton r e c o i l d a t a .
As w i l l be explained, t h e 3 ~ spectrometer
e
g i v e s questionable r e s u l t s
a t l ~ w e rneutron e n e r g i e s (below about 1 M ~ V )when a s t r o n g g m a
f i e l d is present,
A d e t a i l e d explanation of t h e t h e o r y , operation
and a n a l y s i s of t h e d a t a i s given i n t h e following s e c t i o n s .
SUMMARY
Neutron s p e c t r a and f l u x measurements a r e u s u a l l y r e q u i r e d f o r
w c w a t e neutron dose r a t e measurements.
This r e p o r t p r e s e n t s t h e
r e s u l t s of neutron s p e c t r a measurements f o r cckpounds of highexposure piatonium (about 23 pepcent 240F'u) produced by power
reac5ors.
A high exposure PuF4 source gave a peak neutron energy of
1 . 3 MeV, which agrees with previous measurements; a Pu02 source gave
a very broad peak, which can be resolved i n t o a spontaneous f i s s i o n
s ~ e c t r - u nx ~ t ha peak a t about 0.7 MeV and an 180(cr,n) spectrum with
a peak a t about 2.4 MeV.
A 2 1 percent plutonium
-
79 percent
alummum a l l o y gave a neutron spectrum with a peak at about 1.6 MeV.
The average f l u e n c e t o dose conversion f a c t o r s fcr t h e s e sources a r e
a s follows:
r a d 'n19-m2,
PuF4 - 2.5 x 10'~
rad/n/cm2, Pu-A1 a l l o y
Pu02
-
r f i s s l o n spectrum)
2.7 x 10".
-
- 2.6
x 10"
rad/n/cm2, and pure plutonium metal
2.6 x 10"
rad/n/cm2.
THEORY AND OPEBATION OF THE SPECTROMETERS
Theary a f Operation of t h e 3 ~ Neutron
e
Energy Spectrometer
e
d e t e c t o r c o n s i s t s of two c a r e f u l l y matched
The 3 ~ neutron
sll:con
s u r f a c e b a r r i e r d e t e c t o r s s e p a r a t e d by 1 mm and s e a l e d i n t o
a d m b l e walled aluminum can I n t o which 3 ~ gas
e can be admitted.
Each s u r f a c e b a r r i e r d e t e c t o r h a s a s e n s i t i v e a r e a of about 2 cm2
and a d e p l e t i o n depth of 250 microns a t 50 v o l t s b i a s .
volume of t h e d e t e c t o r head i s about 280 mm3,
The s e n s i t i v e
The 1 mrn s e p a r a t i o n
r e s u l t s i n some p a r t i c l e l o s s around t h e edges and some s t r a g g l i n g
and l o s s of r e s o l u t i o n f o r high gas p r e s s u r e s ( g r e a t e r t h a n 10 atm)
.
F a s t neutrons i n t e r a c t w i t h t h e 3 ~ gas
e i n t h e d e t e c t o r head by
t h e following r e a c t i o n :
2He3 + on'
+
1 ~ 3
+ 1 ~ +1 0.764 MeV
Ignoring s m a l l energy l o s s e s due t o s t r a g g l i n g , t h e energy of t h e
i n c i d e n t neutron i s e q u a l t o t h e energy d e p o s i t e d i n t h e s u r f a c e
b a r r i e r d e t e c t o r s by t h e t r i t o n and proton minus t h e Q value of t h e
reaction.
The r e l a t i v e i n t e n s i t y of t h e neutron spectrum i s d e t e r -
mined by c o r r e c t i n g f o r t h e change i n neutron c r o s s s e c t i o n w i t h
energy.
The upper l i m i t of u s e f u l n e s s of t h i s device i s about 5 t o 6
MeV due t o ( 1 ) competition of 3 ~ e ( n , d r) e~a c t i o n s f o r e n e r g i e s
above 4.4 MeV, ( 2 ) t h e range of r e a c t i o n products from h f g h e r energy
neutrons r e q u i r e s a g r e a t e r d e p l e t i o n depth i n s i l i c o n , and ( 3 )
l a r g e depths of s i l i c o n r e s u l t i n cornpetition from neutron induced
reactions i n the silicon.
