Radiative strength function in 96Mo reanalyzed

0.09 MB·English

by  M. Guttormsen

Radiativestrength function in96Mo reanalyzed
M.Guttormsen,1∗ R.Chankova,2U.Agvaanluvsan,2E.Algin,5 L.A.Bernstein,2 F.Ingebretsen,1T.Lo¨nnroth,6
S.Messelt,1 G.E.Mitchell,3,4 J.Rekstad,1 A.Schiller,7 S.Siem,1 A.C.Larsen,1 A.Voinov,7,8andS.Ødega˚rd1
1 Department of Physics, University of Oslo, N-0316 Oslo, Norway
2 LawrenceLivermoreNational Laboratory, L-414, 7000 EastAvenue, Livermore, CA94551, USA
3 North Carolina State University, Raleigh, NC 27695, USA
4 Triangle Universities Nuclear Laboratory, Durham, NC 27708, USA
5 Department of Physics, Osmangazi University, Meselik, Eskisehir, 26480 Turkey
6 Department of Physics, A˚bo Akademi, FIN-20500 Turku, Finland
7 Department of Physics and Astronomy, Ohio University, Athens, Ohio45701, USA and
8 FrankLaboratoryofNeutronPhysics,JointInstituteofNuclearResearch,141980Dubna,Moscowregion,Russia
8 Theradiativestrengthfunctionsof96Mohavebeenreanalyzed.Theenhancedg strengthforEg <3−4MeV
0 isconfirmed.
0
2 PACSnumbers:PACSnumber(s):24.30.Gd,24.10.Pa,25.55.Hp,27.60.+j

