Session W22 - High Spin Stakes.
MIXED session, Tuesday morning, May 02
102C,

[W22.001] Transition Strengths in Odd-Odd ^86Nb

High-spin states in odd-odd ^86Nb were populated using the ^58Ni(^32S,3pn) reaction at 135 MeV. Recoiling ^86Nb nuclei were stopped in a thick Ta backing. Prompt multi-\gamma coincidences with evaporated charged particles were detected using Gammasphere and the Microball at LBNL. Mean lifetimes of 51 levels in ^86Nb were measured using the Doppler-shift attenuation method. Most of the transition quadrupole moments inferred from the lifetimes lie in the range of 1.0 to 2.7 eb, which would correspond to quadrupole deformations \beta _2 of 0.10 to 0.27 in a rotational model. Comparison with Hartree-Fock-Bogolyubov cranking calculations suggests triaxial to oblate shapes. Strongly alternating magnetic dipole transition strengths were observed in the yrast positive-parity bands.

[W22.002] Spectroscopy of ^92-95Zr

The level structures of ^92,93,94,95Zr have been studied in the fission of the compound nucleus ^197Pb, formed in the ^24Mg + ^173Yb reaction at 134.5 MeV using the Gammasphere array. Based on coincidence with known transitions in the complementary Mo fragments, new sequences of transitions have been assigned to ^93,95Zr. The previously known level scheme of ^94Zr has been considerably extended to higher excitations, and exhibits structural similarities to the level scheme of ^92Zr up to spin 10\hbar. The lower part of the level schemes of ^93Zr and of ^95Zr can be interpreted as the coupling of a d_5/2 neutron to the lower levels of ^92Zr and/or ^94Zr, and of ^96Zr, respectively. The observed experimental states are compared with theoretical shell-model calculations which reproduce well the excitation energies of the positive parity states in the even-A Zr isotopes, but underestimate the negative parity states and fail to reproduce the excitation energy of the states in the odd-A Zr isotopes.

[W22.003] Band Structures in ^129Nd.

High-Spin states in ^129Nd were populated using the ^92Mo(^40Ca,2pn) reaction with a beam energy of 184 MeV. The Gammasphere spectrometer at Argonne was used in conjuction with the Microball array to measure gamma rays selected for the charged-particle exit channels of interest. In our analysis, the level schemes of both nuclei have been greatly extended from what was previously known up to spins of I\approx36\hbar. Evidence of a possible \nu i_13/2[660]1/2 crossing with the d_5/2[402]5/2 band in ^129Nd is shown. Band characteristics such as alignment behavior, B(M1)/B(E2) ratios, and DCO ratios are presented.

[W22.004] Three-band mixing in ^131Nd

High-spin states in ^131Nd were populated in the reaction ^94Mo(^40Ca,2pn) at a beam energy of 180 MeV. The gamma rays and charged particles were detected with the Gammasphere and Microball arrays, respectively. Over 170 new transitions were placed in a level scheme that consists of seven rotational structures. An unusual low-spin interaction is observed in the band that is adiabatically crossed by the highly deformed \nu i_13/2 sequence at higher spins. Similar low-spin crossings have been observed in the A\sim 175 region and have been interpreted as the mixing of three bands. This three-band mixing model was applied to the bands in ^131Nd and the results will be presented. The conclusions of this analysis also aid in understanding why the second minimum \nu i_13/2 structure interacts strongly with a normal deformed band.

[W22.005] Experimental indications of chirality near A\sim130

Several A\sim130 odd-odd nuclei were investigated to search for chiral partners of the \pih_11/2\nuh_11/2 \Delta I=1 yrast bands. The N=75 ^130Cs and^132La isotones of previously studied ^134Pr[1] were populated via ^124Sn(^10B,4n) at 47 MeV and ^132Sb(^13C,4n) at 64 MeV, respectively; ^132,134Cs were populated via ^130Te(^6Li,4n) at 38 MeV and ^130Te(^7Li,3n) at 33 MeV, respectively, as additional Cs isotopes at higher neutron Fermi levels. In all experiments, six Compton-suppressed Ge detectors in conjunction with a 14-element BGO multiplicity filter were used. Time-gated prompt \gamma-\gamma coincidences measured with backed targets at the Stony Brook Tandem/LINAC facility were sorted into 2D matrices. As of this writing, \Delta I=1 \pih_11/2\nuh_11/2 side bands were identified in all cases except ^134Cs. DCO analyses established that the links between the side and yrast bands were of M1/E2 mixed character, and thus defined relative spins and parity for the side bands. These newly identified side bands will be presented as indications of chirality. [1]C.M. Petrache et al., Nucl. Phys. A697, (1996) 160.