F o r t u n a t e l y , t h e neutron s p e c t r a of
plutonium compounds c o n t a i n very few neutrons above 5 MeV.
Overall
r e s o l u t i o n of t h e spectrometer depends upon energy, and t h e peak
width from thermal neutrons i s l e s s t h a n 100 keV (FWHM). The
e f f i c i e n c y of t h i s device i s q u i t e low
p e r neutron p e r square c e n t i m e t e r .
-
about
to
events
A f a s t coincidence system i s used i n an attempt t o record only
e t o exclude
t h e t r l t o n and proton from a neutron r e a c t i o n wlth 3 ~ and
gamma and &her- neutron interactions.
Unfortunately, t h e s u r f a c e
b a r r i e r d e t e c t o r s a r e a l s o q u i t e s e n s i t i v e t o gamma r a y s and t o
neutron induced events i n t h e s i l i c o n .
To minimize t h e "background"
from t h e s e extraneous s o u r c e s , i t i s necessary t o have a very f a s t
coincidence system,
The fast coincidence system used with t h e 3 ~ dee t e c t o r head i s
sham
In
F l g w e I.
Care must b e e x e r c i s e d i n t h e s e l e c t i o n of t h e
charge s e n s i t i v e p r e a m p l i f i e r s , f o r t h e l a r g e input capacitance of
t h e s u r f a c e b a r r i e r d e t e c t o r s can c r e a t e s e r i o u s e l e c t r o n i c n o i s e
problems.
The linear a m p l i f i e r s a r e operated with 1.8 microsecond
tlme c o n s t a n t s f o r doubly d i f f e r e n t i a t e d p u l s e s t o s a t i s f y t h e
multichannel analyzer Input s i g n a l requirements.
The s i n g l e channel
analyzer d i s c r i m i n a t o r s a r e s e t t o t r i g g e r only on p u l s e s eorrespond;ng t a protons and t r i t o n s from thermal neutron r e a c t i o n s and a r e
c p e r a t e d i n t h e crossover pick-off mode t o minimize j i t t e r .
The
~ o i n z l d o n c eu n i t has a 100 nanosecond ( 2 t a u ) r e s o l u t i o n time.
Smaller r e s a l u t i o n times would be d e s i r a b l e t o reduce t h e random
rsolncldence from gamma and ~ i ( n , p )r e a c t i o n s , b u t t h e s l i g h t J i t t e r
~ n t r o d u e e dby e l e c t r o n i c n o i s e makes it necessary t o have r e l a t i v e l y
large resolution times,
C a l i b r a t i o n of t h e ?He Spectrometer
The spectrometer was c a l i b r a t e d by using n e a r l y monoenergetfc
n e u t r ~ n sproduced by Tlp,n) and ~ ( d , n )r e a c t i o n s from a 2 MeV
Van de Graaff a c x l e r a t o r
These r e a c t L o n ; a r e eamparatively f r e e
of s t r s n g gamma r s y p r o d u c t l a n whlch might , n t e r f e r e w l t h t h e energy
calibraflon.
( about
S l n c e t h e s f f l c ~ e n c yof t h e d e t e c t o r i s very iow
to
e v e n t s per n,'cm2
produce a s many n e u t r a n s a:, p o s s l b l e
, thick
t3
t h r g e t s were used t o
lmprove c o u n t l n g s t a t i s t i c s .
The r e s u l T r n g n e u t r o n peaks 3 b s e r - ~ e dwere q u ~ t ebr;ad,
sc t h a t the
l e a d l n g edge; were used f o r r.alibratl:,n purposes I n come ~ n s t a n c e s .
