Tibor Kibedi

Multi-quasiparticle and rotational structures in 179W: Fermi alignment, the ifK-selection rule and blocking

Abstract High-spin states in 179 W have been studied following the 170 Er(su13C, 4n) reaction. Rotational bands up to ifI 53 2 have been identified, based on 1-, 3-, 5- and 7-quasiparticle structures. Different alignment mechanisms compete in the generation of angular momentum at the yrast line. A Fermi-aligned if( i 13 2 ) 2 structure, coupled to high- K and to the if 7 2 − [514] orbital, forms the negative-parity yrast sequence above if I π = 31 2 − . This may be called a “t-band” since its description within the cranking model requires a “tilting” of the cranking axis. The anomalous decay of the if K π = 35 2 − 5-quasiparticle isomer is explained as arising from destructive interference of transition amplitudes coupling to the Fermi-aligned structure. Detailed analysis of the excitation energies of the multi-quasiparticle states indicates the quenching of both neutron and proton pair correlations, by comparison with blocking calculations.

Low-spin non-yrast states and collective excitations in 174Os, 176Os, 178Os, 180Os, 182Os and 184Os

Abstract Excited states in the range of isotopes 174, 176, 178, 180, 182, 184 Os, populated in the β-decay of 174, 176, 178, 180, 182, 184 Ir parent activities have been identified using γ-ray singles and coincidence techniques, utilising the high efficiency of a Compton-suppressed array, and conversion electron techniques. Many new states at low excitation energies have been identified, complementing the level schemes previously established from in-beam studies. The new states include excited 0 + states in 174 Os, 176 Os 178 Os and 180 Os. A large body of data on decay properties, spins and parities, and relative E2 and E0 matrix elements has been obtained. The systematics of the quasi-β and quasi-γ bands is discussed. Some detailed analyses in terms of the schematic band-mixing model are presented, incorporating reproduction of the yrare states in the fits. In particular, the proposed interpretation of the anomalous low-frequency alignment gains in the yrast positive-parity states, as a consequence of mixing with a low-lying intruder band, is confronted. The B (E0)/ B (E2) ratios are over-estimated by this model although their form (spin and isotope dependence) is reproduced. Better agreement is obtained within the IBM, which uses however reduced effective charges to match experiment. Absolute measurements of the B (E0) and B (E2) values may be necessary to distinguish between the models and test conjectures of shape differences.

K-forbidden transitions from multi-quasiparticle states

Abstract Four-quasiparticle, K π = 12 + states in 172 Hf and 178 W, with half-lives less than 2 ns, are found to decay to their respective K π = 0 + ground-state bands, in competition with transitions to intermediate- K , two-quasiparticle structures. All decay transitions are weakly hindered. When taken together with other K -isomer decay rates, an overall trend of decreasing hindrance with increasing excitation energy is evident. Estimates based on density-of-states considerations approximately reproduce the trend of the data, but also indicate the need to include other K -mixing effects.

Lens-mode operation of a superconducting electron spectrometer in (HI, xn) reactions

An electron spectrometer, consisting of a superconducting solenoidal magnet transporter and a Si(Li) detector, is described. The spectrometer has been used in lens mode, with an envelope baffle system and with the field swept under computer control. The efficiency obtained for the optimum energy at a given field is ∼ 5.9(5)%. Model calculations have been carried out to describe the transmission and efficiency of the swept lens, as well as the effect of the electron angular distribution on conversion-coefficient determinations. The performance of the spectrometer has been tested with radioactive sources and several in-beam experiments using (HI, xn) reactions.

Non-yrast states and shape co-existence in light Pt isotopes

Abstract Low-lying states in the even-even light platinum isotopes 176Pt, 178Pt, 180Pt and 182Pt have been populated using β+ /EC decay from parent gold nuclei, created in (HI,xn) reactions. State energies, spins and parities and γ-ray branching ratios were determined using γ-ray and electron spectroscopy. Whereas non-yrast states were observed in 178Pt, 180Pt and 182Pt, none were seen in 176Pt. The excitation energies of the observed states are analysed in terms of a band-mixing model, yielding the moments of inertia of the unperturbed bands. Branching ratios and ground-state-band quadrupole moments are calculated and compared with experimental values. The results indicate that the two lowest-lying 0+ states in each of the light Pt isotopes are formed from the mixing of two intrinsic states of different deformation, and other low-lying states can be described as admixtures of rotational states built on these intrinsic states, and on γ-vibrational states.

Incomplete fusion as a spectroscopic tool

Particle- coincidence studies have been a promising but not well exploited means of using incomplete fusion reactions to gain access to states in heavy nuclei near stability. At beam energies near the Coulomb barrier, fusion of the heavy fragment from break-up of the projectile with emission of non-equilibrated particles or other charged particles can be used to study relatively neutron-rich nuclei which cannot be reached by fusion, evaporation reactions with stable beams. Qualitative features which make the reactions attractive as a spectroscopic tool include a spin input which is higher than that achievable if fusion reactions were carried out with beams equivalent to the massive fragment and a correlation between the angle of emission of the light fragment and the number of evaporated neutrons which assists channel identification.

