Establishment of M1 multipolarity of a 6.5 mu_N^2 resonance in 172-Yb at E_gamma=3.3 MeV

0.21 MB·English

by  A. Schiller

Establishment of M1 multipolarity of a 6.5 µ2 resonance in 172Yb at E = 3.3 MeV
N γ
A. Schiller,1,∗ A. Voinov,2 E. Algin,1,3,4,5 J.A. Becker,1 L.A. Bernstein,1
P.E. Garrett,1 M. Guttormsen,6 R.O. Nelson,7 J. Rekstad,6 and S. Siem6
1Lawrence Livermore National Laboratory, L-414, 7000 East Avenue, Livermore, California 94551
2Frank Laboratory of Neutron Physics, Joint Institute of Nuclear Research, 141980 Dubna, Moscow region, Russia
3North Carolina State University, Raleigh, North Carolina 27695
4Triangle Universities Nuclear Laboratory, Durham, North Carolina 27708
5Department of Physics, Osmangazi University, Meselik, Eskisehir, 26480 Turkey
6Department of Physics, University of Oslo, N-0316 Oslo, Norway
7Los Alamos National Laboratory, MS H855, Bikini Atoll Road, Los Alamos, New Mexico 87545
Two-step-cascade spectrain 172Ybhavebeenmeasured after thermalneutroncapture. They are
compared to calculations based on experimental values of the level density and radiative strength
4 function(RSF)obtained from the173Yb(3He,αγ)172Ybreaction. Themultipolarity ofa6.5(15) µ2N
0 resonance at Eγ =3.3(1) MeV in theRSFis determined tobe M1 by this comparison.
0
2 PACSnumbers: 25.40.Lw,25.20.Lj,24.30.Gd,27.70.+q

n
a
J Excited nuclei decay often by a cascade of γ rays. sity ρ of intermediate states with spin and parity Jmπ
While the decay between discrete states is determined
7
2 by the details of the nuclear wavefunctions, unresolved I (E ,E )= ΓXimL(E1)ρ(E ,Jπ)ΓXmLf′(E2)
1 atraconnsittiinounosuasrreabdeiasttivdeessctrriebnegdthbyfusntcattiiostnic(aRlScFon)caenpdtslelivkeel if 1 2 XL,XXL′,Jmπ Γi m m Γm
038v dvEaeγln.useiFtoyor.fhtTahrhededγRecrSaaFyysp,(rr(eo∼vbia7ewb–2iel0idtyMinefoV[r1)],a)thgpiervoRevnSidFγe-sirsatgyhoevenemreneragendy +XL,XXL′,Jmπ′ ΓXimLΓ′(iE2)ρ(Em′,Jmπ′)ΓmXΓ′Lfm′(E′ 1). (1)
1 by the giant electric dipole resonance whose parameters
0 aredeterminedfromphotoabsorption[2]. Thesofttailof The sums in Eq. (1) are restricted to give valid combi-
4 nations of the level spins and parities and the transition
the RSF has been investigated by a variety of methods,
0 multipolaritiesXL. Theyarisesinceonedeterminesnei-
mostnotablybyprimaryγ rays[3]. Recently,systematic
x/ studies of the soft RSF have been performedat the Oslo ther the ordering of the two γ rays, nor the multipolar-
e ities of the transitions nor the spins and parities of the
Cyclotron Laboratory using a method based on sequen-
- intermediate levels, hence one has to include all possi-
l tialextraction. With this method itis possible to obtain
c bilities. The two transition energies are correlated by
u the level density and RSF by a deconvolution of a set
E +E =E −E , thus, TSC spectra can be expressed
n of primary γ spectra from a range of excitation energies 1 2 i f
: [4]. TotalRSFs (summedoverallmultipolarities)ofrare asspectraofonetransitionenergyEγ only. TSCspectra
v are symmetric around Esym = (E −E )/2; integration
i earth nuclei can be extracted for Bn > Eγ > 1 MeV γ i f
X over E yields twice the total TSC intensity I if both
[5]. Their common, most striking feature is a resonance γ if
r at E ∼ 3 MeV which is believed to be of dipole na- γ rays are counted in the spectra. The knowledge of the
a γ parities π [11] and π ensures that I depends roughly
ture but whose electromagnetic character is unknown. i f if
speaking on the product of two RSFs around Esym [8],
It has been shown for all investigated rare earth nuclei γ
i.e., f2 +f2 for π = π and 2f f for π 6= π .
