LA-UR- 0 4 - b7 1
Appro ved for public release;
distribution is unlimited.
Title:
Cross Section Standards for Neutron-Induced Gamma-Ray
Production in the MeV Energy Rang e
Author(s) : R. O. Nelson, N . Fotiades , M. Devlin
J . A. Becker, P. E . Garrett, and W. Younes
Submitted to:
International Conference on Nuclear Data for Science and
Technology , Santa Fe, NM Sept . 27 - Oct . 1 , 2004
• Los Alamos
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Form 836 (8/00)
Cross Section Standards for Neutron-Induced Gamma-Ray
Production in the MeV Energy Rang e
R. O . Nelson*, N . Fotiades*, M . Devlin*, J . A. Becker, P. E . Garrett, and
W. Younes t
`LANSCE-3, MS H855, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
tLawrence Livermore National Laboratory, Livermore, CA 94550, US A
Abstract. Gamma-ray cross section standards for neutron-induced reactions are important in enabling the accurate
determination of absolute cross sections from relative measurements of gamma-ray production . In our work we observed
a need for improvement in these standards . In particular there are large discrepancies between evaluations of the
"aTe(n,111'y) cross section for the 847-keV gamma ray . We have performed (1) absolute cross section measurements, (2)
measurements relative to the nat G(ri , ll l'y) 1434-keV gamma ray, and (3) comparisons using measured total and elastic
scattering cross sections to refine our knowledge of the Fe cross section and the closely linked inelastic channel cross
section for Fe . Calculation of integral tests of the cross section libraries may indicate that adjustment of the angular
distributions of the neutron elastic and inelastic scattering may be needed.
INTRODUCTION
Measurements of 7-ray production cross sections
provide data important for a variety of applications .
Examples include determination of (n,2n) cross
sections used in radiochemical diagnostics and
determination of heating and damage in nuclear
reactors and advanced nuclear systems . Iron is a
common structural material and has also been
proposed as providing a reasonable moray cross section
standard. In work with GEANIE [1], the Germanium
Array for Neutron-Induced Excitations at the WNR
facility at LANSCE, to determine neutron-induced
reaction cross sections we observed a need for better
y-ray cross section standards, and in particular found
that accuracy of the Fe 847-keV y ray cross section
was poor . Due to the near equivalence of the inelastic
channel cross section and that for the 847-keV y ray,
these results also test the inelastic channel cross
sections in the evaluated data libraries .
Measurement of neutron-induced y rays from Fe
date to 1935 with the experiments of Lea [2] . Use of
the gamma-ray cross section to infer the inelastic
channel cross section was mentioned as early as 1956
by Day [3] . Since then over 40 experiments have
measured the (n,n'y) cross section at various incident
neutron energies . Despite these efforts agreement
between the various measurements is generally poor,
and various data eva l uations reflect t h e u n certainties in
the data by their differing values as shown in Table 1 .
TABLE 1 . Evaluated Li b rary Valu es for 56Fe Isotopic
Inel astic Channel Cross Sections at E„ = 14 .5 MeV.
Eva l uation Cross Section % D ifference
(mb) from EN DF
ENDFB-VI 681 0 .0
BROND2 610 -10 .4
JEFF 3 .0 724 6 .3
JENDL 3 .3 672 1 . 3
While neutron cross section standards strive for 1%
accuracy, the best available accuracy for y-ray cross
section standards (other than for neutron capture) is
typically 5 to 10% . In the case of the Fe(n,n'y)
reaction, two relatively recent evaluations [4,5] differ
by 26%, while both claim accuracies of 5 to 10% . To
address this problem we have performed (1) absolute
cross section measurements, (2) relative cross section
measurements using the Cr(n,n'y) reaction, and (3) we
use accurately measured neutron total cross section
and elastic scattering data to further validate our cross
sec ti on resul ts . We di sc u ss di screpa n c i es observed in
int egra l
measureme nt s a nd possi bl e so l utio n s .
Pr e limin a r y r es ult s for y rays fr o m Al . S i , and V a l so
ha ve b ee n obtained .
We con ce ntrat e on the 14-M e V incid e nt n e u tro n
en e r gy reg ion be c a u se angul ar d istr ib ut ion s e ffec ts are
s ma ller her e, and b ec au s e the e ffec t s of re sonan ce
structure and n e ut ro n- e n e r gy reso lution do n ot ha v e a
strong influence o n measurements at thi s energy .
