Two new superdeformed (SD) bands have been observed in Tb150 using the reaction P31+124Sn and the GAMMASPHERE detector array. These new bands are interpreted as being excited SD configurations involving a proton and a neutron particle-hole excitation, respectively. The configurations are discussed in terms of the occupancy of high-N intruder orbitals. These data are used to provide information on (i) the contribution to the dynamic moments of inertia due to the occupation of specific orbitals, (ii) the deformation driving effects of the particle-hole states, and (iii) the consequences of these effects on pairing and band-crossings

Asztalos, S.

Beausang, C.W.

Bergstrom, M.

Carpenter, M.

Cederwall, B.

Clarke, S.

Crowell, B.

Deleplanque, M.A.

Diamond, R.M.

Fallon, P.

Henry, R.G.

Janssens, R.V.F.

Khoo, T.L.

Lauritsen, T.

Lee, I.Y.

Macchiavelli, A.O.

Nisius, D.

Stephens, F.S.

Twin, P.J.

Carolina Digital Repository

PHYSICAL REVIEW C VOLUME 52, NUMBER 1 JULY 1995Proton and neutron ewcitations in superdeformed 150TbP. Fallon, C. W. Beausang, S. Asztalos, D. Nisius, R. V. F. Janssens M. Bergstrom M. CarpenterB. Cederwall, S. Clarke, B. Crowell, M. A. Deleplanque, R. M. Diamond, R. G. Henry, T. L. Khoo,T. Lauritsen, I. Y. Lee, A. O. Macchiavelli, F. S. Stephens, and P. 3. TwinNuclear Science Division, Lawrence Berkeley Laboratory, Berkeley, California g$720Oliver Lodge Laboratory, University of Liverpool, L69$BX, United KingdomArgonne National Laboratory, Argonne, Illinois 60/99(Received 15 March 1995)Two new superdeformed (SD) bands have been observed in Tb using the reaction P + Snand the GAMMASPHERE detector array. These new bands are interpreted as being excited SDconfigurations involving a proton and a neutron particle-hole excitation, respectively. The configu-rations are discussed in terms of the occupancy of high-N intruder orbitals. These data are used toprovide information on (i) the contribution to the dynamic moments of inertia due to the occupa-tion of specific orbitals, (ii) the deformation driving egects of the particle-hole states, and (iii) theconsequences of these efFects on pairing and band crossings.PACS number(s): 21.10.Re, 23.20.1 v, 27.70.+qI. INTRODUCTIONThe spectroscopy of superdeformed (SD) nuclei (P2 =0.6) in the A=150 region began with the observation ofa SD band in 2Dy [1]. There are now over 40 SD bandsreported in this region and in most cases their propertiescan be understood in terms of single-particle states in theabsence of pair correlations. Exceptions to this are Gdband 1 [2], 4 Gd band 2 [3,4], Gd band 1 [5], andGd band 5 [6], where the observation at low frequencyof either (i) a rapid rise in the dynamic moment of inertia(9(2)), or (ii) a backbend in the p-ray SD band transitionenergies, has been interpreted as a paired quasineutronor quasiproton band crossing. In addition, more recentdata on excited SD bands in is2Dy [7] have been reportedwhich the authors claim provide evidence for octupole(shape vibration) correlations.In order to gain a better understanding of the role ofcorrelations in A=150 SD bands (e.g. , due to pair cor-relations or shape vibrations), one must first gain a bet-ter understanding of the role of specific single-particlestates in determining the