On-line nuclear orientation studies of neutron deficient Te, I and Cs isotopes

On-line nuclear orientation at low temperature has become an important technique for the study of nuclei far from stability, through measurements of nuclear moments and other quantities of spectroscopic interest. The theory of low temperature nuclear orientation and its application to the study of nuclear structure are reviewed. Of particular importance to the on-line measurement, in which a wide range of short-lived nuclei are available for study, is the question of how fast these nuclei can be cooled to the lattice temperature, and thus oriented. To address this, the theory of nuclear spin-lattice relaxation, relevant to the online technique, is outlined. In particular, quantitative methods to deal with cases in which the spin-lattice relaxation time is comparable with the isotope half-life have been developed and applied. One of the major current interests in nuclear structure physics is to investigate how the neutron-proton interaction influences the structure of nuclei that are transitional, between well established regions of spherical and deformed nuclei. In such nuclei, intruder excitations, which signal the onset of deformation, are observed low in energy. Using the Daresbury on-line isotope separator, an extensive study of the decay of ¹¹⁸I to ¹¹⁸Te has been performed using nuclear orientation techniques, combined with γ - γ and conversion electron spectroscopy measurements. Interpretation of the results obtained for ¹¹⁸Te within the framework of IBM-2, gives strong evidence for the existence of such an ,em>intruder configuration in this nucleus. On-line experiments have also been performed in which a range of neutrondeficient Cs nuclei has been oriented for the first time. In these measurements the hyperfine field of CsFe has been determined as (+)40.8(7) T, and also the Korringa constant for the system ¹²¹Cs^mFe has been measured (using a new technique) to be C_k = 0.059(l6)sK. These results have been applied to the case of ¹¹⁸Cs^m, for which the magnetic moment has been measured to be 5.4(1.1)nm. This large value clearly indicates the presence of the [404]9/2 orbital, which provides further evidence for the existence of intruder states in this region.