EXPERIMENT S
T h e WNR faci lity a t LANSCE and t he GE ANI E
de t ec t o r array ar e described i n Liows ki er al . [6] and
Ne l so n et al . [ I] . r es p ec ti ve l} . In brief, t he L ANSCE
ac cele rator produc es 8 00 - Mel ' pr o ton pul se s with subnanosecond wi d th s that are d e li ver e d to awater- c o o l ed
tungsten neutr o n p ro ducti o n t arget . Spallat io n neutro n s
fro m th e target trave l swe ll- co llimat e d 2 0 .3 4 m flig ht
p a th t o th e G EANI E s ampl e .
neutr o n be a m s p o t i s mea sured with x-ra y image plates
a nd u se d in MCNP s imul 3 ti o n , o f th e measurements to
correct the detector acccptantc . ']'he contribution Of'
neu tron multiple sca tterin g a nd re action s t o the
me a s ure d c ross secti o n s is performed u s in g M( ' NP 181
c a l c ulat io n s o f th e neutron flux in the s am p l e an d
u s ing it tec hniqu e de s crib e d in Nelson et u l . 1 9 1 M( ' NI'
c a lc ulati o n s a re al so used to d e t e rmine y-ra y
a tte nuation i n the sam p l es .
Th e r esul t s of a 1994 measurement of the Felln,n'y)
9 4 7- keV cr oss sec ti o n a re s hown in Fig . I as th e
" Nel s on 94" d a t a po int s . Mos t o f the published
m eas u re ment s are al so show n fo r c ompari s on . A s
expect e d from th e n e ar equivalence alence of the Fe inelastic
c hann e l cross section and the 84 7 - k c 4' pa rtial y- ra y
c ross sec ti o n , th e s hap e of the FNDF inela s tic
excitati o n functi o n a nd th e shape o f our data are eery
s imilar . Thi s i s not tru e for some a t ' the p r ev i o u s
m e a s u re m e nt s . We agree we ll with the excitation
function s h ape of D i c k e n s . et al . I lU) t ha t was used i n
th e E NUF evaluation, h oweve r our cros s s ecti o n s are
ab o ut 2 010 larger than those of Dickens cI al .
In all measurements the y-ray detector efficiency
was determined in s in l using calibrate(] 7-ray sources
,ugh as 5 ' F u an d nii wd radionuclide suurrr> .
1eoo F
Draks 7 0
Fe (n,n 'jj E7 - 0.8 47 Me V
1 600
Th e "T ed n , n i' y) 8=1 7 -k e V c ross s ection a t incident
neutro n en e r g i es
ab ov e 11 . 4 M e V in c lu des
contributions from bo th '" F e1n, n 'y) and '' F e(n,? n y)
reac t io n s . We present bo th th e s umm ed ("'Fe( n ,n'y) +
''Fe (n ,? n y) p h oton cro ss sectio n re s ult bec a u se it i s
u sefu l as a st an da r d cross sec tion , a n d the pure
"'Fe(n . n ' y) c ross s e c ti o n because it prov ide s it m eas ure
o f th e inel as ti c chann e l cro ss secti o n . We a l so p res e n t
b o th n a t ura l a nd i so t o pi c cross sec t io n s fo r
convenience in compari so n s .
Beta-decay of the i''Mn product of the 5"Fe(n,p)
reaction for E„
3 .0 MeV, as well as contributions
from the minor constituents ''Fe(nd) and "Fe(n,t) at
higher energies can pose a background problem for
experiments that do not use pulsed beam techniques .
L achka r 7 4
- ~,
1 4 00 V~
1 200
E
NoI SOn 94
s,vio 76
1000
DlcM ~f 91
•~~'r,
.
% 800
600
Ke ttl e 7 3
aoo
Slm akov 9 3
2 00
0
f
5
7
9
T
11
E„(Mev )
13
15
F I G I RE: I . Absolute cross section measurements (nwst at
125 dogreesl for the "Fe(n .n'y) E .r = 847 keV y rah . Authors
are indicated fir some of the 34 experimental resul t s plotted .
The black cone is the FNI)FB-VI e\aluatiun for the ' P Fe
inelastic channel cross scction multiplied by the 91 .K" . .
abundance of " .Fe to cumert to the . .natural" cross section
for comparison .
A bsolute C ross Sec ti on Meas ureme nts
Ab so lut e y-ray c ross s e c ti o n meas ur em ent s ha ve
b e en ma d e at o ur labor a tor y fo r man y y e a r s . These
mea s urement s u se it fi ss i o n ionization chamber with
both 2"' U a nd '''U foils t o determine th e i n c ident
n e utr o n fluence [ 7 ] . T he thi c kness o f th e U d epos it s
and th e di stributi o n of material h av e b e en ca re full y
measured using seve r a l t ec hn iq ues . Th e pro fil e of the
Re l a ti ve Cross Sec ti o n Meas ure m e n t s
Of the reaction proposed a s y- ray c ro ss section
standards, the Cr( n,n'y) 1 4 3 4 -k e\ cross section at 14 .5
MeV appears to have the most consistent and accurate
measurement s . The evaluation b~ Simako \ , ct u/ . 141
uses 6 measurements, all but one of w hich i s
consistent w ithin unrrr till ntirs . The error in the
evaluated cross section is 5% . There is one
measurement on SZCr(n,n'y) that is also in near
agreement with the natCi measurements and has an
uncertainty of 3 .8% .
