For the first time, six-quasiparticle isomers have been observed in the meta-stable nucleus 18073Ta107. Two new high-spin isomers were populated following deep-inelastic reactions with a pulsed 23892U beam incident on a thick 18072Hf target. Out-of-beam γ-ray events were collected using the Gammasphere germanium detector array. In addition to the known four-quasiparticle isomers, yrast Kπ = (22-) and K≥23 six-quasiparticle isomers have been observed with microsecond half-lives. These are the highest-spin isomers observed using the technique of deep-inelastic excitation. The assignments are compared to predictions made by BCS and Lipkin-Nogami multiquasiparticle calculations

Ahmad, I.

Carpenter, M.P.

Chowdhury, P.

Cullen, D.M.

D'Alarcao, R.

Janssens, R.V.F.

Khoo, T.L.

Kondev, F.G.

Lister, C.J.

Pearson, C.J.

Podolyàk, Zs.

Seweryniak, D.

Shestakova, I.

Walker, P.M.

Wheldon, C.

Wiedenhoever, I.

Carolina Digital Repository

PHYSICAL REVIEW C, VOLUME 62, 057301Inelastic excitation of new high-spin yrast isomers in180TaC. Wheldon,1,* P. M. Walker,1 P. Chowdhury,2 I. Shestakova,2 R. D’Alarcao,2 I. Ahmad,3 M. P. Carpenter,3D. M. Cullen,4,† R. V. F. Janssens,3 T. L. Khoo,3 F. G. Kondev,3 C. J. Lister,3 C. J. Pearson,1 Zs. Podolya`k,1D. Seweryniak,3 and I. Wiedenhoever31Department of Physics, University of Surrey, Guildford GU2 7XH, United Kingdom2University of Massachusetts, Lowell, Massachusetts 018543Argonne National Laboratory, Argonne, Illinois 604394Department of Physics, Oliver Lodge Laboratory, University of Liverpool, Liverpool L69 7ZE, United Kingdom~Received 1 June 2000; published 25 September 2000!For the first time, six-quasiparticle isomers have been observed in the meta-stable nucleus73180Ta107. Twonew high-spin isomers were populated following deep-inelastic reactions with a pulsed92238U beam incident ona thick 72180Hf target. Out-of-beamg-ray events were collected using the Gammasphere germanium detectorarray. In addition to the known four-quasiparticle isomers, yrastKp5(222) and K>23 six-quasiparticleisomers have been observed with microsecond half-lives. These are the highest-spin isomers observed usingthe technique of deep-inelastic excitation. The assignments are compared to predictions made by BCS andLipkin-Nogami multiquasiparticle calculations.PACS number~s!: 21.10.Tg, 23.20.Lv, 27.70.1qcufwatatsondetiasrohentioetodio-edtofs.on-2.5N/pec-on98d 3Ther tortedomrsstallypu-elheec-ei-life.eoriveThe structure and excitations of the rare isotope180Ta areof considerable current interest, for example Refs.@1–3#,arising from its unique status as the only isotope to ocnaturally in an isomeric state. The 11 ground-state has ahalf-life of only 8 hours, whereas an excited 92 state lying at75 keV, has a lifetime of.1015 years. The direct decay othis isomer is highly hindered due to a combination of its loexcitation energy and high spin, relative to the ground stSeveral studies have attempted to explain the productionsurvival of this spin-trap isomer in stellar environmenmost recently Ref.@1#.This odd-odd nucleus with 73 protons and 107 neutrlies in a region of the nuclear chart where large prolateformations are energetically favored. This leads to parconservation of the angular momentum projectionK on thenuclear symmetry axis@4#. The decay of these intrinsic stateto the competing collective rotational levels is hindered, pviding a sensitive probe of the underlying structure of tnucleus.Previous experiments to study the yrast structure of180Tahave used light heavy-ion projectiles, namely,511B and 37Lion targets of176Yb @2,3#. Several new isomers have beeobserved using these partial fusion and fusion-evaporareactions, including a 31ms Kp5152 four-quasiparticlestate @5#. However, this technique is limited to;20 \ ofangular momentum. Recently, experiments using deinelastic reactions@6–8# have enabled states with spins up;30 \ to be reached@9# andK isomers have been populatein neutron-rich nuclei that cannot be accessed using fusevaporation reactions with stable beams and targets@10–12#.The first isomers at spins.20 \, using this method of transfer and inelastic excitation are now reported.