A neutron counter for small samples Page: 4 of 4
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documerted for the powder samples. Longer count times
and statistical tiltering of the data would improve the results.
This evaluation also demonstrated the importance of sample
positioning for improving measurement accuracy.
Differences in geometry between pellets and powder and
differences in the containers both impacted the accuracy of
the measurement results. The presence of HEU in some of
the MOX items caused a measurable bias because of sample
multiplication.
Simultaneous Measurement of Isotopics and 240Pu Effective
M ass ___ _ _ _ _
We combine traditional neutron coincidence measure-
ments with knowledge of the plutonium isotopics to calculate
the plutonium mass from the measured neutron count rates.
The INVS Model IV can be outfitted with a bottom end plug
that allows the insertion of a germanium detector into the
sample cavity. A customized germanium detector is used to
minimize the effects on the neutron counting. Tin filters were
used for the germanium detector because the use of cadmium
would impact the neutron detector efficiency. The bottom of
the sample holder is raised 4.13 cm when the germanium
detector is in position. This configuration :allows simultane-
ous measurement of the passive gamma-ray signal and the
passive neutron signal The sample plutonium mass can he
determined from this measurenent configuration alone.
We are presently evaluating the performance of the
combined INVS and isotopics system at Los Alamos, The
addition of the germanium countei does not have a signifi-
cant effect on the accuracy of the INVS neutron imeasure-
mcnts. The neutron detection rlticiencv is decreased 0.2%
by the gennanum detector Table IV lists preliminary results
for measurements on six samples.
a e . tsoo pics Measurieien Results
anip e II) traction -'tofu effective ouni t ne
town meaSUrec
.. rr?.i03'0.069 ( 0.ST
rrrITrr- WT1 i 0 1.170 f-0 1
I'l ".5 ..) 1 .224C tiIR T
(,1 IM 1 ( 1.0017- T( %
('otit filles of I(NN) lit ((MN) S allow is to measure the
IractiUn of "'101'11 effective with ;m uncertainty of I ii ('C.
We exme I thtal larger samples will te imiasimed with an
ncertainty of appiunlately (.' t it an hu
Isia it n i I tre (il. n) Rale
Sample mulutpication is gernirmaly neghgible loi small
samnipleS it pliniomim onsequntly the nliphtinn ril
irclion is not grnrally ltsrd for INVN da ut irtituno. Ih
INVS Mtelrl IV can Ieu iio mrasmir t''lhu rffectve
mtss and the (:r, n) neution late for these non muluplviig
samples. Ilms ieasiud alpha value can flin i iusid flt ithe
ulk teis that are irpiesetr t by tIhe small samplein i he
INV'S countI lhe hulk iteis coul Ie mr:iutird utsmop
1"trantg villppmnt ,mnd the known .tlplha anailvsi% melhili
/ 1/
I e ri imi t uiii ilnct u n i .ite ilritri ui S he ' 't1i
rlrie mvr IILISS 1or nomuilltplS utp .unplrs, Ihr 4 uit normt rcount rate can he combined with the total neutron count rate
to provide an estimate of a. the ratio of the (C, n) and spon-
taneous fission rates. Since changes in the neutron energy
affect the neutron detection efficiency, the INVS Model IV
can read the 1Ile detector rings separately. The ratio of the
inner to outer total neutron count rates can he used to provide
an efficiency correction for the (a, n) neutrons that generally
differ in energy from the fission neutrons. This method of
estimating a requires an accurate totals measurement, which
implies an accurate value for the background rate. This
method is being evaluated at Los Alamos with impure sam-
pIes of PuO2.
5jMMARY
The measurement goal of high accuracy has been
achieved. Two different field evaluations have demonstrated
hall-percent or better results for the neutron coincidence
counting of small samples. Both on-site laboratory use and
inspectors with portable equipment can take advantage of
this improvement in the technique. The addition of the iso-
topics capability will make the INVS less portable, hut for
some applications the high-resolution gamma-ray spectrome-
try hardware is available at the inspection site. he abiIiy to
count the gamma-ray and neutron signals situaneously IS
attractive to inspectors.
The glove-twx installation of the INVS is being used
frequently. It has the advantage of reducing the %ample
handling by personnel and reducing the waste generated by
the hag out pr'cedure. The increased interest in nmmum g
mixed waste should help direct more at mention t) the INVIS
MIxel IV as a potential emeasureincnt tecimilque.
/l/ "Inventory Sample ('oincidence outertr Mainual." 1I. C).
Menlove, C). R. Ilolhinoks. A. Ramialho, I.ANI. repoi I .%
'5.14 M O CS P ) I HI), (Novetiber 1982).
/21 "l)eadtitnie Reduction tt Thernial Neutron (' ititieice
'otitiers," J. F. Swan , I ANt. repiot I .A 4 tfi MS
(March 191)
/1/ "The liiproved Ilnventory Sample (Coune !NVS Mt.dl
I1 ," M ('. Miller. 11 C). %lenlove. A Atbel ltiliti. I
ltasmn, A. Kestleman, L.ANI, iCport I A 1.111.' (IS1C)
124). (May 1l1')11.
/41/ "MCNIP A (iencial Monte ('ulo t 'ode tor Nruin and
l'iholon lranspott." I.ANI irpit I A I C'It Rev '
/ti/ "I'roiiiiaic I e-Is of iec I lgh Aucmus Cnhnmrd
Neutron/( tutln a Ihictuor (C )SI. C'o uicr )," II ) Nir lin '.
IR Weihn, M C)ugiri, I- Sena. II Ci Wagnel, I illien.
K Naver. to it pImsente'I at I N'h U SAIR O)A SmolSuu on
Saleguatis ,uid Niiulear Matetial Managlmgenl. Rlumet, Mis
/(V "An Itvaluattin of the INVS iodel IV Ni ii'r
tuountci," J K Spimnkl Ii II) Menloveri. Mi' Nlr.l
P A RA s-,u, I.ANI irpoit I A 1.'.1( MS I lan.o v 111 t)
/// "tI' Awilysus of Nrtuuin C'uui ulure I to %'ritit
1111t SpSa5 lCittut I Lsion .mi t -ISStultblr Isololir." 1 1
Mcnlovr. R Abeim /.adch. R /lhl, I ,\NI whim0 I \
1111111 %IS ( Alil',I 119'))
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Sprinkle Jr., J. K.; Menlove, H. O.; Miller, M. C.; Russo, P. A. & Jorgensen, C. A neutron counter for small samples, article, January 1, 1993; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc1209053/m1/4/: accessed May 8, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.