The Application of Monochromatic Photoneutron Spectroscopy In An Examination of Nuclear Energy Levels of ¹⁸⁰Ta, ²⁰²Tl And ²⁰⁴Tl

Abstract

<p>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.</p> <p>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.</p>