that the total RSF is most readily decomposed into a E1 M1 i f E1 M1 i f
I depends also on the level density. This usually pre-
sum of the Kadmenski˘ı-Markushev-Furman (KMF) E1 if
vents drawing firm conclusions from TSC experiments
model [6], a spin-flip M1 model [7], and the aforemen-
alone [10]. A combined analysis of Oslo-type and TSC
tioned soft dipole resonance [5]. The knowledge of the
experiments, however, enables one to establish the sum
character of this resonance is essential for its theoreti-
and product,respectively,ofallcontributionstof and
cal interpretation. Experimentally, it can be determined M1
f at energies of the soft resonance, thus determining
from a two-step-cascade (TSC) measurement [8]. E1
its character. For this goal, the partial widths of Eq. (1)
TheTSCmethodisbasedontheobservationofdecays
are expressed via
fromaninitialstatei to afinalstate f via one,andonly
one, intermediate level m [9, 10]. A convenient initial ΓXL (E )=f (E )E2L+1D (2)
x→y γ XL γ γ x
stateisthatformedinthermaloraverageresonancecap-
ture (ARC); the final state can be any low-lying discrete in terms of RSFs and level spacings D . Eq. (2) actu-
x
state. TSC spectra are determined by the branching ra- ally gives only the average value of the Porter-Thomas
tios of the initial and intermediate states (expressed as distributed partial widths [12]. The total width Γ is the
ratios of partial to total widths Γ) and by the level den- sumofallpartialwidths. Again,thesumisonlythesum
2
FIG.1: Leftpanel: totalleveldensity(filledcircles),constant-
temperature extrapolation (solid line), level density at Bn
fromaverageneutronresonancespacing(filledsquare)[7],and
leveldensityfromcountingofdiscretelevels(jaggedline)[19].
Right panel: total RSF (filled circles), fit to the data, and
decomposition into RSFs of different multipolarities (solid
lines). Inclusion of the soft resonance in the fit decreases
χ2 from ∼ 7.4 to ∼ 1.3. Since this value is close to unity,
red
inclusion of additional non-statistical structures cannot sig-
nificantly improvethe fit.
FIG. 2: Upper panel: energy-summed coincidence spectrum
of meanvalues, however,the distribution of totalwidths
from the 171Yb(n,γγ)172Yb reaction. Peaks are labeled by
with many components is almost δ-like [12]. The level theenergyofthefinalstate. Peaksdenotedby71Geand29Si
density fora givenspinandparityis calculatedfromthe aredueton-captureinthedetectorandintheglassampoule,
total level density by [13] respectively. SE and DE stands for single and double escape
peaks, respectively. Lower panel: TSC spectra to the 2+
ρ(E ,Jπ)=ρ(E )1 2Jx+1exp −(Jx+1/2)2 , (3) state. The slight asymmetry is due to the energy-dependen1t
x x x 2 2σ2 (cid:20) 2σ2 (cid:21) resolution of thedetectors.
where σ is the spin cut-off parameter, and we assume
equalnumbersofpositiveandnegativeparitylevels. This KMF E1 model, a spin-flip M1 model, and a soft dipole
assumption and Eq. (3) have been verified from the dis- resonance [5]. Here, we have improved on the normal-
crete level schemes of rare earth nuclei [14]. Thus, all izationof the level density and the RSF and included an
quantities for calculating TSC spectra are based on ex- isoscalar Lorentzian E2 model [18] giving
perimental data.
The combinedanalysisis applied to the nucleus 172Yb f =K(f +f )+E2f +f (4)
tot E1 M1 γ E2 soft,
which has been investigatedby the 173Yb(3He,αγ)172Yb
reaction in Oslo and by the 171Yb(n,γγ)172Yb reaction where K is a scaling factor of the order of one. Since
at the Lujan Center of the Los Alamos Neutron Science quadrupole transitions populate levels within a broader
Center(LANSCE). TheOslodatahavebeenreportedin spin interval than dipole transitions, Eq. (4) is of an ap-
[4, 5]. Thus, only a short summary is given. The ex- proximative nature. Given the weakness of quadrupole
periment was performed using a 45-MeV 3He beam on a transitions and the level of experimental uncertainties,
metallic, enriched, self-supporting target. Ejectiles were however, this approximation is believed to be sufficient.