The energy c a l i b r a t l ~ n1 s shown ;n Figure 2 f c r the 3 ~ dee t e c t o r
o r i e n t e d w i t h t h e s u r f a c e b a r r l e r d e t e c t ~ r sp a r % l l e l t o t h e n e u t r o n
beam.
The n s n - l i n e a r r e g l o n bel3w about 0.3 MeV 1s b e l l e v e d t o b e
due t o d l f f e r e n c e s i n t h e f;.rward s c a t t e r of t h e t r l t o n and p r o t o n
from f a s t n e u t r o r r e a c t r o c s arid due t o d l f f e r e n c e s i n t h e e f f i c i e n c y
of t h e s11rfa1:e b a l r l e r d e t t : t L x s
surfaces.
wlth a t h i n l a y e r ,f g o l d on t h e i r
p ~ ! s e g e ~ i e r a t o rwas u3ed t a p r c v l d e a secondary
A pre:lsLon
c a l l b r a t l o n zf tke s p & : t r ~ m e t e r t o a s c ~ r r a ~whether
n
electronic gain
shifts 3 c c u r r e d .
T h e s p e c t r o m e t e r w a s r e z d l ~ b r a t e ds e v e r a l t i m e s ,
and no s l g c i f ~ c a n tchsnges were n c t c d ;vsr
The 3 ~ detep?\r
e
;hqee-m,nth
perlods.
he&d e x h l b l t s same ang,,isr r e s p m s e .
For
exposures m r d e with !..O MeV n e u t r o n s , th- a e p a r h t l o n between t h e
thermal neut,ro= p a i r snd !' MeV next1 2n peak i n r r e s s e a ~s t h e 3 ~ e
det,e.-tar he;d
UF.;
r . - - s t ~ df r j m ;p c l s ~ ' ; x i with t h l s ~ r f a c eb a r r i e r
l
d e t e c t o r s g x ~ r d ; : I ~:
t h e r.el:Lr:n
b a r r l e r d e t e r ~ , ~perpendi:ulsr
c~
betweer! t h e p ~ a k slr,i:rca-;ed
paslt:~nc:d
&r '$5'; ana
d e + e c t a r was p L L ed
ht
1~
9:
bemL t 3
3
p a s l t l - r i w ~ t ht h e s u r f a c e
t; t h e n e u t r a i beam.
The s e p a r a t i o n
about 5 pr r.:er.+, wncr? + h e d e t s c t c r was
ic:~eiistd
pb-ut
12
perpeqd- ula--4
pt:<E3T
' t
., :n.;'
when t h e
:leuCrari beam.
The n e u t r o n peaks sometimes c o n t a i n e d only &bout 100 counts p e r t e n
channels, s o t h a t t h e counting s t a t i s t i c s a r e r e l a t i v e l y poor.
The ? ~ sep e c t r o m e t e r a l s o e x h i b l t s p u l s e p l l e - u p problems i n
s t r o n g gamma f i e l d s .
Exposures made w i t h a 1 2 c ( d , n ) r e a c t i o n pro-
ducing l , h 8 MeV n e u t r o n s gave u n u s u a l r e s u l t s
--
t h e n e u t r o n peak
o c c u r r e d i n a channel corresponding t o 1 . 7 MeV n e u t r o n s .
The gamma
dose r a t e w a s n o t measured w h i l e t h e a c c e l e r a t o r was o p e r a t i n g , b u t
a f t e r t h e a c c e l e r a t o r was s h u t down, t h e r e s i d u a l t a r g e t dose r a t e
was about 100 mR/hr.
With t h e s e k i n d s of dose r a t e s , it i s q u i t e
probable t h a t some summing o c c u r r e d ,
Experimental Procedures f o r Operation and Data A n a l y s i s o f
t h e 3 ~ Sep e c t r ~ m e t e r
To a c q u i r e n e u t r o n s p e c t r a l d a t a , t h e 3 ~ dee t e c t o r head i s
f i l l e d w i t h 80 t o 100 p s i a of 3 ~ gas
e which has been p u r i f i e d t o
remove t r i t i u m ( p u r i t y of 1 p a r t 3~
o r "foreground" exposure i s made.
per
loi1
The 'He
p a r t s 3 ~ e and
)
"gross"
i s t h e n removed and a
"background" count i s made t o compensate f o r e v e n t s from gamma,
s i ( n Y p ) , and o t h e r ~ n t e r a c t f o n s . A f i t of t h e d a t a i s made and t h e
"b&ckgrouncil' s u b t r a c t e d .
