Summary remarks and future prospects for on-line nuclear orientation

Abstract

Nuclear orientation (NO), lik~ other sub-specialty fields including perturbed angular correlations and Mossbauer effect, has existed for many years as a basic technique for studying hyperfine interactions, which is often rather loosely defined as the interaction of nuclei with their environment. These techniques thus have the potential to elucidate problems in nuclear physics and in solid-state physics. Indeed, early low temperature NO work in the mid-1950's revealed many new properties of paramagnetic salts and also established many nuclear spectroscopic parameters, including spins, moments, and radiation multipolarities of certain relatively simple and long-lived radioactive decay schemes such as B~ and S~n. (A review of the development of the field is given by Postma and Stone fl].) In the late 1950's two important advances occurred: the parity-violating B~ beta-decay experiment of Wu et al. [2], which showed that. NO could be used to attack nuclear problems of fun-~-am~ta] interest, and the discovery that ferromagnetic metals could be used as host lattices for NO, which greatly extended the range of orientable nuclei by replacing the need to incorporte them chemically into a paramagnetic salt with the need to introduce them physically as a component of a dilute alloy. The late 1960's saw two further technological developments--the dilution refrigerator and the Ge(Li) detector--that boosted the capability of NO as a nuclear spectroscopic tool. The NO investigator could now study decay schemes at high resolution and could do so for long periods, permitting even very weak transitions to be precisely measured. For more than a decade, NO laboratories continued to turn out precise and useful nuclear spectroscopy data, refining, elaborating, and often correcting previous knowledge, but it is clear that this work did not reveal any new or unexpected features of nuclear structure and that the frontiers of research of low-energy nuclear physics lay elsewhere. One such frontier is the study of the short-lived decays of nuclei far from stability. Following the development of isotope separators on-line with accelerators and reactors over a decade ago, studies of nuclei far from stability have provided many new insights and led to numerous discoveries in the shapes, motions and structure of nuclei. These discoveries include, for example, the coexistence of both near-spherical and deformed shapes in one nucleus, unexpected new regions of very strong deformation, the importance of reinforcing shell gaps for both protons and neutrons on the nuclear shape, and the importance of new shell gaps at different proton and neutron numbers on the structure of nuclei. There are other important discoveries in this field which do not bear on NO studies, such as proton radioactivity of nuclear ground states, beta-delayed twoand three-neutron decay and beta-delayed two-proton decay. The importance of these discoveries and the broad ranges of unexplored nuclear terrain still