Non-yrast spectroscopy and high-spin states of reflection-asymmetric Th222 (travel and subsistence)

The nucleus at the heart of every atom is made up of two types of particles - neutrons and protons. It is well known that nature tends towards symmetry, and for that reason it may at first be thought that the neutrons and protons should stick together to form a spherical ball of nuclear matter. In many cases this is correct - many atomic nuclei are indeed spherical, but generally it is not true. In general, there is a complex interplay of interactions between the neutrons and protons which conspire to cause the nucleus to take on a non-spherical shape. Deformed shapes of atomic nuclei have been known and studied for many years. Nuclei often take on what is known as a prolate shape, similar to a rugby ball, or an oblate shape, like squashed sphere. In some areas of the nuclear chart, the neutrons and protons interact in such a way as to make the nucleus reflection asymmetric, giving it a pear shape. Such pear-shaped or 'octupole deformed' nuclei occur in different regions across the nuclear chart, but the most reflection-asymmetric shapes are found in the light-actinide nuclei - in the light radium (Z=88), thorium (Z=90), and uranium (Z=92) nuclei. In analogy with reflection-asymmetric molecules such as HCl, reflection-asymmetric nuclei have rotational excitations which form a sequence of positive- and negative-parity states in a rotational-like structure. Furthermore, because the pear-shaped nucleus has a 'pointed 'end, the 'lightening-rod effect' comes into play: the charged particles in the nucleus (protons) tend to gather in the pointed end of the pear shape where the radius of curvature of the equipotential surface is smallest. This causes the centre of charge to be displaced from the centre of mass, giving rise to an intrinsic electric dipole moment, giving rise to strong electric-dipole gamma-ray transitions. These experimental signatures of pear-shaped nuclei are well defined and have been observed in around twenty nuclei, in the light actinides, where octupole correlations are at their strongest. In almost all cases, the spectroscopic knowledge extends to knowledge of yrast states in the ground-state rotational 'octupole' band. There are only very few cases in which non-yrast states have been observed, and in no cases in the even-even isotopes has a second octupole band been observed. It would therefore be very interesting to make a comprehensive non-yrast high-spin nstudy of nuclei in this region. There are several other motivations for studying nuclei in this region: firstly, there are longstanding predictions of highly-elongated ('superdeformed') structures in the light actinides, and secondly, a new interpretation of the structure of reflection-asymmetric nuclei in terms of the condensation of rotationally-aligned octupole phonons has recently been put forward. An UWS-led experiment has been successfully proposed at Argonne National Laboratory in the UWS, to study the high-spin excitations of 222Th, and neighbouring nuclei such as 220Ra and 219Ra. Following the reaction of a 18O beam on a 208Pb target, these nuclei will be formed, and their de-excitation gamma rays will be detected with Gammasphere. However, instead of producing the thorium and radium nuclei to be studied, it is far more likely that the interaction of the O18 beam on the Pb208 target will induce fission accompanied by multiple gamma rays. In order to select the reaction products of interest from the background due to fission, the Washington University high-efficiency recoil detector HERCULES will be used. The results of the experiment will enable the high-spin properties of Th222 and neighbouring nuclei to be studied, which will be of significant interest in physics on a worldwide scale, and will help maintain the UK's longstanding position at the forefront of gamma-ray spectroscopy.