The r e s u l t a n t d a t a a r e t h e n c o r r e c t e d f o r
t h e changes i n 3 ~ cer o s s s e c t i o n w l t h energy t a k e n f r o m F f g w e 3"(1)
To o b t a f n a c c u r a t e s p e c t r a , ~t was n e c e s s a r y t o I s o l a t e t h e
s p e c t r ~ m e t e rarid ta c o n s t r u c t a jig t o s h l e l d and p o s i t i o n t h e 3 ~ e
d e t e c t o r head f cr a c c u r a t e "background" c o u n t s ,
The j i g c o n s i s t s o f
a one-lnch l e a d s h l e l d t o r e d w e %he gamms b a c k g r e m d w l t h a 1/8inch f i n s h l e l d
53
reduce any l e a d X-rays p r e s e n t ,
The 'He d e t e c t o r
-.
0
q;r
[I)
C
-0rl
c,
[I)
[I)
head f i t s i n s i d e an aluminum c y l i n d e r t o maintain t h e same geometry f o r
t t b a ~ k g r ~ u n rdut nf s .
F i g u r e 4 shows a semilogarithmic p l o t of t h e d a t a o b t a i n e d by
e
head i n t h e l e a d s h i e l d e d j i g and
p o s i t i o n i n g t h e 3 ~ spectrometer
exposing it f o r a two-week p e r i o d t o a plutonium f l u o r i d e sample t h r e e
inches i n diameter by t h r e e inches h i g h c o n t a i n i n g 539 grams of h i g h
.
ft
was
exposure plutonium ( 2 2 p e r c e n t 2 4 0 ~ u ) The t ' g r ~ ~ smeasurement
nade w i t h 100 p s i a of 3 ~ gas
e f i l l i n g ; t h e "background" measurement
was made w i t h t h e d e t e c t o r head evacuated.
Note t h e g r e a t number of
"background" e v e n t s a t lower e n e r g i e s due t o random coincidences of
gamma r a y s and ~ i ( n , p )r e a c t i o n s i n t h e semiconductor d e t e c t o r s .
S u b t r a c t i n g t h e f i t t e d v a l u e s of t h e "background" exposure from t h e
f i t t e d v a l u e s of t h e "gross" exposure r e s u l t s i n very l a r g e s t a t i s tical e r r o r s a t lower neutron e n e r g i e s .
Figure 5 shows t h e "back-
ground" c o r r e c t e d curve obtained by s u b t r a c t i n g f i t t e d v a l u e s .
The
p o i n t s w i t h e r r o r b a r s a r e t h e r e s u l t s o b t a i n e d by u s i n g t h e a c t u a l
"gross" values r a t h e r t h a n f i t t e d v a l u e s .
To o b t a i n t h e neutron
energy spectrum, it i s n e c e s s a r y t o c o r r e c t f o r t h e changes i n t h e
3 ~ e ( n , p c) r~o s s s e c t i o n w i t h energy a s shown i n F i g u r e 3,
The s o l i d
l i n e i n F i g u r e 5 i s obtained by m u l t i p l y i n g t h e "backgroundft
c o r r e c t e d curve (shown a s a dashed l i n e ) by t h e i n v e r t e d c r o s s s e c t i o n
f c r t h e 3 ~ e ( n , p )r e a c t i o n normalized t o a v a l u e of 1.00 f o r t h e peak
o c c u r r i n g a t 1 . 7 MeV i n t h e 3 ~ e ( n , p )c r o s s s e c t i o n .
