J.C. Willmott

Lifetimes of high spin rotational states

Abstract Lifetimes of 8 + , 10 + and 12 + states in fourteen rotational and quasi-rotational doubly even nuclei have been measured with a Doppler broadened lineshape technique. In addition, the lifetime of the 14 + state in 164 Dy has been determined. The B (E2) values have been extracted and compared with rotational model predictions. All the 8 + → 6 + and 10 + → 8 + transitions and most of the 12 + → 10 + transitions are well described by the model. However, the 12 + → 10 + transitions in 162 Dy, 164 Er and 170 Yb are found to be retarded significantly. It is Suggested that the hindrances may be related to the known backbending properties of two of these nuclei.

Study of 170,171,173Ta isotopes at high spinṡ

Abstract High-spin data are presented for 170 Ta and 171 Ta for the first time, and the known level scheme of 173 Ta is extended to higher spins. Anomalies found in the crossing frequencies for the alignment of a pair of neutrons are considered. At high spin a large and constant value of J (2) , with J (2) > J (1) , is found for two 171 Ta bands and the implications of this are discussed. The π i 13/2 [660]1/2 + , α = + 1/2 orbital is identified as it crosses the rotational band built on the [402]5/2 + , α = + 1/2 orbital in 171 Ta. Magnetic transition probabilities obtained from measured branching ratios show the effects of the aligning pair of neutrons and the spectator orbital. The data are interpreted using cranked shell-model calculations and a semiclassical vector-coupling scheme developed by Donau and Frauendorf.

Configuration dependent pairing from band crossing frequencies

Abstract Data are presented on the crossing frequencies of odd-parity rotational bands in 161 Er. The crossing frequencies are shown to be correlated with the sign and magnitude of the quadrupole moments of the valence neutron orbits in odd- N rare earth nuclei. This can be understood in terms of configuration-dependent “blocking” of pairing-energy contributions requiring the inclusion of quadrupole terms in the pairing-correlation energy.

High-spin, discrete line spectroscopy of 162Yb and 164Hf and the systematics of N = 92 isotones

Abstract The high spin states of 162 Yb and 164 Hf have been studied using 122 Sn( 44 Ca, 4n) and 120 Sn( 48 Sn( 48 Ti, 4n) reactions respectively. The resulting new level schemes for 162 Yb and 164 Hf are presented and compared with existing data for 160 Er. The systematic behaviour of these three N = 92 isotones at large angular momentum yields information both on modifications of the single-neutron spectrum of states resulting from the polarisation of the nuclear field and on the single-proton spectrum of states.

A study of the energy levels of 75Se

Abstract A level scheme has been established for 75Se by observing the γ-rays and internal conversion electrons following the decay of 75Br, and spins and parities have been assigned to some of the states. A search has been made for a low-lying 1 2 − state which systematically occurs in odd-A nuclei with N between 39 and 47, but no candidate has been found. It has been demonstrated that a doublet occurs at 286.5 keV, and that the two components probably possess J π = 3 2 − and 5 2 − .

Low-lying positive-parity states in 71Ge

Abstract The 20 ms 71 Ge isomeric activity has been positively identified with a 198.4 keV 9 2 + level, by measuring the conversion coefficient of the transition involved. The energies and relative intensities of the γ-rays following the decay of 71 As have been measured, and log ft values calculated. A level scheme for 71 Ge is proposed and compared with the schemes deduced from the (d, p) scattering measurements of Goldman and the (p, nγ) measurements of Malan. It is deduced that the first anomalous positive-parity state in 71 Ge is the 5 2 + 525.1 keV level.

The decay of 73Br

Abstract The energies and relative intensities of the γ-rays following the radioactive decay of 3.3 min 73Br produced by the 59Co(16O, 2n) reaction have been measured using Ge(Li) detectors. No isomeric transition with half-life greater than 1 ms has been observed in 73Br. A level scheme for 73Se is proposed. The results are discussed in the light of observed correlations of the variations in the B(E2) values and the excitation energies of the first 2+ states in doubly even nuclei and the excitation energies of the first 9 2 + states in odd-proton nuclei in the region Z, N = 28 and Z, N = 50.

The decay of 73Se

Abstract The energies and relative intensities of the γ-rays following the radioactive decay of 7.1 h 73Se and 42 min 73mSe produced by the 70Ge(α, n) reaction have been measured using Ge(Li) detectors. The excitation functions of the 11 most prominent γ-rays following the 73Se decays have been studied. A level scheme for 73As is proposed and compared with those of 75As and 75As.

An isomeric state in 134Nd

Abstract States of 134 Nd have been excited by the reaction 118 Sn( 20 Ne, 4n), and the first four members of a band based on the ground state have been identified. An isomeric state at 2294 keV with a half-life of 410 μs has been observed. This is found to decay to the 8 + member of the ground-state band by an E1 transition which is hindered by 10 10 and is shown to possess J π = 8 − . A weak M2 transition with a hindrance of 10 7 is observed to the 6 + member. Isomerism involving an 8 − state is shown to be a common feature of N = 74 isotones, and a possible two-neutron quasiparticle configuration is identified.

High-spin states in 172Hf

The reaction 128Te(48Ca,4n) has been used to extend the yrast sequence of 172Hf up to Ipi =30+. In addition, two negative-parity sidebands have been observed to Ipi =24- and 25-. The aligned angular momentum of these bands is seen to increase approximately linearly with rotational frequency at high frequencies.