Core-excitations in Po
Introduction
Over twenty years ago the Stockholm group established the high spin level scheme of Po up to an energy of 4354 keV, provided a shell model interpretation of the scheme and showed that the 31/2− isomer at 4266 keV was a core excited state (Bergström et al., Rensfelt et al. [1], [2]). Since that time, there has been no further attempt to investigate the scheme to higher excitation energies and establish the positions of other core excited states. This is surprising because Po, with two protons and one neutron hole outside the doubly closed shell Pb126 core is a particularly simple system and the results of multiparticle shell model calculations are readily compared to the experimental data. The magnetic moment of the 17/2− and 31/2− isomers were determined by Häusser et al. [3], and Rensfelt et al. [2] respectively, while the quadrupole moment of the 17/2− state was measured by Dafni et al. [4]. Martin [5] summarises all nuclear structure information on Po as known up to 1991. No reports relevant to the subject of the present work have been published since.
In this paper we report the extension of the high spin level scheme of Po to an excitation energy of 8391 keV and a spin of (47/2−). The results have been interpreted in terms of the multi particle shell model and the role played by core excited states along the yrast line has been determined. The data relevant to Po has been obtained by analyses of experimental results from an investigation of the high spin states of Po. That work has already been published [6] as have the results for the nuclei Pb and Hg (see Refs. [7], [8]) which were obtained from the same data set.
Section snippets
Experimental procedure
Details concerning the different experiments are summarized in the first paper in this series [7]. The prime aim of the experiments had been to investigate the high spin structure of Po using the Hg(Be, 5n)Po reaction. Following an excitation function measurement, a bombarding energy of 62 MeV, which optimized the population of Po, was chosen for all subsequent experiments. Pulsed beam–γ-timing and γ–γ-time measurements using the CAESAR array [9] were carried out as well as angular
Results and level schemes
The level scheme deduced from our work is given in Fig. 1 in which level energies are given to the nearest keV. For completeness, Table 1 lists all electromagnetic transitions (involving high spin states) assigned to Po from the present and previous work [1], [2], together with their assigned position and precise energies. Relative intensities observed in the present work together with the a2 coefficient in the Legendre polynomial expansion describing the angular distribution of each gamma
Empirical Shell Model (ESM) calculations
The Empirical Shell Model (see for instance, Refs. [6], [7], [10]) has been used to interpret the observed nuclear structure of Po. Calculations were carried out for two classes of states: those arising from the valence particles alone and those which occur as a result of core excitation. Fig. 5 illustrates the orbitals which were considered, while Table 4 lists the theoretical levels identified with those experimentally observed. Fig. 6 gives an overview of the calculated level energies
Conclusions
We have investigated the properties of high spin states of Po to an excitation energy of nearly 8400 keV and spins up to J=(47/2). Empirical Shell Model calculations have enabled us to understand the major features of the decay scheme and to confirm that core-excited states intrude onto the yrast line at an excitation energy of 4266 keV. Furthermore, they dominate the yrast sequence above this energy. The properties of the E3 transitions in Po which have been discussed above support the
Acknowledgements
The first author thanks the academic and technical staff of the ANU 14UD accelerator facility for willing assistance offered over the course of many visits and the staff at Laboratori Nazionali Legnaro for their patience with someone whose Italian should have been much better.
References (11)
- et al.
Nucl. Phys. A
(1976) - et al.
Nucl. Phys. A
(1983) Nuclear Data Sheets
(1991)- et al.
Nucl. Phys. A
(1997) - et al.
Nucl. Phys. A
(1994)
Cited by (0)
- 1
Visiting Scientist, Laboratori Nazionali Legnaro, 1998.
- 2
Joint appointment, Department of Physics, Faculties, Australian National University.