High-K isomers: some of the questions

EPJ Web of Conferences, Jan 2016

High-K isomers exemplify the coexistence of individual-particle and collective motion in atomic nuclei. Here, the topic is briefly outlined, and some open questions are discussed. These include violations of the K quantum number; the high-spin limit to K isomerism; the fission stability of K isomers; possibilities for manipulation and control of K-isomer decay rates; and access to K isomers in neutron-rich nuclei.

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High-K isomers: some of the questions

EPJ Web of Conferences High-K isomers: some of the questions P. M. Walker 0 0 Department of Physics, University of Surrey , Guildford GU2 7XH , United Kingdom High-K isomers exemplify the coexistence of individual-particle and collective motion in atomic nuclei. Here, the topic is briefly outlined, and some open questions are discussed. These include violations of the K quantum number; the high-spin limit to K isomerism; the fission stability of K isomers; possibilities for manipulation and control of K-isomer decay rates; and access to K isomers in neutron-rich nuclei. 1 Introduction This conference celebrates the scientific life and work of George Dracoulis, who sadly passed away in 2014. I was privileged to be his first PhD student. During my PhD, we made an experimental study of high-spin states in 172Hf [ 1, 2 ], and we thus embarked on a life-long journey exploring long-lived, excited states in deformed nuclei, the so-called K isomers. Amongst our many joint publications are a 1999 Nature review of isomers [3]; a 2001 review focussed on high-K isomers [ 4 ]; and finally an in-depth isomer review, soon to be published [ 5 ], that Dracoulis worked on until shortly before he died. He also published other related reviews, notably his recent Nobel symposium paper [ 6 ], and a new and comprehensive K-isomer tabulation [ 7 ]. The present paper introduces a selection of open questions relating to K isomers, where much research remains to be done. As we develop our understanding and push towards the outer reaches of the nuclear chart, one of the experimental issues that needs to be kept in mind was stated by Dracoulis [ 6 ]: “There is always a problem to be aware of with the study of isomers ... and that is that the popular techniques for their identification can be compromised if the lifetimes become very long, as might occur, paradoxically, in the more interesting cases.” First, though, some terminology needs a brief explanation. The K quantum number represents the projection of the nuclear angular momentum onto its deformation axis (symmetry axis) with the deformed shape being prolate in the vast majority of cases. High K values can be made by broken-pair, deformation-aligned excitations, with each broken pair increasing the number of quasiparticles (unpaired nucleons) by two units. Electromagnetic transitions from (multi-)quasiparticle states are called “K forbidden” if the change in K exceeds the multipole order of the transition, i.e. if ΔK > λ. However, due to K-mixing mechanisms (rotational, vibrational or statistical [ 4 ]) such transition are hindered, rather than strictly forbidden. The degree of forbiddenness is defined as ν = ΔK − λ, and the reduced hindrance is expressed as fν = (FW )1/ν, where FW is the Weisskopf hindrance factor [ 4, 7 ]. In this way, fν represents the hindrance per degree of K forbiddenness, giving some measure of the effect of K-mixing processes, with large fν values corresponding to little K mixing. 2 Erosion of the K quantum number There is no strict definition of the half-life needed for a nuclear excited state to be termed an “isomer”. Nevertheless, it is clear that short half-lives generally correspond to low fν values. With germanium γ-ray detectors, it becomes difficult to determine half-lives of less than about 10 ns. One such case from the early work of Dracoulis and Walker [ 1, 2 ], alluded to above, is the half-life of the two-quasiproton, Kπ = 6+ isomer in 172Hf. The curve fitting necessary to obtain the 5 ns value is illustrated in Fig. 1. Perhaps surprisingly, after nearly four decades, this 172Hf isomer is still at the edge of accessibility, with regard to having a measureable half-life: the corresponding state in the lighter adjacent even-even isotope, 170Hf, only has a half-life limit, measured to be <5 ns [ 9 ]. The advent of fast-timing LaBr3, γ-ray detectors (see, for example, Ref. [ 10 ]) may well remove this impasse, and open the door to a range of shorter-lived K isomers. Nevertheless, as of now, there are five even-even hafnium isotopes with Kπ = 6+ isomers that have known half-lives, as given in Table 1. The most recently determined value is for 180Hf, now listed in the 2015 review of Kondev et al. [ 7 ]. As seen in Table 1, the half-lives range over almost four orders of magnitude, and the fν values for the E2 decay branches vary by a factor of six. Clues to the observed behaviour come from inspection of the dependence of fν on the product of the valence nucleon numbers, NpNn, revealing a strong correlation [ 11, 12 ], as illustrated in Fig. 2. (Note that the new fν value for 180Hf is in good accord.) Qualitatively, the behaviour seems to be reasonably simple: small values of NpNn correspond to weak collectivity, which can reasonably be associated with poor K conservation, hence small fν values. However, a quantitative understanding of this relationship remains elusive. In contrast, a recent analysis by Chen et al (...truncated)


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P. M. Walker. High-K isomers: some of the questions, EPJ Web of Conferences, 2016, 123, DOI: 10.1051/epjconf/201612301001