M.P. Carpenter

Structure of 52,54Ti and shell closures in neutron-rich nuclei above 48Ca

Abstract The level structure of 5422Ti32 has been explored for the first time by combining β-decay measurements from fragmentation products with prompt γ-ray spectroscopy following deep inelastic reactions. The latter technique was also instrumental in tracing 52Ti30 to higher spin. The data provide new tests of effective interactions for full pf-shell calculations in neutron-rich nuclei above 48Ca. The data indicate the presence of a significant subshell gap at N=32 and comparisons between theory and experiment suggest an additional shell closure at N=34 in Ca and Ti isotopes.

Single step links of the superdeformed band in Pb-194 : A measure of the absolute excitation energy, spin and parity of the superdeformed states

Abstract The EUROGAM array has been used to investigate the decay out of the yrast superdeformed (SD) band in 194 Pb. Six single step decays from the lowest observed SD states to low-lying states at normal deformation (ND) have been identified. From this observation, the excitation energy of the SD band in 194 Pb is established at 4877 ± 1.5 keV for the 6 + SD state. The most probable spins and parities of all members of the SD band are also deduced assuming that the SD states have even spin and positive parity.

Excited superdeformed bands in 191Hg

Abstract Two weakly populated rotational bands have been observed in 191 Hg with properties (energy spacings, moments of inertia and lifetimes) very similar to those of the previously reported superdeformed band. Based on cranked Woods-Saxon calculations, these structures are interpreted as the first excited bands in the superdeformed minimum of 191 Hg. Comparisons between the data and the calculations highlight the role of specific orbitals at large deformations.

OPENING UP THE A 180 K-ISOMER LANDSCAPE : INELASTIC EXCITATION OF NEW MULTI-QUASIPARTICLE YRAST TRAPS

Abstract New high-spin isomers in 175 Lu, 181 Ta and 186 W target nuclei have been strongly excited by pulsed 238 U beams. The use of inelastic excitation opens up an uncharted region in the neutron-rich K-isomer landscape. First results establish the key role of multi-quasiparticle configurations in the high-angular-momentum structure.

Direct evidence of octupole deformation in neutron-rich 144Ba

The neutron-rich nucleus ^{144}Ba (t_{1/2}=11.5u2009u2009s) is expected to exhibit some of the strongest octupole correlations among nuclei with mass numbers A less than 200. Until now, indirect evidence for such strong correlations has been inferred from observations such as enhanced E1 transitions and interleaving positive- and negative-parity levels in the ground-state band. In this experiment, the octupole strength was measured directly by sub-barrier, multistep Coulomb excitation of a post-accelerated 650-MeV ^{144}Ba beam on a 1.0-mg/cm^{2} ^{208}Pb target. The measured value of the matrix element, ⟨3_{1}^{-}∥M(E3)∥0_{1}^{+}⟩=0.65(+17/-23) eb^{3/2}, corresponds to a reduced B(E3) transition probability of 48(+25/-34)u2009u2009W.u. This result represents an unambiguous determination of the octupole collectivity, is larger than any available theoretical prediction, and is consistent with octupole deformation.

First evidence for triaxial superdeformation in 168Hf

Abstract Three superdeformed (SD) bands have been found in 168Hf. Lifetime measurements revealed a large quadrupole moment, Q t ∼11.4xa0e b , for the strongest band (TSD1). Theoretical calculations using the Ultimate Cranker code predict high-spin SD minima (ϵ2∼0.43) with stable triaxial deformations of γ∼+20° and γ∼−15°. The measured Qt value suggests that band TSD1 corresponds, most likely, to a deformation with a positive γ value. This constitutes the first evidence for triaxial superdeformation in an even proton system.

Identification of excited states in 140Dy.

Excited structures in the proton-rich nucleus 140 Dy have been established following the decay of an 8 − isomer. The excitation energy of the isomer is established to be 2.16 MeV with a half-life of 7.3 ± 1.5 µs. The isomer decays into the yrast line at the 8 + state, revealing a rotational band with a deduced deformation of β2 = 0.24(3). The isotope 140 Dy is the daughter of the deformed proton emitter 141 Ho. The new information obtained here supports the role of deformation in proton emission and the previous assignments of single-particle configurations to the two proton emitting states in 141 Ho. In addition, the reduced hindrance factor measured for the isomer is consistent with the trend observed in the N = 74 isotones. uf6d9 2002 Elsevier Science B.V. All rights reserved.

Spin ∼60 ℏ in 156Dy: competition between collective and terminating structures at very high-spin

AbstractThe highest-spin discrete states x0e.I;60 and E ;30 MeV in normal deformed nuclei have been observed in the x rare-earth isotope Dy using the Sn S,4n reaction in conjunction with the GAMMASPHERE spectrometer. The 156 124 x0e 36 .positive parity yrast sequence appears to evolve smoothly from a prolate collective towards an oblate non-collectivex0e. x0e .shape, in contrast to the sudden shape change observed in the isotone 158 Er. Terminating states are identified in the negativeparity sequences at I p s52 y and 53 y and specific multi-particle-hole configurations are assigned by comparison withcranked Nilsson-Strutinsky calculations. An order of magnitude increase in the interaction strength between close lyinghigh-spin weakly collective structures is determined compared with that found in the lower-spin strongly collective domain.These results give valuable insight into the interplay between collective and terminating structures, and their competition foryrast status in the 40y60 spin range. q1998 Elsevier Science B.V. All rights reserved.

Feeding and decay of superdeformed states

Abstract The mechanisms for feeding and decay of superdeformed (SD) bands are examined. Data connected with both processes in 192 Hg are compared with model calculations. The calculations successfully reproduce the data, suggesting that the mechanisms for both processes are understood. Constraints on the energy of the SD band energies and on the well-depths at low and high spins have been obtained. At the point of decay around spin 10, we suggest that the SD band 1s 3.3–4.3 MeV above the normal yrast line and that the well depths at spin 10 and 40 are 0.5–1.3 and 3.5–4.5 MeV, respectively.

Lifetimes of the superdeformed band in 192Hg

Lifetimes of states in the superdeformed band in 192Hg have been measured using the Doppler-shift attenuation and the recoil-distance methods for the high-spin and the low-spin regions, respectively. Line shapes and decay curves were obtained from high-fold γ-ray coincidence spectra measured with the EUROGAM array. The transition quadrupole moments, derived from the lifetimes, are constant with an average value of Qt = 18.6 b down to the bottom of the band where it decays to the normal states. This suggests that the superdeformed potential-energy minimum is still present at that point and that the decay proceeds by tunneling.