*Present address: Department of Physics, Oliver Lodge Labtory, University of Liverpool, Liverpool L69 7ZE, UK.†Present address: Department of Physics and Astronomy, Unsity of Manchester, Manchester M13 9PL, UK.0556-2813/2000/62~5!/057301~4!/$15.00 62 0573re.nd,s-l-np-n-A 1.6 GeV pulsed 92238U beam provided by the ATLASaccelerator facility at Argonne National Laboratory was usto bombard a thick target ('40 mg cm22) of 72180Hf, backedby >50 mg cm22 of natPb. The beam energy was chosenbe 15% above the Coulomb barrier, consistent with Re@11,12#. The target was enriched to 98.2% in180Hf, theheaviest stable isotope, to favor the population of neutrrich products. The natural pulsing period of the beam is 8ns, and this was swept on two longer time ranges with OOFF conditions of 8.25ms/16.5 ms and 2 ms/4 ms~corre-sponding to 100/200 and 24 250/48 500 beam bursts, restively!. All of the recoils were stopped at the target positiat the focus of the Gammasphere array, comprised ofg-ray Compton-suppressed germanium detectors anCompton-suppressed planar x-ray germanium detectors.trigger condition required at least one germanium detectofire in the beam-off period. These delayed events were sointo g-g, g-x-ray andg-time matrices and ag-g-g cube,allowing level schemes and half-lives to be deduced frcoincidence relations. A wide variety of targetlike isomehave been observed from69171Tm up to 77189Ir, together with alarge number of nuclei from the fission of uranium. The mointense out-of-beam decays are those from the inelasticexcited isomers in the180Hf target @12#, but the one-protontransfer channels to tantalum nuclei are also strongly polated.A new isomer was observed in180Ta, feeding the knownstates in theKp5192 band based on the 2899 keV lev@2,3# via transitions with energies of 492 and 370 keV. Tlevel scheme is shown in Fig. 1. From the coincidence sptrum gated by the 365 keV transition@Fig. 2 ~top!# it can beseen that the new 492 keVg ray is much weaker than th370 keV and subsequent decays ('25% of theg-ray inten-sity!. A probable explanation for the difference in coincdence intensity is the presence of an intermediate half-The time window forg-ray coincidences to be in the samevent remains open for 800 ns. A half-life of 2.06 .5ms fora-r-©2000 The American Physical Society01-1-thcee8i-kayoingre10.6fsheateedfle-entyletstis.forthesi-esateb-th athe-heThethetBRIEF REPORTS PHYSICAL REVIEW C 62 057301the 3678 keV level would allow'25% of the coincidencesto fall inside of the 800 ns window. This half-life is confirmed by the absence of the 492 keV transition whencoincidence window is reduced to 20 ns@Fig. 2 ~middle!#.~The transitions below the 22 ns isomer also have reduintensity in this spectrum.! The 492 keV transition can bcleanly observed by gating on the 410 keVg ray and pro-jecting the coincidence events observed between 40 andns earlier, as shown in Fig. 2~bottom!.All of the g rays below the 4170 keV level are contamnated@see, for example, Fig. 2~top!# precluding a half-lifemeasurement using theg-time matrix. In addition the timespectra for the 16.5ms range are distorted, perhaps asresult of the high in-beam rate. Therefore, eight-g matricesgated by 1.5ms time intervals~with 1.6 ms separating thecentroids!, for times .2 ms, were constructed. The peaareas for the transitions above the 31ms isomer, obtainedfrom spectra gated by the 365, 410, and 431 keV decwere plotted as a function of time, negating the problemcontaminants. To correct for the distortion, the resultspectrum was divided by a normalized half-life curve corsponding to a long-lived product@100ms). ~This techniquereliably reproduced known half-lives, for example, the61 ms isomer in 180Hf @12# was measured here to be 9ms.) The result is shown in Fig. 3 yielding a lifetime ot1/251765 ms. The large uncertainty arises from the preence of the'2 ms half-life component for the 3678 keVlevel and a sensitivity to the profile used to correct for tdistortion.FIG. 1. Decay scheme for the new six-quasiparticle isomer180Ta observed in the present work. The spins and parities for stbelow the (222) 3678 keV level are taken from Refs.