Primary versus secondary gamma intensities in 171Yb(n_th,gamma)

0.1 MB·English

by  A. Schiller

Primary versus secondary γ intensities in 171Yb(nth,γ)
A. Schiller,1,2,∗ A. Voinov,3,4 E. Algin,2,5,6,7 L.A. Bernstein,2
P.E. Garrett,2,8 M. Guttormsen,9 R.O. Nelson,10 J. Rekstad,9 and S. Siem9
1National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan 48824
2Lawrence Livermore National Laboratory, L-414, 7000 East Avenue, Livermore, California 94551
3Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701
4Frank Laboratory of Neutron Physics, Joint Institute of Nuclear Research, 141980 Dubna, Moscow region, Russia
5North Carolina State University, Raleigh, North Carolina 27695
6Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708
7Department of Physics, Eskisehir Osmangazi University, Meselik, Eskisehir, 26480 Turkey
8Department of Physics, University of Guelph, Guelph, Ontario N1G 2W1 Canada
9Department of Physics, University of Oslo, N-0316 Oslo, Norway
10Los Alamos National Laboratory, MS H855, Bikini Atoll Rd., Los Alamos, New Mexico 87545
6
0 Thetwopublishedliteraturevalues[Greenwoodetal.,Nucl.Phys.A252,260(1975)andGelletly
0 et al., J. Phys. G 11, 1055 (1985)] for absolute primary γ intensities following thermal neutron
2 capture of 171Yb differ in average by a factor of three. We have resolved this conflict in favor of
Greenwood et al. bya measurement of primary versus secondary intensities.
n
a
PACSnumbers: 23.20.Lv,25.20.Lj,25.40.Lw,27.70.+q
J