SD band properties. In partic-ular the role of the high-N intruder orbitals has receiveda great deal of attention. At near spherical deforma-tions (P2 = 0) intruder orbitals are those which, due tothe spin-orbit interaction, are lowered in energy &om theN+ 1 oscillator shell to the ¹hshell. At superdeforma-tions (P2 0.6) levels originating from the %+2 oscilla-tor shell appear close to the Fermi surface. The influenceof these high-N intruder orbitals was first discussed byBengtsson and co-workers [6] and later by Nazarewicz etal. [9]. It was found, for A=150 SD bands, that the 8(2)moments of inertia are sensitive to the occupation of spe-cific intruder orbitals. By comparing the calculated andexperimental G{ ~ values it has been possible to assign in-truder configurations to the SD bands and hence classifythese nuclei by the number of intruder orbitals occupied.It is also important to obtain information on the non-intruder orbitals close to the Fermi surface. This becomesespecially evident when one wishes to discuss the phe-nomenon of "identical" 2 SD bands [10—12]. For example,in the A=150 region, the occupation of the 7r[301]1/2,v[411]1/2, or v[514]9/2 orbital has been linked with theobservation of identical bands. An understanding of thesingle-particle properties of these and similar orbitals istherefore necessary.In this paper we report data from an experiment tostudy the excited SD structures in the N=85, Z=65 nu-cleus Tb. Two new SD bands are observed, whichwe interpret as excited SD bands in ~ Tb based on pro-ton (band 2) and neutron (band 3) excitations relativeto band 1 (the yrast SD band [13]).These data are usedto provide information on (i) the contribution to the 9(2)moments of inertia &om the alignment of speci6c orbitals,(ii) the deformation driving efFects of the particle-holestates, and (iii) the consequences of these efFects on pair-ing and band crossings, within the &amework of crankedWoods-Saxon calculations.II. EXPERIMENTHigh spin states in Tb were produced at theLawrence Berkeley Laboratory 88-inch cyclotron usingthe reaction P + Sn at a beam energy of 167 MeV.The labeling of SD bands as band 1, band 2, etc. refers tothe labeling adopted in the referenced works.In general, identical bands are those which have either in-teger or half-integer alignments relative to each other. Thealignment is the difFerence in the total angular momenta at agiven rotational frequency.0556-2813/95/52(1)/93{6)/$06. 00 52 93 1995 The American Physical Society94 P. FALLON et al. 52The target consisted of two isotopically enriched 600pg/cm2 self-supporting foils. The subsequent p-ray de-cays were studied using the GAMMASPHERE detec-tor array [14] which consisted at this time of 28 high-resolution, 80% eKcient, escape-suppressed Ge detec-tors. The event trigger condition required at least three(Ge fold & 3) escape-suppressed Ge detector signals toregister in coincidence. The total number of events withGe fold & 3 was 1 x 109 which yielded approximately3 x 10 expanded triple events. Data analysis was carriedout using both three-dimensional E~-E~-E~ correlationcubes and single-gated two-dimensional E~-E~ correla-tion matrices.N 0 roiIa Nv lOIAEO CO~vTve700 000 1100 1300 1500 1700Energy (keY)III. RESULTS AND DISCUSSIONIn