@2,3#.05730ed00as,f--No 779 keV decay was observed from the 3678 keV stto theKp5192 bandhead, but an upper limit can be obtainfor the partialg-ray half-life from the maximum intensity othis transition. This yieldst1/2g .10 ms for the unobserved779 keV transition. Comparisons with the Weisskopf singparticle estimate show that such a half-life is only consistwith l>3 transitions~within a generous margin of 103) sug-gesting a spinI>22 for the 3678 keV state. From intensitbalancing with the 410 keVM1 transition, the total electronconversion coefficient for the 370 keV decay isaT(exp)50.0960.08. This is consistent with electric quadrupo@aT(E2)50.04# and electric octupole assignmen@aT(E3)50.16#, considering only those multipolarities thalead to a spin greater than 21\ for the'2 ms isomer. (M2,M3, and higher multipoles are ruled out by this analys!Therefore, the most likely spin and parity assignmentsthe 3678 keV state areKp5222 and 231. Examining theWeisskopf units~W.u.! for the 370 keV decay favors theE2assignment, as anE3 multipole would imply;300 W.u.,which is exceptionally fast for a noncollectiveK-forbiddentransition. Thus, the preferred tentative assignment for3678 keV level isKp5(222).There are two possible scenarios for the 492 keV trantion observed above the'2 ms isomer. Firstly, it could bethe direct decay from the isomeric state, consistent withl<3 transitions. Alternatively, the 4170 keV level could btheinesFIG. 2. The top panel shows delayedg rays gated by the 365keV transition in 180Ta with an 800 ns coincidence window. Thmiddle panel shows the ‘‘logical and’’ of two delayed spectra, otained from single gates on the 312 and 410 keV transitions winarrow610 ns coincidence condition.~A ‘‘logical and’’ comparesthe counts per channel in two individual spectra, and displayssmaller of the two per channel@13#. This helps to reduce contaminant g rays.! The 492 keV decay is no longer present and tintensity of the transitions below the 22 ns isomer is reduced.bottom spectrum shows the ‘‘early’’ transitions which precede410 keVg ray by between 40 and 800 ns. Known contaminangrays are labeled by their energy and associated nuclide.1-2ityei-with70om.agi-aethspos-lescteenee41-mula-BRIEF REPORTS PHYSICAL REVIEW C 62 057301first excited member of the rotational band associated wtheKp5222 state, which could be fed by a low-energy~un-observed! transition from the'17 ms isomer, represented bthe offset n,60 keV in Fig. 1. Such a decay would bcompatible withl<2. In both of these situations the transtion rate favors a quadrupole assignment by comparisonWeisskopf estimates. The most likely spin assignment for'17 ms isomer is 23, 24, or 25\.In addition to the first observation of the 3678 and 411n keV isomeric states, a new 1137 keVK-allowed transi-tion, with a branching ratio of 7%, has been observed frthe 22 ns isomer to the 31ms isomer. This is shown in Fig4. The 2587 keVI 518 level has previously been assignedtentative positive parity@2,3#. The absence of a competinquadrupole decay to the 1791I p5162 state favors anE3multipolarity for the 1137 keV transition, supporting a postive parity assignment for the 22 ns isomer.Predictions for the excitation energies, spin and paritysignments for low-lying six-quasiparticle isomers have bemade using Lipkin-Nogami@2# and BCS calculations@3#~see Fig. 5!, at deformations of«250.237, «450.056 and«250.242,«450.087, respectively. These calculations bopredict yrast Kp5241 and Kp5222 six-quasiparticlestates with configurationsn$ 1121@615#, 921@624#, 722@514#%FIG. 3. Peak areas for the transitions above the 31ms isomer in180Ta, plotted as a function of time. The intensities were obtainfrom spectra gated by the 365, 409, and 431 keV decays~see textfor details!. The solid line through the data represents a ocomponent fit with the half-life shown.FIG. 4. Gamma-ray spectrum produced from summing the sptra double gated by the 492 keV decay with the 312, 370, andkeV transitions. The 1137 keV transition can be clearly seen.05730hthes-n^ p$ 922@514#, 721@404#, 521@402#% and n$ 921@624#,722@514#, 722@503#% ^ p$ 922@514#, 721@404#, 521@402#%, re-spectively. The predicted energies areE24153775 keV andE22253188 keV from Ref.