n
a INTRODUCTION we estimated the statistical multiplicity at excitation energy
J
E byintroducinga lowerg -raythresholdE0 andaneffective
0
3 The Oslo grouphas reportedunexpectedenhancementsin excitationenergyE−Eentrygiving
the radiative strength functions (RSF) of low energy g -rays
] for the 44,45Sc, 50,51V, 56,57Fe and 93−98Mo nuclei [1, 2, 3, hMgstati=(E−Eentry)/hEgi>E0, (1)
x
-e 4m]e.thTohde[s5e],rehsauvletsg,awinheidchbrhoaavdeibneteernesotbintaitnheednuwcilteharthpehyOssiclos where hEgi>E0 is the average energy of the g spectrum for
l Eg >E0. TheEentryparametermimicstheexcitationenergyat
c community. The96Monucleushasbecomea benchmarkfor whichthestatisticalg transitionsenterthegroundband. This
u
otherexperimentalgroupstryingtoverifyorfalsifytheOslo
treatmentisapplicabletorareearthnuclei,wherethedetector
n
results. Unfortunately,96Mowas notanalyzedin an optimal
[ systemefficiencyforthelowestgroundstatebandtransitions,
way, and we have therefore decided to reanalyze these data typically the 4+ →2+ and the 2+ →0+ transitions, is low.
1 sets. However,for96Motheenergyofthelowestgroundbandtran-
v
7 sitionsare detectedeasily. Therefore,in the presentanalysis
6 weusethestraightforwardexpressionforthetotalg -raymul-
6 REANALYZEDDATAANDRESULTS tiplicity
4
1. The two data sets were recorded from the hMgtoti=E/hEgi, (2)
0 96Mo(3He,3He′g )96Mo and 97Mo(3He,ag )96Mo reac-
8 wherewesimplydividetheexcitationenergybytheaverage
tions. In this work we use the same key parameters for the
0 energyoftheg spectrum.
normalizations as previously [4, 6], namely a level density
:
v of r (B ) = 71800 MeV−1 and an average total radiative Both the level density and the radiative strength function
n
Xi widthofhG (B )i=150meV(froms-waveresonances)atthe will be slightly modifiedby the new multiplicity expression.
n
Figures 1 and 2 compare the new level densities and radia-
neutronbindingenergyB . Experimentaldetails andfurther
r n
tive strength functions with previous data [4, 6]. We see a
a referencesarefoundinRefs.[4,5,6].
verygoodresemblancebetweenthepick-upreactionandthe
Thenewanalyzesconcerntwomajorpoints. Intheoldex-
inelastic scattering reaction. The error bars include statisti-
tractionofRSF,weincludedtheg -rayenergiescloseorbelow
cal errors only. The new level densities are very similar to
the strong 778 keV 2+ →0+ ground band transition. This
thepreviousones. ThesameistruefortheRSFs,exceptthat
transitional region in the experimental (Eg,E) matrix is not
theupbendislesspronouncedduetotheexclusionofthe778
properly subtracted in the first-generation procedure [7] and
keVtransition. Thevaluesofthedatapointscanbefoundat
isnowexcluded.
http://ocl.uio.no/compilation/.
The second point concerns the estimate of the g -ray mul-
There have been some misunderstandings concerning the
tiplicity as functionof excitation energy, which is an impor-
descriptionoftheRSFupbendforlowg energiesgivenby[7]
tant quantity both for normalizingthe g spectra with respect
toeachotherandweightingthehigher-ordergenerationspec- 1
trainthesubtractionprocedure[7]. Inthepreviousanalyses, fupbend= 3p 2h¯2c2AEg−b, (3)
whereA andbarefitparameters,andEg isgiveninMeV.The
formulawaschoseninordertofitthelow-energydatabyonly
∗Electronicaddress:[email protected] twoparameters. We wouldliketostressthatthisdescription
2
No. DE-FG03-03-NA00074, No. DE-FG03-03-NA00076
and U.S. Department of Energy Grant No. DE-FG02-97-
ER41042.
[1] A. C. Larsen, M. Guttormsen, R. Chankova, T. Lo¨nnroth,
S. Messelt, F. Ingebretsen, J. Rekstad, A. Schiller, S. Siem,
N.U.H.Syed,andA.Voinov,Phys.Rev.C76,044303(2007).
[2] A. C. Larsen, R. Chankova, M. Guttormsen, F. Ingebretsen,
T. Lo¨nnroth, S. Messelt, J. Rekstad, A. Schiller, S. Siem,
N.U.H.Syed,A.Voinov,andS.W.Ødega˚rd,Phys.Rev.C73,
064301(2006).
[3] A.Voinov,E.Algin,U.Agvaanluvsan,T.Belgya,R.Chankova,
M. Guttormsen, G.E. Mitchell, J. Rekstad, A. Schiller and
S.Siem,Phys.Rev.Lett93,142504(2004).
[4] M.Guttormsen,etal.,Phys.Rev.C71,044307(2005).
[5] A.Schiller,L.Bergholt, M.Guttormsen, E.Melby, J.Rekstad,
and S. Siem, Nucl. Instrum. Methods Phys. Res. A 447, 498
(2000).
[6] R.Chankova,etal.,Phys.Rev.C73,034311(2006).
[7] M.Guttormsen,T.Ramsøy,andJ.Rekstad,Nucl.Instrum.Meth-
odsA255,518(1987).
FIG.1:Experimentalleveldensitiesfromthe(3He,a )(filledcircles)
andthe(3He,3He’)(open circles)reaction. Thedatafromthenew
analysisiscomparedwithpreviouslypublisheddata[6].
should notbe used for g energieslower than the experimen-
tal data points. In the extremecase whenEg →0, it is clear
that the description is totally unrealistic as it gives wrong g
multiplicity.
SUMMARYANDCONCLUSIONS
The radiative strength function of 96Mo has been reana-
lyzedgivingaslightlylesspronouncedenhancementatlower
g -ray energies. The data points at and below the 778 keV
2+ →0+ transition have been omitted. Since extraction of
leveldensityiscoupledtotheradiativestrengthfunction,new
leveldensitieshavealsobeenpresented.
Financial support from the Norwegian Research Council
(NFR) is gratefully acknowledged. Part of this work was
performedundertheauspicesofthe U.S.DepartmentofEn-
ergybytheUniversityofCalifornia,LawrenceLivermoreNa-
tionalLaboratoryunderContractW-7405-ENG-48.A.V.E.A, FIG.2: Experimentalradiativestrengthfunctionsfromthe(3He,a )
U.A,andG.E.MacknowledgesupportfromtheNationalNu- (filled circles) and the (3He,3He’) (open circles) reaction. The
clearSecurityAdministrationundertheStewardshipScience data from the new analysis is compared with previously published
Academic AlliancesprogramthroughDOE Research Grants data[7].