total, three SD bands have been observed in thisdata set, one of which (band 1) has been reported previ-ously [13] and assigned as the yrast SD band in ~soTb.In this experiment two new bands were assigned to ~5oTbthrough observed coincidences with low-lying normal de-formed yrast Tb transitions. The transition multipo-larities in band 1 were shown [13] to be consistent withan E2 character. However, in this work we were not ableto determine the multipolarity of transitions in bands 2and 3 and the rotational nature of these bands, as well astheir SD character, was inferred Rom the regular transi-tion energy spacings and the intensity profiles. The tran-sition energies and relative inband intensities are listedin Table I for all three SD bands. Band 1 was foundto carry 1% of the total ~s Tb decay Hux. The twonew SD bands (bands 2 and 3) have intensities which areapproximately 25% and 10% relative to that of band 1,FIG. 1. Spectrum generated by a sum of E~-E~ coincidencegates on transitions in Tb SD band 1. A * denotes knownnormal deformed transitions in Tb.respectively, and therefore we make the assumption thatthese bands are built on excited SD configurations. Inthe following sections we will concentrate on the proper-ties and interpretations of the two new SD bands, anduse the fact that the dynamic moments of inertia (9&2))are sensitive to the occupation of specific high-N intruderorbitals [8,9] in order to identify the most likely configu-rations for the two new SD bands in Tb.A. Band 1The latest results for band 1 are presented for com-pleteness and the spectrum is shown in Fig. 1. Thisband has been extended over the previous work [13] byTABLE I. Table of transition energies (keV) and inband relative intensities for the three SDbands in Tb. For each band the intensities are normalized to the maximum intensity carried bythat band.E~596.8 (0.2)647.4 (0.1)697.7 (0.1)748.2 (0.1)799.2 (O.l)sso.s (o.1)902.1 (O.l)954.1 (0.1)1006.9 (0.1)1059.6 (0.1)1112.4 (0.1)1165.5 (0.1)1218.8 (0.1)1272.3 (0.1)1326.4 (0.2)1380.3 (0.2)1434.9 (0.2)1489.4 (0.3)1544.1 (0.7)1599.6 (1.0)Tb SD band 1Intensity0.26 (0.05)0.57 (0.03)1.o2 (o.o2)1.04 (0.02)1.02 (0.02)0.89 (0.03)1.01 (0.02)0.99 (0.02)0.98 (0.02)0.96 (0.02)0.90 (0.02)0.88 (0.02)0.90 (0.02)0.67 (0.02)0.48 (0.02)0.38 (0.02)0.24 (0.02)0.17 (0.03)0.10 (0.02)o.os (o.o3)E662.5 (0.2)716.1 (0.2)769.7 (0.3)823.5 (0.3)87?.3 (0.3)930.5 (0.2)983.7 (0.2)1037.9 (0.2)1090.8 (0.3)1143.1 (0.3)1196.1 (0.3)1247.6 (0.4)1299.? (0.4)1351.4 (0.6)1403.3 (0.8)1453.0 (1.1)Intensity0.64 (0.06)0.88 (0.05)1.10 (0.06)1.15 (0.05)0.93 (0.05)0.82 (0.05)0.97 (0.05)0.91 (0.05)o.9s (o.os)0.64 (0.04)0.70 (0.05)0.47 (0.04)0.58 (0.04)0.52 (0.04)0.31 (0.03)o.17 (o.os)Tb SD band 2 '"Tb SD877.0 (0.4)888.3 (0.3)929.4 (0.3)977.0 (0.3)1026.7 (0.5)1077.0 (0.4)1126.2 (0.3)1177.9 (0.5)1229.7 (0.6)1282.4 (0.6)1333.3 (0.7)1387.1 (0.9)1440.5 (1.0)[1494.0 (1.2)]band 3Intensityo.68 (o.o6)0.78 (0.06)1.0S (0.07)0.88 (0.10)1.01 (0.10)1.08 (0.09)0.91 (0.08)0.82 (0.08)1.03 (0.10)o.so (o.1o)0.65 (0.06)0.32 (0.05)0.20 (O.G8)0.10 (O.G5)52 PROTON AND NEUTRON EXCITATIONS IN SUPERDEFORMED '5 Tb 95OQVOv —10--—8-=IC9[651]1/2[642]5/27l~ ~72—11--bQ I0.0 0.12 0w —3 Q--bQC+.QQ[301]1/2C4—5.0--2...---- -.