@2# and E24154133 keV andE22253314 keV from Ref.@3#. ~Note that these energieinclude residual nucleon-nucleon interactions.! These calcu-lated states have the most favored residual interactionssible as given by the Gallagher-Moszkowski coupling ru@14#, which result in a lowering of the energies with respeto unfavored configurations. Saitohet al. @3# point out thatthese energies are ‘‘only accurate to within6400 keV butthat it is reasonable to assume that thisKp5222 level liesbelow the 212 level of the 192 band’’ at 3737 keV. As nolow-lying spin 23\ states are predicted it is likely that th'2 ms isomer is the calculated 222 intrinsic level, whichdoeslie below the 212 member of theKp5192 band. TheKp5(222) state qualifies as anyrast trap, as it lies below allof the I 521 levels, forcing it to decay by al.1 transition.The energy of the'17 ms state observed at 41701n keV isin good agreement with the predictedKp5241 level. @Thered-c-0FIG. 5. Observed~Expt.! and calculated intrinsic four- and sixquasiparticle states in180Ta. The energies of Calc. I are taken froRef. @2# and Calc. II are taken from Ref.@3#. Residual nucleon-nucleon interactions are included in the energies of both calctions. See text for details.1-3heee-hii-testetheithra-byon09-BRIEF REPORTS PHYSICAL REVIEW C 62 057301are no low-lyingK525 states predicted and aKp5262 state~discussed later! is calculated to be 1029 keV higher than tKp5241 state@2#.#Dracoulis et al. @2# have calculated the energies of thmost favored eight-quasiparticle states. The lowest of this a Kp5262 level with the configurationn$ 921@624#,722@514#, 722@503#, 522@512#, 322@512#% ^ p$ 922@514#,721@404#, 521@402#%, predicted at 4804 keV, including residual interactions. No candidates for transitions from tpossible isomer to the 41701n keV state have been identfied in the present study, but this together with higher staprovide a challenging objective for future studies..id-on.L05730sessIn summary, two new high-spin six-quasiparticle yraisomers have been populated in the nucleus180Ta, usingdeep inelastic reactions with a238U beam. These are thhighest spin and highest seniorityK isomers yet identifiedwith deep inelastic reactions. The excitation energies ofnew intrinsic states are found to be in good agreement wpredictions of multiquasiparticle calculations, and configution assignments have been made.This work was supported by the U.K. EPSRC andthe U.S. Department of Energy, Nuclear Physics Divisiunder Contracts No. DE-FG02-94ER40848 and W-31-1ENG-38.@1# D. Belic et al., Phys. Rev. Lett.83, 5242 ~1999!, and refer-ences therein.@2# G.D. Dracoulis, S.M. Mullins, A.P. Byrne, F.G. Kondev, TKibédi, S. Bayer, G.J. Lane, T.R. McGoram, and P.M. Davson, Phys. Rev. C58, 1444~1998!.@3# T.R. Saitohet al., Nucl. Phys.A660, 121 ~1999!.@4# A. Bohr and B. R. Mottelson,Nuclear Structure (Benjamin,New York, 1975!, Vol. 2.@5# G.D. Dracoulis, F.G. Kondev, A.P. Byrne, T. Kibe´di, S. Bayer,P.M. Davidson, P.M. Walker, C. Purry, and C.J. PearsPhys. Rev. C53, 1205~1996!.@6# R. Broda, M.A. Quader, P.J. Daly, R.V.F. Janssens, TKhoo, W.C. Ma, and M.W. Drigert, Phys. Lett. B251, 245,.~1990!.@7# R. Brodaet al., Phys. Rev. Lett.74, 868 ~1995!.@8# I.Y. Lee et al., Phys. Rev. C56, 753 ~1997!.@9# J.F.C. Cockset al., Phys. Rev. Lett.78, 2920~1997!.@10# K. Krumbholzet al., Z. Phys. A351, 11 ~1995!.@11# C. Wheldonet al., Phys. Lett. B425, 239~1998!; Eur. Phys. J.A 5, 353 ~1999!.@12# R. D’Alarcaoet al., Phys. Rev. C59, R1227~1999!.@13# D.C. Radford, Nucl. Instrum. Methods Phys. Res. A361, 297~1995!.@14# C.J. Gallagher and S.A. Moszkowski, Phys. Rev.111, 1282~1958!.1-4

Inelastic excitation of new high-spin yrast isomers in 180Ta

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Inelastic excitation of new high-spin yrast isomers in 180Ta

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