Date of Award

1994

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Edward F. Zganjar

Abstract

Nuclei in the neutron deficient Z $\le$ 82 region have attracted considerable attention since research at UNISOR first observed the nearly degenerate nuclear shape coexistence in $\sp{184-188}$Hg. The deformed prolate shapes in this region are believed to result from a proton pair excitation into the $h\sb{9/2}$ shell model orbital which lies above the closed proton shell at Z = 82. In the region far from stability (where the neutron shell is half filled), the degeneracy of the shape coexistence is maximum. These prolate deformed intruder states in the Hg isotopes have been identified in radioactive decay studies for neutron numbers 104 to 108. It has been demonstrated that enhancement of E0 transitions between coexisting shapes is a common feature and that a measure of this enhancement can be used to investigate the origins of nuclear shape coexistence. In $\sp{186}$Hg and $\sp{188}$Hg, E0 transitions that connect the coexisting shapes have been identified, but no determination of the mixing between these configurations was done since the measurement of the lifetime of these states is a difficult experimental problem. That difficulty was overcome in this work however, and lifetime measurements were carried out in $\sp{186}$Hg and $\sp{188}$Hg. The data show that mixing of these shape coexisting configurations is quite large at the mid-neutron shell and decreases with departure from that point. Theoretical predictions do not indicate the existence of this deformed configuration in Hg isotopes heavier than $\sp{188}$Hg. Experiments carried out in this work, however show a weakly populated intruder band in $\sp{190}$Hg which then disappears or changes character in $\sp{192}$Hg. Superdeformed bands in Hg isotopes have been identified in several Hg isotopes by in-beam spectroscopy, but have never been placed in the energy level scheme. In beta decay, based on energy and angular momentum considerations, it may be possible to populate the superdeformed bands and precisely locate them in the energy level schemes. A new technique, utilizing internal pair formation, was developed to provide greater sensitivity. While superdeformed states were not observed, the data enable one to set sensitive limits on the possible population of the superdeformed states in $\sp{190,192}$Hg from $\sp{190,192}$Tl beta decay.

Pages

163

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