The measurements described here used relatively
thin "a`Fe and "a`Cr plates . This reduces corrections for
neutron multiple scattering and reactions as well as
corrections for y-ray attenuation in the samples . In this
relative measurement, only the thickness of the
samples, the relative efficiency for the y rays and the
number of counts in the moray peak are important.
Corrections to the ratio include : y-ray attenuation in
the sample, multiple scattering and reactions in the
sample, angular distribution effects, and internal
conversion . All of these corrections were at the few
percent level or less . A very minor correction was
made for the 56Fe(n,n(xy)12 Cr reaction and the
contribution of the 54Fe(n,3Hey)12 Cr to the production
of the 1434-keV y ray was ignored .
Other y rays that may provide suitable cross section
standards and for which we have preliminary results of
relative measurements include : "Al, E'Y = 3004 keV ;
nat
n at
Sl, Ey = 1779 keV ; and V , Fes, = 320 keV .
The 53Cr(n,2ny)52Cr reaction contributes to the
cross section of the 1434-keV y ray for E , > 9 .5 MeV .
The 53Cr(n,2ny)12Cr reaction is included in the cross
section value of 695 +/- 35 mb from the evaluation of
Simakov, et al . [4] that we use to obtain the value for
the Fe cross section from our relative measurement .
Because the slopes of the inelastic cross sections as
a function of energy are very similar for Fe and Cr at
incident-neutron energies near 14 MeV, the relative
measurement is insensitive to small changes in the
incident neutron energy . We used a neutron energy bin
with a half width of 0 .5 MeV centered on 14 .5 MeV to
obtain the cross section ratio . With a three day
measurement we obtained 1 .8% and 0 .8% statistical
uncertainties for the Fe 847-keV and Cr 1434keV grays, respectively . The sample thicknesses were
388 mg/cm2 for Cr and 164 mg/cm2 for Fe .
The results are presented in Table 2 where we give
both the measured "a`Fe(n,n'y) cross section including
the contributions of the 57 Fe(n,2n) reaction, and the
56
Fe(n,n'y) isotopic cross section where we have
subtracted the 57Fe(n,2n) contribution of 22 mb using
the ENDF/B-VI value at 14 .5 MeV corrected for the
terrestrial natural abundance of 57Fe of 2 .119% .
T ABLE 2 . C ross Secti on s for th e Fe 84 7- keV y Ray at E„ = 14.5 MeV.
Data Set
6( n1F e) (mb)
s6 Fe(n,n' y)+S 7Fe(n ,2ny)
a("a`Fe) ( mb )
6(56Fe) ( mb )
56Fe( n, n 'y) n at u ral 5 6Fe(n, n 'y) is o top ic
This Work - Absolute 705 +/- 56 683 +/- 57 744 +/- 62
This Work - Relative 684 +/- 45 669 +/- 46 730 +/- 50
Simakov et al. Evaluation 785 +/- 4 8
Savin et al . Evaluation 621 +/-6 2
ENDF/B-VI Inelastic Channel 68 1
INELASTIC SCATTERING CHANNEL
CROSS SECTION S
For 56Fe, a very large fraction of the inelastic decay
cascades down through the first excited state . From
this we assume that our value for the 56Fe(n,n'y) cross
section at 14 .5 MeV provides a lower limit on the 56Fe
inelastic channel cross section that is very close to the
true value . In Table 2 we compare our values with
those from the ENDF evaluated data library . We note
that the ENDF/B-VI cross section is 7% lower than
our value .
The evaluations are constrained so that the sum of
the various channel cross sections (elastic, inelastic, np, n2n, etc) must equal the accurately (< 2%) measure d
neutron total cross section. If we increase the inelastic
cross section by 7% then we must decrease the elastic
channel cross section by 7% of the inelastic cross
section to maintain the correct total . In Fig. 2 we show
the ENDF/B-VI elastic scattering cross section for 56Fe
with the available measurements . The dashed line
shows the ENDF elastic cross section reduced as
discussed above . The improved agreement between the
evaluation and the elastic scattering data support a
revision of these cross sections .
INTEGRAL MEASURE M ENTS AND
EVALUATED DATA LIBRARIE S
Int eg r a l n e ut ro n s p ec tra l meas urement s with pul se d
sph e r es and for nuclear re a c to r pr ess ure vessels, in th e
past ha ve o ft e n s h o wn more hi g h en e rg y ne u tron s th a n
calcu l a ted u si n g th e data libra ri es, and h e n c e ap p ear t o
co n tr adi c t our re sult o f a lar ge r in e la stic c ross se ction .