Channel Number
FIGURE 4
Semilogarithmic Plot of "roreground" and "Background"
Data for the 3 ~ Spectrometer
e
Exposed to a PuF4 Source
8k
t
a
5
Z
2C
0
VI
w
3u
u
u
H Q)
Erc a
VI
C
0
E
k
u
3
Q)
Z
Theory and Operation of t h e 'Li
Neutron Energy Spectrometer
The 6 ~neutron
i
d e t e c t o r c o n s i s t s of two s i l i c o n s u r f a c e b a r r i e r
diodes placed f a c e t o f a c e w i t h a t h i n f i l m of ' L ~ F evaporated onto
t h e f a c e of one of t h e d i o d e s .
Neutrons i n t e r a c t w i t h t h e 'Li
according t o t h e r e a c t i o n :
3Li6
+
on1
+
2~e4
+ 1 ~ 3+ 4.78 MeV
As i n t h e case of t h e 6 ~ sep e c t r o m e t e r , t h e neutron energy i s d e t e r mined by measuring t h e energy of t h e r e a c t i o n p r o d u c t s .
The ' L i ( n , a )
r e a c t i o n has such a h i g h Q value t h a t t h e r e a r e few "background"
e v e n t s from gamma and neutron induced events i n t h e s i l i c o n .
a relatively slow coincidence system can b e used.
The s e n s i t i v i t y
of t h e 6 ~d eit e c t o r i s l e s s t h a n t h a t of t h e 3 ~ dee t e c t o r .
t h i c k n e s s of t h e 6
Hence,
I f the
~l a y ie r i~s i n c r e a s e d t o i n c r e a s e t h e neutron
s e n s i t i v i t y , t h e r e s o l u t i o n of t h e d e t e c t o r d e t e r i o r a t e s .
For a
150 microgram/square c e n t i m e t e r t h i c k l a y e r of ' L ~ F , t h e width of
t h e peak r e s u l t i n g from slow neutrons i s typically 300 keV (FWHM).
The 6 ~ spectrometer
i
has a u s e f u l o p e r a t i n g range from 0 . 5 MeV t o
about 10 MeV due t o competing n u c l e a r reactions.
competing r e a c t i o n 1s t h e ' L i ( n , a )
energy of 1,72 MeV.
The most important
dn' r e a c t i o n , whlch has a t h r e s h o l d
On t h e average, t h e energy of t h e outgolng
neutron i s around 1 t o 2 MeV, s o t h a t t h e energy d l f f e r e n e e between
t h e 'Li(n,a)dn'
and ' ~ i ( n , a )r e~a ~
c t i o n s i s about 8 MeV.
For
neutron s p e c t r a w i t h a l a r g e number of neutrons above about 5 MeV,
t h e s p e c t r a below 1 o r 2 MeV may be i n e r r o r . ( 2 )
The 6 ~spectrometer
i
was a l s o c a l i b r a t e d by exposure t o
monoenergetic neutrons produced by a Van de Graaff a c c e l e r a t o r .
However, t h e r e s u l t s a r e more u n c e r t a i n because of t h e lower s e n s i t i v i t y of t h e 6 ~spectrometer
i
and because of e l e c t r o n i c g a i n s h i f t
problems
.
The d a t a a n a l y s i s of t h e 6 ~spectrometer
i
i s s i m i l a r t o t h a t of
e
t h e 3 ~ spectrometer.
The "background" i s much lower because of t h e
i
high Q value of t h e 3 ~ i ( n , a )r e a c t i o n , s o t h a t t h e 6 ~spectrometer
p e s u l t s a r e probably more a c c u r a t e a t lower neutron e n e r g i e s c l o s e
t o 1 MeV.
The d a t a must be c o r r e c t e d f o r changes i n t h e c r o s s s e c t i o n
with energy a s shown i n Figure 3. ( 3 )
Since t h e 6 ~spectrometer
i
has
poorer r e s o l u t i o n , one must b e c a r e f u l t o s u b t r a c t out t h e thermal
neutron peak b e f o r e applying c r o s s s e c t i o n c o r r e c t i o n s t o t h e d a t a .