Spectroscopy of 175Ir and 177Ir and deformation effects in odd iridium nuclei

Abstract Excited states in 175Ir and 177Ir have been identified using (heavy-ion, pxn) reactions and γ-ray spectroscopic techniques. Rotational bands based on intrinsic states arising from the odd-proton parents, h 9 2 , h 11 2 , i 13 2 and d 5 2 were established to high spin except for the 5 2 + [402] bands. Only the h 9 2 bands show sharp alignment gains compatible with alignment of ( i 13 2 ) 2 neutrons. The smooth apparent alignments of the i 13 2 proton bands can be attributed, at least qualitatively, to a much larger deformation. The complex alignment gains observed in the h 11 2 bands in these and other iridium isotopes in the range 173–181Ir are consistent with the effects of mixing with a deformed intruder (equivalently, a low-spin shape change). This conjecture is tested against the in-band decay properties using a three-band model. Examination of the band structure suggests a significant gamma deformation at very low spin, before the change to a more deformed configuration occurs.

High-spin states and intrinsic structure in 174Os and 175Os: Alignments and strong interaction

Abstract High-spin states have been identified in 174Os and 175Os using (HI, x>n) reactions and gamma-ray techniques. Rotational bands built on low-lying intrinsic states in 175Os have been observed for the first time. The intrinsic states are identified as a mixed i 13 2 neutron state (nominally 7 2 + [633] ) and the 1 2 − [521] and 5 2 − [512] Nilsson configurations. Differences in the alignment of the two signatures of the decoupled band built on the 1 2 − [521] state and the comparison between the signature splitting of routhians and if B(M1)/B(E2) values in the i 13 2 band suggest softness towards gamma deformation. Crossing transitions between the negative-parity bands at intermediate spin have been observed and the interaction strength derived from the branching ratios. A comparison of the properties of bands in 174Os and 175Os shows some expected similarities, but also surprising differences which point towards the need for a more comprehensive treatment in strong interaction cases. Depending on the choice of reference the alignment in the i l3 2 band might show degenerate BC and AD crossings at a relatively low frequency. This may be consistent with the alignment observed in the negative parity side bands in 174Os.

Non-yrast states and shape co-existence in 172Os

Abstract Previous studies of 172 Os noted an anomaly in the behaviour of the moment of inertia of the yrast band at low spin. A phenomenological model of shape coexistence based on interacting rotational bands was proposed to explain this anomaly and this model predicted low-lying non-yrast states. In order to test these predictions, the β-decay of 172 Ir has been used to populate 172 Os. Excited states have been observed and classified into positive-parity “quasi-β” and “quasi-γ” bands and a negative-parity band. The energies of the quasi-β band states are seen to be in general agreement with the predictions of the phenomenological model and the model is refined to take into account the new data. The bands involved are determined to have significantly different moments of inertia.

Intrinsic states and collective structures in 181Ir

One- and three-quasiparticle states, and associated rotational bands have been identified in the odd-proton nucleus 181Ir. They were populated using the 169Tm(16O, 4n)181Ir reaction and established using a variety of time-correlated γ-ray and electron techniques. The 92−[514] and 52+[402] intrinsic states (from the h112 and d52 parents) were found to be metastable with mean of 193 and 430 ns, respectively. A third isomer, with Kπ = 232+ and a meanlife of 42 ns was identified and attributed to a one-quasiproton, two-quasineutron configuration, as were two other high-K band heads. The B(M1)B(E2) and gK − gR values extracted from the in-band decay properties of the observed rotational bands, based on these and other intrinsic states, were used to identify and test the proposed configurations. Interference effects were observed between states of the band assigned to the α = +12 signature sequence from the i132 configuration, with those from a second α = +12 sequence, and the states from one of the high-K bands (leading to complicated branchings in the decay scheme). The α = +12 sequence of the h92 proton band was extended to higher spins than known previously and its (unflavoured) α = −12 sequence identified. Both show alignments and alignment gains which are essentially well-behaved and consistent with a stable quadrupole deformation and small γ-asymmetry. In contrast, the h112 and d52 bands show complex alignment gains at low frequencies. These, and the in-band decay properties can be reproduced in a three-band model. However, essentially all bands, except for the h92 one-quasiparticle cases, exhibit an underlying alignment increase with frequency if reference parameters appropriate to the h92 band are used. Comparisons between the one-, two- and three-quasiparticle bands in which different orbitals are blocked, suggest the presence of alignment gains due to the h92 protons but conflicting evidence remains. Some of the alignment effects may be artefacts of larger deformations, or alternatively a reduction in pairing.