[W22.006] Energy-degenerate levels in the A\sim130 region: search for chiral partners

Comparative studies of \pih_11/2\nuh_11/2 bands in odd-odd N=75 isotones were performed using collective nuclear models. Investigations were concentrated on the previously known yrast bands, as well as recently identified side bands built from the same configuration. The study was motivated by the observation of the energy-degenerate levels in ^134Pr, which possibly indicate dynamical chiral symmetry in rotating nuclei~[1]. Experimental Routhians for \pih_11/2\nuh_11/2 bands were found to be inconsistent with predictions of the Principal Axis Cranking (PAC) model. A phenomenological approach was developed based on a core-particle-hole coupling model to address the experimental observables. The model includes a triaxial core, single particle Hamiltonians, and quadrupole-quadrupole interactions; level energies and electromagnetic properties for odd-odd nuclei are calculated. Parameters of the model were adjusted to reproduce experimental results for ^132La. With this set of parameters, the model predicts the existence of degenerate levels at spins near \sim14\hbar for rigid rotational cores with \gamma \sim 30^\circ. Comparisons between experimental data presented in the previous abstract and these calculations will be discussed. [1 mm] [1]S.~Frauendorf and J.~Meng,~Nucl.~Phys.~A617~(1997)~131.

[W22.007] High-spin structure of ^136Pm: Evidence for chiral symmetry in nuclei?

Recent theoretical calculations predict the existence of chiral symmetry states in doubly-odd triaxial nuclei [1]. To search for these states, an experiment was performed to study high spin states in ^136Pm. The reaction used was ^116Sn(^24Mg,p3n)^136Pm with beam energies of 130 and 135 MeV. Gamma rays were detected using the YRAST ball detector array, which at the time consisted of 21 Compton suppressed coaxial Ge detectors and 4 segmented clover detectors, at the Wright Nuclear Structure Laboratory, Yale University. The known yrast and side bands were extended and several new levels comprising a new side band were observed. The new side band feeds into the yrast band by a series of mixed M1/E2 transitions. This side band is similar to that previously found in the neighboring nucleus ^134Pr [2] which was interpreted by Frauendorf [1] as evidence for possible chiral symmetry, and to new bands recently observed in the N=75 isotones ^132La and ^130Cs [3]. Analysis and results will be presented. [1] S. Frauendorf and J. Meng, Nucl. Phys. A617, 131 (1997). [2] C.M. Petrache, et al., Nucl. Phys. A597, 106 (1996). [3] K. Starosta, et al., to be published.

[W22.008] Multiplet Structure Of Nuclei Near A=130

Experimental data for the low-lying states of Sn and Te isotopes as well as of the chain with N=75 are analyzed in the framework of particle- anharmonic core coupling (PACC) model. The identified multiplets are compared with the energies obtained within the boson-fermion model (Kota V.K.B.- Proc.Intern. Conf. on Nucl. Str., Dubrovnik,(World Scientific), 1986, p.259) and within the classification scheme proposed in (Balantekin A.B., I.Bars,F.Iachello, Phys.Rev.Lett. v.47,1981,p.19).

A correspondence of the states with 0-, 1-, 2-... quasiparticle nature established by applying a generalized PACC- model and the states which build a boson-fermion multiplet defined^2 by the quantum number \cal N (equal to the sum of boson and fermion quantum numbers) in the system A , A+1 is found. The possibility is discussed for a deeper understanding of the physical meaning of supersymmetric model parameters and their determination by means of the experience stored in dealing with geometrical models.

[W22.009] Lifetime of Exotic Bands in ^168Hf

lifetime measurements using the DSAM technique have been performed for the presumably triaxial superdeformed (TSD) band newly identified in the ^168Hf nucleus (P.G. Varmette et al., Bull. Amer. Phys. Soc. 44, No.1 (1999) 1297.). From this data set, lifetimes of some of the TSD states have been extracted, and the quadrupole moment of the TSD band derived. The results will be used to determine the quadrupole deformation parameter and to distinguish between band structures of positive and negative values of the gamma degrees of freedom, that is, to confirm the original assumption of triaxiality of this SD band. Data analysis is still in progress, results will be presented. Work supported in part by US DOE under grants DE-FG02-95ER40939 (MSU) and W-31-109-ENG-38 (ANL).

[W22.010] Climbing up the isomer ladder in neutron-rich ^181Hf

Neutron-rich Hf (Z=72) nuclei, long predicted to be fertile terrain for high-spin K-isomers, have just started to be accessible via transfer reactions with very heavy beams (R.D'Alarcao et al., Phys. Rev. C59, R1227 (1999).). Here we present the first identification and spectroscopy of three-quasiparticle (3qp) isomers in the neutron-rich ^181Hf nucleus. A 1585 MeV ^238U beam from ATLAS was incident on a thick target of enriched ^180Hf, the most neutron-rich stable isotope. The beam was swept at two different time intervals (8.25 \mus on/16.5 \mus off and 2 ms on/4 ms off). With the exception of a short prompt run, the majority of the data were collected in the beam-off intervals with the Gammasphere array (98 HPGe and 3 LEPS). Delayed \gamma-\gamma-\gamma cubes were analyzed and relative time difference techniques were used to correlate \gamma-rays across known isomers with half-lives as long as 100 \mus. X-\gamma coincidences allowed additional Z-identification. Systematics of 3qp excitations in the A=180 region will be discussed.

Part W of program listing