Date of Award

11-1982

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Supervisor

Professor T.J. Kennett

Abstract

Monochromatic photons from in-core neutron capture sources of aluminum, copper, iron, nickel and chromium were used to initiate the (Υ,n) reaction on the ¹⁸¹Ta, ²⁰⁵Tl and ²⁰³Tl nuclei. The use of this method allows precise measurement of the Q values for the ¹⁸¹Ta(Υ,n) ¹⁸⁰Ta (7579 ± 2 Kev) and the ²⁰⁵Tl(Υ,n)²⁰⁴Tl (7549 ± 1 Kev) reactions. Comparison of the ¹⁸¹Ta(Υ,n) data with the ¹⁸⁰Ta(n,Υ) data confirms the existence of an isomeric state in ¹⁸⁰Ta at 73 ± 2 Kev. Over twenty previously unobserved levels in ¹⁸⁰Ta were populated with typically ²Kev precision in energy determination. Well-documented levels ²⁰⁴Tl were observed to also be populated.

The total (Υ, cross sections were measured and the gamma ray strength functions were deduced for the stronger (Υ,n) channels. The small incident gamma ray energy spread of a few ev allows excitation of individual excited state resonances in ²⁰⁵Tl, while averaging over many compound resonances in ¹⁸¹Ta is observed. The number of degrees of freedom for a Porter-Thomas distribution of reduced widths was also estimated and gave some credence to the deduced Υ-ray strength functions. Attempts to assign spin and parity in the residual ¹⁸⁰Ta and ²⁰⁴Tl nuclei were thwarted by the assumption of a dominant electric-dipole absorption cross section, and by the half-integer ground state spins of the ²⁰⁵Tl and ¹⁸¹Ta targets.


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