identifiedandtheirenergiesmeasuredusingparticletele- The improved data, the fit to the total RSF, and its
scopesat45◦. Incoincidencewithαparticles,γrayswere decomposition into different multipolarities are given in
detected in an array of 28 NaI detectors. From the re- Fig. 1. The parameters for the E1 RSF are taken from
action kinematics, α energy is converted into E , and γ [5], those for the M1 and E2 RSFs from [7], where we
x
cascade spectra are constructed for a range of Ex bins. use the fE1/fM1 systematics at ∼ 7 MeV giving val-
Theγspectraareunfolded[15]andtheprimaryγspectra ues in agreement with ARC work [20]. The fit parame-
areextractedusingasubtractionmethod[16]. Thespec- ters are: the constant temperature of the KMF model
traaredeconvolutedintoaleveldensityandatotalRSF T = 0.34(3) MeV, the normalization coefficient K =
by applying the Brink-Axel hypothesis [17]. The level 1.7(1), and the three parameters of the soft resonance
density is normalizedby comparisonto discrete levels at E = 3.3(1) MeV, Γ = 1.2(3) MeV, and σ = 0.49(5) mb
low E and to the average neutron resonance spacing at [21].
x
B [4]. The RSF is normalized using the average total For the 171Yb(n,γγ)172Yb experiment, we used ∼1 g
n
widthofneutronresonances,andis decomposedintothe of enriched, dry Yb O powder encapsulated in a glass
2 3
3
ampoule, mounted in an evacuated beam tube and irra-
diated by collimated neutrons with a time-averagedflux
of ∼ 4×104 neutrons/cm2s at ∼ 20 m from the ther-
mal moderator. γ rays were detected by two 80% and
one shielded and segmented ∼ 200% clover Ge(HP) de-
tector, placed at ∼12 cm from the target in a geometry
tominimize angularcorrelationeffectsandcontributions
from higher multiplicity cascades. Single and coincident
γ rays were recorded simultaneously. The experiment
ran for ∼ 150 h yielding ∼ 107 coincidences. The rela-
tive detector efficiencies from 1–9 MeV were determined
bytwoseparaterunsof∼12heach,beforeandafterthe
171Yb(n,γγ)172Yb experiment, using the 35Cl(n,γ)36Cl
reactionanditsknownγintensities[22]. Also,astandard
calibrated 60Co source has been measured to adjust the
relativecurvestoanabsolutescale. The energy-summed
coincidencespectrum(Fig.2,upperpanel)showsdistinct
peaks corresponding to TSCs between B and several
n
low-lying states. The two strongest peaks have ∼ 4000
counts each. TSC spectra (lower panel) were obtained
by gating on four peaks. Relative intensities of primary
versus secondary γ rays were determined from singles
FIG.3: Left: experimentalvalues(hatchedareas)forTSCin-
spectra and are in agreement with Ref. [20]. Absolute tensitiestofinalstates(fromtoptobottom)2− at1198keV,
1
primary intensities were determined by using new data 1− at 1155 keV,2+ at 79keV,andthe0+ groundstatecom-
1 1 1
on absolute secondary γ-ray intensities [23] and subse- pared to calculations as function of R. Included are statis-
quent scaling of primaryintensities to these values using tical errors and systematical errors from normalization and
detection efficiency, the latter two being correlated for all fi-
therelativeintensitiesof[20]. Theseabsoluteprimaryin-
nal levels. Solid and dashed lines correspond to M1 and E1
tensities are ∼ 20% higher than in [20]. TSC intensities
hypotheses for the soft resonance. Right: combined χ2 for
red
are normalized to (i) the absolute primary intensity and
all four TSC intensities as function of R for the M1 and E1
secondarybranchingratioofone,strong,individualTSC hypotheses (upper panel). Experimental (filled circles) and
and (ii) by effectively estimating the number of neutron calculated TSC spectra to the 2+ state (middle panel) and
1
captures during the experiment from secondary singles 0+ state (lower panel) for theM1 hypothesisat R=0.4 and
1
lines, their absolute intensities, andabsolute detector ef- the E1 hypothesis at R=0.8. At ∼2 MeV, Porter-Thomas
fluctuationsin theexperimental spectra become visible.
ficiencies. Both methods give equal results within the
error bars.