-C4 ~ QC4)C0.2~ WC/}0.3 0 4640.5 0.6 0.7 0.8 0.9""'"'""'-'-"'-"~ ~ ~ -W. .0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9Rotational Frequency hto (MeV)FIG. 2. Neutron and proton single-particle energies as afunction of rotational frequency (hey) for a fixed deformationPz = 0.59, P4 —0.09. The Routhians are labeled by 7r = +,o =+1/2 (solid lines); m = +, o. =—1/2 (dotted lines); 7r = —,o =+1/2 (dot-dashed lines); s = —,o =—1/2 (dashed lines).B. Band 2The spectrum of band 2 is shown in Fig. 3 and its 9~ ~moment of inertia is plotted as a function of the rota-tional &equency in Fig. 4. Also included in Fig. 4 arethe 8( ) values for Dy band 1 [16,17] (which is theZ + 1 isotone of Tb) and Tb band 1 (dashed line).Clearly, Tb band 2 has a very difFerent 9~ ~ curve com-pared with Tb band 1. Therefore we do not associateband 2 with the same intruder configuration as Tbband 1. We note, however, that the 8~ ~ values in Tbband 2 and Dy band 1 exhibit a similar upward slopeas a function of increasing ~, and that their magnitudestwo transitions. Figure 2 shows the proton and neutronsingle-particle orbitals as a function of rotational &e-quency (Ru) for a fixed deformation Pq ——0.59, P4 —0.09,appropriate for i5oTb band 1 and taken from Ref. [9].The calculations are based on a Woods-Saxon potentialwith the "universal" parameters given in [15]. Band 1 inisoTb has been assigned [9,13] a m6 v7 high-N intruderconfiguration (i.e., three N=6 proton and one N=7 neu-tron intruder orbitals occupied).It is clear &om Fig. 2 that excited SD configurations(relative to band 1) may involve changes in either protonor neutron orbitals and more specifically changes in theoccupation of the high-N intruder orbitals.1500eC4hltOt1000CaO500Cl OlOloChCOCSOlC)T TTCOCbTTCb0)CO CVt04T0500 700I900 1100Energy (keV)1300 1500FIG. 3. Spectrum generated by a sum of E~-E~ coincidencegates on transitions in Tb SD band 2. A *denotes knownnormal deformed transitions in Tb.85Q)6580"Q)OCQ) 75--0OE6$70a 0.2Tb band 210—%0.3 0.4 0.5 0.6 0.7 0.8Rotational Frequency h'ro (MeV)FIG. 4. Dynamic moments of inertia, as a function of rota-tional frequency for Tb band 2 (filled circles), Dy band1 (open squares), and Tb band 1 (dashed line).dier by a roughly constant amount. Figure 5 illustratesthe contribution to the 9& & moment of inertia &om thefour lowest N=6 proton intruder orbitals (adapted from[8]). The favored signature of the [651]3/2 (cr = + z) pro-ton intruder orbital (labeled m6s in Fig. 5) generates asmall and slightly decreasing contribution to the Qt ~ mo-ment of inertia within the &equency range Lu = 0.4 —0.8MeV. Thus, it is proposed that the Tb band 2 con-figuration can be described in terms of a hole in thisorbital (vr6s) relative to the i Dy band 1 core configu-ration [i.e., (is Dy)„, ,tm([651]z) i]. This correspondsto a particle-hole excitation &om the favored signature ofthe [651]3/2 (n=+ z) proton intruder orbital into the un-favored signature [651]3/2 (n=-~), i.e., n6s ~ n64. Onemay then describe 0Tb band 2 as the unfavored sig-nature partner to ~ Tb band 1. The reduced intensityof band 2 relative to band 1 is consistent with the largesignature splitting for the sr[651]3/2 orbital as shown inFig. 