0
1 Primary, high-energetic (>∼ 3.6 MeV) and secondary, The experiment was divided into two runs. During the
low-energetic, (<∼ 2.4 MeV) γ-ray intensities from the firstrun,thetargetampulehungroughlydiagonally,hav-
1
171Yb(n ,γ)reactionhavebeenpublishedbytwogroups ing a ∼45◦ angle with the vertical and a 90◦ angle with
v th
5 inthepast[1,2]. Whiletheyagreeonabsolutesecondary thebeam. Duringthesecondrun,thetargetampulewas
1 intensitiesandrelativeprimaryintensities,valuesforab- mounted parallel to the beam. Two efficiency calibra-
0 solute primary intensities differ in averageby a factor of tions, running ∼ 12 h each, were performed with cor-
1 ∼ 3. The potential problem with [1] is that the authors responding orientations of ampules containing ∼ 1g of
0 of this study only measured relative primary intensities; NaCl and ∼1g of dried AlCl , respectively.
3
6
0 these relative intensities were then brought to an abso- The main purpose of the experiment was to measure
/ lute scale by normalizing to a previous measurement on γ-γ coincidences, however, downscaled (by a factor of
x absolute primary intensities [3, 4]. The potential prob- 100) singles γ rays were also recorded. More details of
e
- lem with [2] is that primary and secondary γ rays were the main experiment are given in [7, 8]. While the ex-
l measured with two distinct detector systems, hence, a perimentwasnotaimedatmeasuringabsoluteintensities
c
u systematic error in the absolute efficiency calibration of anditlackedthesensitivitytodeterminerelativeprimary
n the high-energy γ detector could have resulted in wrong or secondary intensities better than in [1, 2], the consis-
: primary intensities. The ENSDF compilation [5] gives a tentefficiencycalibrationofourdetectorsoverarangeof
v
i slight preference for the primary intensities of [2], how- 8 MeV gave us the opportunity to investigate the main
X ever, it also mentions that the large disagreement be- point of disagreement between [1] and [2] by measuring
r tween the two groups is not understood. primary versus secondary intensities. For this reason,
a
Wehaveperformedmeasurementsonthe171Yb(n ,γ) we have divided summed peak areas from all three de-
th
tectors for individual primary and secondary γ rays by
reaction at the Los Alamos Neutron Science Center
(LANSCE) with two large-volume (∼ 80%) Ge(HP) de- the summed relative detector efficiencies and relative in-
tectorsplacedatbeamheightatanangleof∼110◦ with tensities from [5] to produce divided yields (see Fig. 1).
the beam, and one actively shielded ∼ 200% clover de- Divided yields are then averaged for all primary and all
secondary γ rays to produce average divided yields.
tector placed vertically over the target. All three de-
tectors were at a distance of ∼ 12 cm from the target. The ratio of averagedivided yields between secondary
The target consisted of ∼ 1 g of enriched (>∼ 95%), dry and primary γ rays is 17.5(7) and 18.4(7) for the two
171Yb O powder encapsulated in a glass ampule which runs,respectively,or17.9(5)combined. Thisratiohasto
2 3
was mounted in an evacuated beam tube and irradi- be compared to the ratio of normalization factors given
ated by collimated neutrons with a time-averaged flux by[1,2]andquotedin[5]tobringtherelativesecondary
of ∼ 4×104 n/cm2s over ∼ 150 h. The relative detec- andprimaryintensitiestoanabsolutescale. Theseratios
tor efficiencies from 1–9 MeV were determined by the are 0.063/0.00107 ≈ 58.9±50% systematic uncertainty
35Cl(n,γ)36Cl reaction and its known γ intensities [6]. for[2]and0.054/0.00287≈18.8(4)for[1]. Forthelatter,
theerrorwascalculatedassumingthatthequotedfactors
have uncertainties in the order of their least significant
digits. Hence, we can conclude that [1] gives correctval-
∗Electronicaddress: [email protected] ues of primary intensities (or rather a correct conversion
2
FIG.1: Dividedyieldsofsecondary(leftpanels)andprimary(rightpanels)γrays. Thelinesshowaveragevaluesofthedivided
yields(solid lines) andtheirerrorbands(dashedlines). Upperandlowerpanelscorrespondtodiagonalandparallel mountsof
thetarget ampules, respectively, and different chlorine salts used for theefficiency calibration (see text).
factortobringrelativeprimaryintensitiestoanabsolute oratory. This facility is funded by the U.S. Depart-
scale), while [2] does not. To be more precise, we have ment of Energy under Contract W-7405-ENG-36. Part
only shown that [1] has the correct ratio of absolute pri- of this work was performed under the auspices of the
mary versus secondary intensities; with our experiment U.S. Department of Energy by the University of Cal-
wecannotruleoutthepossibilitythatbothprimaryand ifornia, Lawrence Livermore National Laboratory un-
secondary intensities from [1] might be off by the same derContractW-7405-ENG-48,andLosAlamosNational
factor. However,sincetheredoesnotseemtobeastrong Laboratory under Contract W-7405-ENG-36. Financial
disagreementbetweendifferentmeasurementsofabsolute support from the Norwegian Research Council (NFR)
secondary intensities, we do not expect this to be a seri- is gratefully acknowledged. A.V. acknowledges support
ous problem. The 1.5σ disagreement between our work from a NATO Science Fellowship under project number
and [1] (provided the error estimate for the latter is cor- 150027/432and from the National Nuclear Security Ad-
rect) might be attributed to systematic uncertainties in ministration under the Stewardship Science Academic
our efficiency calibration due to the parameterization of Alliances program through U.S. Department of Energy
the efficiency curve, absorption in the target, and other Research Grant No. DE-FG03-03-NA00074. E.A. ac-
minor corrections which were all neglected here. knowledgessupportbyU.S.DepartmentofEnergyGrant
ThisworkhasbenefitedfromtheuseoftheLosAlamos No. DE-FG02-97-ER41042. We thank Gail F. Eaton for
NeutronScienceCenterattheLosAlamosNationalLab- making the targets.
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