TSC spectra are compared to calculations according
to Eq.(1) assumingeither electric or magneticcharacter of ratios R = σth (0−)/σth . These calculations show,
n,γ n,γ
for the soft resonance [8]. Due to Porter-Thomasfluctu- however, that only the TSC intensity to the 0+ state
1
ationsofTSCintensities,TSCspectraarecompressedto has a strong dependence on this ratio. Total experimen-
∼300keVenergybinsandonlya∼2.4MeVbroadenergy tal and calculated TSC intensities are shown in the left
intervalinthemiddleofthespectraistakenintoaccount panels of Fig. 3. The calculations assuming E1 for the
[10]forcomparison. Correctionsduetonon-isotropican- soft resonance do not reproduce the experimental inten-
gular correlations of TSCs have been estimated to be sities consistently for any value of R. Good agreement
less than ∼3% andare thus neglected. Contributions to is achieved assuming M1 with the additional condition
the thermal radiative neutron capture cross section σth of R ∼ 0.4 for the 0+ final state. However, it has to be
n,γ 1
− −
fromthe twopossiblespins (0 and1 )involvedinneu- emphasized that the conclusion of an M1 multipolarity
tron s-capture on 171Yb are uncertain. The compilation for the soft resonance can be established from the TSC
[24] assumes 0− for the sub-threshold resonances which intensities to the 2+, 1−, and 2− states independently,
1 1 1
contribute 88% to σth . Another 4% comes from 0− res- irrespectiveofthevalue ofR. Possiblesystematicuncer-
n,γ
onances above threshold, giving in total a 92%contribu- taintiesintheabsolutenormalizationcannotchangethis
tion of 0− states. On the other hand, there is no strong conclusion, since in the case of the final state 2+, one
1
evidence that all contributing sub-threshold resonances wouldneedadecrease whileatthesametime,forthe 1−
1
have 0−. Examination of hard primary γ-rays [20, 25] finalstateonewouldneedanincrease intheexperimental
reveals many strong transitions populating 2+ levels, in- TSCintensitiesinordertoaccommodatetheE1hypoth-
dicatingthatasizeableportionofσth stemsfrom1−res- esis. The combined χ2 for all four TSC intensities as
n,γ red
onances. Therefore, we performed calculations for a set function of R is also given. The M1 hypothesis yields
4
the global minimum for R = 0.4±0.25 with χ2 = 0.1 150027/432. E.A.acknowledgessupportbyU.S.Depart-
red
whereas the minimal χ2 for the E1 hypothesis is ∼2.7 ment of Energy Grant No. DE-FG02-97-ER41042. We
red
for R ∼ 0.8. Finally, we show the TSC spectra to two thank Gail F. Eaton for making the targets.
final states compared to calculations using the M1 hy-
pothesis at R = 0.4 and the E1 hypothesis at R = 0.8.
No further conclusions have been drawn from this com-
parison, however.
∗
Electronic address: [email protected]
The integrated strength of the soft resonance is ex-
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N
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N
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N [19] R. Firestone and V.S. Shirley, Table of Isotopes, 8th ed.
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N
centroids of the resonances increase weakly with mass Phys. A252, 260 (1975).
number. [21] Thecitedparametersaremeanvaluesobtainedfromthe
173Yb(3He,αγ)172Yb and 172Yb(3He,3He′γ)172Yb reac-
ThisworkhasbenefitedfromtheuseoftheLosAlamos
tion data.
NeutronScienceCenterattheLosAlamosNationalLab-
[22] C. Coceva, A. Brusegan, and C. van der Vorst, Nucl.
oratory. This facility is funded by the U.S. Depart- Instrum. Methods Phys.Res. A 378, 511 (1996).
ment of Energy under Contract W-7405-ENG-36. Part [23] R. Firestone, privatecommunication.
of this work was performed under the auspices of the [24] S.F. Mughabghab Neutron Cross Sections, (Academic
U.S. Department of Energy by the University of Cal- Press, New York,1984), Vol. I,part B.
ifornia, Lawrence Livermore National Laboratory un- [25] W. Gelletly et al., J. Phys.G 11, 1055 (1985).
[26] E.LippariniandS.Stringari,Phys.Rep.175,103(1989).
derContractW-7405-ENG-48,andLosAlamosNational
[27] A. Zilges et al., Nucl. Phys. A507, 399 (1990); A519,
Laboratory under Contract W-7405-ENG-36. Financial
848 (1990).
support from the Norwegian Research Council (NFR)
[28] A. Schiller et al., preprint nucl-ex/0011018.
is gratefully acknowledged. A.V. acknowledges support [29] S. Siem et al., Phys. Rev.C 65, 044318 (2002).
from a NATO Science Fellowship under project number