A so luti o n to thi s p ro blem ma y lie in adju stin g th e
an gular distributions o f the neut ro n e la s ti c and
inel as ti c sc att ering . New hi g her accur acy pulsedsphere data s uch a s thos e of Ma sse y , et id . [ II ] m ay
aid in re s ol v in g these que s ti o n s . Rec e nt measurements
of C hri s t o d o ul o u e' 1 u! . [ 1 21 o f th e ang ul a r di s tributi o n s
of th e e l as ti c a nd in e l as ti c scattering of n e utr o n s fr o m
F e do s how some discrepancies . It i s no ted th a t a t the
v ery forw ar d an g l es the e xpe rime nt und e re s timate s the
ela s tic cross s ec ti o n due t o the va ri a tion with a n g le of
the incid e nt n eut ro n e nergy .
Only the recently updated JE F F 3 .0 evaluated
library is in good agreement with the present results at
En =14 .5 MeV, although this is near a local maximum .
The ENDF 13-VI . JENDI . 3 .3 and BROND 2 library
values are all lower, the first two by about 7% and the
latter by 17 11 ,, .
ACKNOWLEDGMENT S
Thi s wo rk was p er fo rm e d un der th e au s pices 0 f thr
U . S . D epart me nt o f E n e rgy b y th e Uni v er s i ty o f
Californ ia , L os A lam os Nati o nal Laboratory under
C ontra c t N o . W-7405-EN6-30, and the Lawrence
Li v ermor e N a ti o n a l Lab o ra t ory und rr Contract N o . 1\ '740 5- EN 6 -4 9 . Thi s wo rk ha s benefited from the u s e
o f the L os Alamos Neutr o n Science ('enter at LANL .
Thi s fa c ilit y i s fund e d b y the U .S . Department of
Energ y under C o ntrac t N o 1h -" 405 _ E N (i - 36 .
2 .5
Kinney 68
• &nchurg I
• ke pi 8 2
- E NDFB-V
2
REFERENCE S
F NM n o7•i NI
1 .5
a
b
i
0 .5
o-
13
2.
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Nelson, K . U ., Becker, I . A ., Archer, I) . F ., E3rmstcin, 1A ., Johns . G . D . Wilburn . W . S . . Yuunes, W ., Drake . U .
M ., Rundberg, R . S ., W'endrr, S . A ., Ilauschild, K .,
Yates S . W ., and (i,irrett . P . 1 ,. .
International
CnrtJ o-e n re nm N'rrrlrun• /)u!u Ji )r Sc irit tV unh l
1'c rhnnlu tr . 1y1) 7. Italian Ph .N :ical Societ y ( ' unl 'crcnrc
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Nelson, R . U ., A 'u r leur Plrl '.v ic \ Nex 'e , 7 (1997) 1 I .
Lea . D . E . . Pru e Rol-al Sew, L wtdun. Serie s A . 151), 6 3766 R .( 1 93 5 ) ,
3 . Da~ . R . B ., Pit* i Y . Rev . 1112, 767-787 (1956) .
F IC U RE 2 . Cross section measurements for
'" Fe ( n,n) elastic scattering are shown with the current
ENDF ,! E3 -V I elastic scattering cross section (upper
curve) and with the ENDF values reduced by 7' i,, of
the inelastic channel c ro ss section (lower curve) .
CONCLUSIO N
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l mvrn m . . 1le lhm l s ,-I 33 0 , 220 ( 1991) .
n.
The data presented here give a more accurate value
for production of the 847-keV y ray, and a consistent
set of results for neutron inelastic, elastic and total
reaction cross section values for "'(-c . We plan to
improve the accuracy of the measured thickness of our
Cr sample and of the relative efficiency measurements
to further refine our cross section result . Additional
work on interpreting integral measurements on Fe and
the evaluated data libraries appears to he needed .
M( "J P -- A ( tzn e ral Mon t e ( ur l a N -Part i c l e fr ans r u rt
Code, J . F . B r ir s mc is tr r . I di! ur . Lo s A lamos ?rationa l
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Young, P . G . . ,1't«cl . Sri . unJ l~:ag . 1 3 8 . 1 0 5 20 1111 .
10 . Dickens, J . K ., Todd . J . I I , Larson . D . C' ., Re r un
UKNI . ;TM-1 It,i 1 .ORi~L(I(0'~ll) .
II .Thomas N . Masse~, Steen N1 . Grimes, Alla n l) .
Carlson . James M . Adams . Michael 1 . Wenner . and
Alirera Haghighat, runliibuti~~n to this conference .
12 . C . G . C'hristod0ulou, N . C . 'lirlig .~ni :, and G . F . K n u ll ,
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