J. C. Waddington

Gamma-ray spectroscopy of 126Ba

Abstract States of 126Ba up to spin 36+ were populated in the reaction 96Zr(34S, 4n)126Ba at 155 MeV and up to spin 20+ in the reaction 116Sn(13C, 3n)126Ba at 56 MeV. Gamma-ray spectroscopy was performed with the 8π spectrometer, an instrument comprising 20 Campton-suppressed HPGe detectors and 71 BGO ball elements. A level scheme organized into fifteen rotational bands is proposed on the basis on the γγ-coincidence and γ-ray angular correlation data. The competing π h 11 2 and v h 11 2 band crossings are investigated and interpreted in terms of cranked shell model and total routhian surface calculations. Amongst the topics discussed are (i) a comparison of DCO and spin orientation techniques for determining spins and multipolarities. (ii) the addivity property of quasiparticle energies or routhians, and (iii) analysis of B(M1)/B(E2, ratios between signature partner bands. The possibility of couplings between vibrational and aligned quasiparticle structures is explored.

Collective properties of 48Cr at high spin

Abstract High-spin states of the nucleus 48 Cr have been studied via particle-γ-γ spectroscopy, following the 28 Si( 28 Si,2α) reaction. A 44-element particle-detector array was used to isolate 48 Cr residues and to reduce γ-ray Doppler broadening. The collective band built upon the ground state has now been firmly established to spin 16 + , the highest possible in the isolated f 7 2 shell, and lifetimes of the four highest states have been measured from Doppler shifts. Although some of the ground-state band properties are well reproduced by recent fp -shell model calculations, a sharp reduction in E2 transition rates at the backbend is not.

Studies of superdeformation in the gadolinium nuclei

Abstract We have used the 8π γ-ray spectrometer at the Chalk River TASCC facility to study superdeformed rotational bands in the chain of isotopes 145–149 Gd. The five bands already known have been extended to higher spin, while four new excited bands have been discovered. The use of very similar reactions, data-acquisition parameters and data-analysis techniques, combined with the enlarged set of data, has allowed us for the first time to approach the topic of superdeformation in the A = 150 mass region from a comprehensive point of view. Transition energies of neighbouring nuclei were compared to extract effective aligned spins of the added particle. Such a comparison requires knowledge of relative nuclear spins, which we have to assume. Nevertheless, we find that a very illustrative picture emerges making it possible to understand all nine bands in the Gd isotopes within a very simple scheme of orbital assignments.

High-spin states in 183Pt

Abstract High-spin states in 183 Pt have been studied for the first time using the reactions 154 Sm( 34 S, 5n) and 170 Yb( 16 O,3n). Rotational bands built on the Nilsson configurations 1 2 − [521], 7 2 − [514] and 9 2 + [624] were observed up to spin values of 39 2 − 49 2 h . Quasiparticle alignments and band crossing frequencies were investigated in these bands. A large signature splitting was observed in the νi 13 2 - band structure . The experimental results were compared with total routhian surface calculations, in which the shape of the nucleus could be followed as a function of rotational frequency for different quasiparticle configurations.

Rotational bands and shape changes in 124Ba

Abstract High-spin states in 124 Ba were populated via the 94 Zr( 34 S, 4n) 124 Ba and 110 Cd( 16 O, 2n) 124 Ba reactions at 145 MeV and 60 MeV, respectively. The yrast band has been extended to 34 ħ and seven side bands have been observed. Gamma-ray angular distributions with respect to the nuclear spin axis have been used along with directional correlation (DCO) ratios to establish spins and multipolarities. The competing π h 11 2 and ν h 11 2 crossings in this region are investigated and an interpretation of the observed crossings is proposed within the framework of the cranked shell model (CSM) and total routhian surface (TRS) calculations.

Channel selection for high spin γ-ray spectroscopy studies via total energy measurements in fusion-evaporation reactions

Abstract A channel selection method for high spin γ-ray spectroscopy studies based on the measurement of the total energy of all radiations (both charged particle and γ-ray) emitted in heavy-ion fusion reactions is presented. The method is applicable to all reactions in which charged-particle evaporation from the compound system dominates, and is particularly effective in isolating the weakly populated low particle multiplicity channels that leave the final nucleus with the greatest spin and excitation energy. The method is illustrated using data taken with the 8π γ-ray spectrometer and the miniball 4π charged-particle detector array at the Chalk River Tandem Accelerator Superconducting Cyclotron (TASCC) facility. Channel-to-total ratios are improved over those obtained with charged-particle detection alone by factors as large as 46 without significant loss of statistics for the selected channel.

Dynamic moment of inertia of the 192Hg superdeformed band at high rotational frequencies

Abstract The superdeformed band in 192Hg has been extended to higher transition energies from a new analysis of a large set of double and triple coincidence data. Contrary to the results of cranked shell model calculations including monopole pairing, the dynamic moment of inertia I (2) is found to continue to increase with rotational frequency.

Collective γ-vibrational bands in 165Ho and 167Er

Abstract The nuclear structures of 165Ho and 167Er have been investigated by means of Coulomb excitation. These nuclei excited at moderate spins exhibit γ-vibrational bands with K π = 11 2 − , 3 2 − in 165Ho and K π = 11 2 + in 167Er. The γ-vibrational bands in 165Ho are found to be isospectral; heaving very nearly identical in-band γ-ray energies. Gamma-ray branching ratios are analysed to extract information on collectivity and Coriolis mixing. Experimental results are compared with calculations performed with the Cranked Shell Moedl + RPA + particle-vibration coupling and by invoking the generalized intensity relations (GIR) in the unified model scheme. Although this model explains many features of the data, puzzling aspects such as identical transition energies for the bands in 165Ho remain unexplained. The role of the K quantum number in identical bands is discussed.

The l-forbidden m1 decay of the 2522 keV level in 39K

Abstract The mean lifetime of the first excited 1 2 + level in 39 K has been measured to be τ = 93 ± 13 fs by high recoil velocity DSAM. The level was excited by bombarding a 4 He-implanted Ta target with a 100 MeV 39 K beam. The lifetime was determined by fitting the Doppler-broadened lineshape of the 2522 keV gamma-ray transition to the 3 2 + ground state. Together with the measured B (E2), the B (M 1; d 3 2 → S 1 2 ) for this l -forbidden M1 transition was determined to be 0.0100 ±0.0027 μ 2 N . A theoretical calculation, taking into account core polarization, delta excitation and mesonexchange current contributions, underestimates the B ( M )1) by more than a factor of six. The experiment also measured the lifetimes of the 3019 keV and 3883 keV levels to be 20 ±4 fs and 11 ± 6 fs, respectively.

Properties of superdeformed bands in 153Dy

Two new superdeformed (SD) bands in Dy-153, in addition to the three previously observed, have been studied with the GAMMASPHERE detector array. Assignments to single-neutron orbits in the SD potential are made based on the band properties. Evidence for a band interaction at very high spins, possibly involving the first N = 7 proton intruder orbital, is observed in the strongest populated SD band. Furthermore, evidence far a Delta I = 2 staggering is also found in this SD band.