2.Recently [18],a similar proton excitation (7r6s —+ n'64)has been proposed as an explanation for band 3 in ~Tb.In this case, Tb band 3 has a 8~ ~ moment of inertia96 P. FALLON et al. 521000Og ]0-Oo I+K~ IKO0~ &++I4cCfIC41//g40 0.5 1.0Rotational Freqnency 5M (Me&)800-OP Nj600-OcuT ooci400ooo-iII800 900 - 1000 1100 1200 1300Energy (keV)TIA1h[ti14M 15M 1600FIG. 6. Spectrum generated. by a sum of E~-E~ coincidencegates on transitions in Tb SD band 3. A *denotes knownnormal deformed transitions in Tb.FIG. 5. Single-particle contributions to the dynamic mo-ment of inertia (9 ) from the four lowest N=6 proton in-truder orbitals (adapted from Ref. [8]). The labels 1 and 2 (3and 4) refer to the two signatures of the [600]1/2 ([651]3/2)orbital.which exhibits a similar slope but with a reduced mag-nitude relative to that of Dy band 1.Other likely candidates for a simple particle-hole exci-tation involving proton orbitals would most likely requirea hole in the [301]1/2 level. In analogy to the pair ofidentical bands in i Dy and Tb' [10],one may expectthat an excited SD configuration in Tb which involvedthe [301]1/2 orbital would result in a SD band with nearidentical transition energies relative to those observed inDy band 1. However, no SD band with these prop-erties has been identified in this data set. This suggeststhat in oTb, as compared with i Tb, the [301]1/2 or-bital lies lower in energy relative to the [651]3/2 intruderlevel. Hence the 7r63 ~ vr64 excitation is more favorable.C. Band 3The spectrum of Tb band 3 is shown in Fig. 6 andits 8~ ~ moment of inertia, together with that for Tbband 1, is shown in the upper part of Fig. 7 as a functionof rotational &equency. In the lower part of Fig. 7 the9(2) curves for Gd band 2 [4] and Gd band 1 [2]are presented. For either the Gd or Tb isotope pair, theexcited SD band in the %=85 nucleus exhibits a 9~ ~curve which falls below that of the yrast SD band in theN = 86 isotope at the higher rotational &equencies. Atlow &equencies the excited SD bands ( soTb band 3 andi4sGd band 2) exhibit a 9(~) which exceeds that seen inthe N + 1 yrast SD sequence (isiTb band 1 and Gdband 1). It is proposed that both isoTb and i4sGd areassociated with the same neutron particle-hole excitationand that this is most likely an excitation from a non-intruder N=6 orbital (either [642]5/2 or [651]1/2) to anN=7 high-N intruder ([770]1/2).Since the interpretation for Tb band 3 is closely re-lated to that of Gd band 2, it is instructive to dis-cuss the Gd bands first. In Gd band 1 —whichhas a vr6 v7 intruder configuration —the increase inQ(2) at ~ 0.4 (Fig. 7 lower) is associated [2] with an%=7 quasineutron band crossing. The Gd SD band2 configuration is assumed [3] to be based on a neutronparticle-hole excitation &om the [642]5/2 (or [651]1/2)orbital into the N=7 intruder orbital ([770]1/2 n=+1/2).Therefore this band also has a 7r6 v7 intruder configu-120 .110100)0) 90-a) 80.70e 120 .E110.06$E 100-90-Tb BAND 3Tb BAND 1Gd BAND 2Gd BAND 170600.4 0.5 0.6 0.7 0.8Rotational Frequency hro (Me V)FIG. 7. Dynamic moments of inertia as a function of rota-tional frequency for (upper) Tb band 3 (filled circles) andTb band 1 (open squares), and (lower) Gd band 2 (filledcircles) and Gd band 1 (open squares).52 97ar e reduction ln tMe@. One clearly observes a g7 uasineutron eros ' gsinteraction streng& m ~2 p 6 ~ A =th for the N=7 q0 56f ation is reduce o7 and the discus-obeom the comparison g150Th band 3 can als esion above it is suggo of a neutron &c m either„der configurat o" (' '. ""h [642]5/2 or [651]1/jntruthe1~ coupled to a hod that the sharp rise &rbjtal. It ls pr p0 45 MeVneutron o 1s —at ~nd 3 Q~ ~ values .h N —7 quasjneu-T a'involving t ejred band cross»gdz-jving efFects oo a Parro the deformatloI1 oTbtrons. In this scena ~lier deformation forthe neutron pair gt low frequency-'rin at —0jncreases in t yf the presence o pbackbends, ls givends. The precise nareiations In =h dynamic (vibratiopajr costatic, remains unclear.1.5"——— ——1.0" i0.5"O.o" p, =0.60O.5&.5-=1.00.5o.p"p =o.58-0.5~ MQ. ~0.5D 150TbS IN SUpERDN FXCITATIOpROTON AND NEUTQ.O" ti =056206 0705 2 03 Q4 0 ~ROt~t1()nal Freque"&y m (MeV)of rotationafunction8 quasineu«on n energies as .ofFI~mber pf =8' he6 at deformations0.10, 0.09, i11..t„.th f., t, =h N 7't dreduction in e 'h d formation is re uaire d band crossing, ast e eIV. SUMMARYs have been observe lnIn summary, twoeen interpre et d as being ase oTb Thes~ haveutron' ns involving a proits afi—151D b nd13 72 ' ""der ""fe in the auration. T eto have am6 v inrorbital, is calcu-assumed[651]1/2 neutronhole, in thehe deformation for1 d 1' htl reduce tN=7is thus interpreted as acrossing.ACKNOWLEDGM ENTSnd stafF of the 88-k the operators a arate-h cyclotron. 1tiscussions wites finan-lncd. One of usis work.fully acknowledged.in the course of thisNo. D. E-AC03-76SF00098 and o.Pin Gd band 2 at Lu—0.45ration an d the backbendh the same N=7d. '"Gd b d~Fig. 7 sower isas observed in1 dKh SDb ddftlon s reict apared wld'G (P = .585e 642 5/2 ortion riving11 d foste that the sma er eGdb d1 ltor la.hhl dtsma er1' Gd 1 1the neu ron Fermi level inreasea iiolti dr orbitals and t e ne rges incran e sationsh F'.. .t,.t,.l.t.=0a cran eld 1 ltd t~"BCS pair fie ca cand with a constant98 P. FALLOW et a1. 52[3][4][6][7][8][9][10]P.J. Twin et a/. , Phys. Rev. Lett. 5'7, 811 (1986).P. Fallon et a/. , Phys. Lett. B 218, 137 (1989); 257, 269(1991).B. Haas et a/. , Phys. Rev. C 42, R1817 (1990).S. Flibotte et a/. , Phys. Rev. Lett. 71, 688 (1993).S. Lunardi et a/. , Phys. Rev. Lett. 72, 1427 (1994).P. Fallon et a/. , Phys. Rev. Lett. 73, 782 (1994).P.J. Dagnal et a/. , Phys. Lett. B 335, 313 (1994).T. Bengtsson, S. %berg, and I. Ragnarsson, Phys. Lett. B208, 39 (1989).W. Nazarewicz, R. Wyss, and A. Johnson, Nucl. Phys.A50$, 285 (1989).T. Byrski et a/. , Phys. Rev. Lett. 64, 1650 (1990).[11][12][13][14][15][16][18]W. Nazarewicz, P.J. Twin, P. Fallon, and J.D. Garrett,Phys. Rev. Lett. 64, 1654 (1990).F.S. Stephens et a/. , Phys. Rev. Lett. 64, 2623 (1990);65, 301 (1990).M.A. Deleplanque et at. , Phys. Rev. Lett. 60, 1626(1989).I.Y. Lee Nucl. Phys. A520, 641c (1990).S. Cwiok, J. Dudek, W. Nazarewicz, J. Skalski, and T.Werner, Comput. Phys. Commun. 24, 379 (1987).G.-E. Rathke et aL, Phys. Lett. B 209, 177 (1988).D. Nisius et aL, Phys. Lett. B 848, 15 (1995).B. Kharraja et a/. , Phys. Lett. B $41, 268 (1995).

Proton and neutron excitations in superdeformed Tb150

https://cdr.lib.unc.edu/downloads/z603r727f

Proton and neutron excitations in superdeformed Tb150

Abstract

Similar works

Full text

Available Versions

Carolina Digital Repository