This can be done by p l o t t i n g t h e thermal peak on a semilogarithmic
graph and l i n e a s l y e x t r a p o l a t i n g t h e edge of t h e peak t o h i g h e r
neutron e n e r g i e s .
RESULTS
The experimentally measured neutron energy s p e c t r a from v a r i o u s
plutonium compounds a r e shown i n Figures 6 through 1 0 .
The r e l a t i v e
neutron intensity ( w i t h dimensions of neutrons ) i s p l o t t e d a g a i n s t
.
t h e neutron energy (wit3hdimensions of M ~ V ) The a r e a under t h e
curves i n each of t h e s e f i g u r e s i s normalized t o a value of 100
neutron-MeV ( i . e . , t h e curves a r e d i s t r i b u t i o n f u n c t i o n s whlch have
s r d i n a t e values i n p e r c e n t ) .
The plutonium compounds used f o r t h e s e measurements were i n t h e
form of l o o s e powders, which were s e a l e d i n s i d e of two t h i n walled
s t e e l cans.
A summary of t h e plutonium sources used i s shown i n
Table 1. The r e s u l t s of t h e s e measurements a r e summarized i n Table
2 , and a summary of published s p e c t r a l d a t a i s included i n Table 3
f o r comparTson.
F i g u r e 6 shows t h e neutron s p e c t r a o b t a l n e d from a 6 ~neutron
i
spectrometer exposed t o a l a r g e , low e x p o s w e ( 6 p e r c e n t 2 4 0 ~ u )
plutonium t e t r a f l u o r i d e source ( s o u r c e A i n Table 1 ) . The a r e a under
each curve i s t h e same, s o t h a t t h e e f f e c t s of t h e one-inch L u c i t e
moderator a r e apparent.
The moderator i n c r e a s e s t h e peak width,
removes a few higher energy n e u t r o n s , and c r e a t e s a few lower energy
neutrons.
The peak occurs a t 1.1 MeV, which i s somewhat lower t h a n
o t h e r measurements i n d i c a t e ( s e e Table 3 ) .
The energy c a l i b r a t i o n
f o r t h e s e measurements i s somewhat u n c e r t a i n .
Flgure 7 shows t h e neutron spectrum o b t a i n e d from a 3 ~ neutron
e
spectrometer n t h a high exposure (22 p e r c e n t 2 ' 0 ~ u ) plutonium t e t r a f l u o r i d e s o u r c e , d e s i g n a t e d a s source B i n Table 1. Note t h a t t h e
energy peak a t 1 , 3 MeV i s i n e x c e l l e n t agreement w i t h p r e v i o u s i y
published d a t a o b t a l n e d from n u c l e a r emulsions and s t i l b e n e c r y s t a l s
( p r o t o n r e c o i l ) shown i n Table 3.
The spectrum from a high expzsure (22.7 p e r c e n t 2 ' 0 ~ u ) plutonium
d i o x i d e s o u r c e , d e s i g n a t e d a s source C , i s p r e s e n t e d i n Figure 8.
The oxide c o n t a i n s n a t u r a l l y occTZring oxygen; no attempt was made t o
change t h e neutron y i e l d from 1 8 0 ( a , n ) by l s o t o p i c enrichment.
The
spectrum b e b w about 1 . 5 MeV l a u n c e r t a i n because of competition from
TABLE 1
NEUTRON SOURCE DATA
Source
Identification
A
PuF4
Mass
Plutonium
765 g
Isotopic Composition in Weight Percent
238~u
'Pu
23
2 x 1 0 ~ ~9 3
2 4 0 ~ ~241PU
6
0.5
Size of
2 4 2 ~ u Source
3-1/2" dia.
x 4-1/8"
Yield
Neutrons /see
5 x lo6
a
Ld
a,
rl
-r-i
0
.A
k
.rl
d
hO
.d
rn