Speaker
Description
We investigate $^{207}$Tl, the one-proton-hole neighbor of doubly magic $^{208}$Pb — one of the most thoroughly studied doubly magic nuclei, whose first excited state is a well-established octupole phonon vibration. A deep-inelastic experiment has confirmed the existence of a 17/2$^+$ state in $^{207}$Tl [1], previously proposed theoretically to be a particle-octupole vibration coupling (POVC) between the $^{208}$Pb core and a proton hole ($\pi h_{11/2}^{-1} \times 3^-$) [2].
Even though a 17/2$^+$ state has been identified, its lifetime — and hence its B(E3) value — has not been measured. A measurement of the equivalent high-spin state in $^{207}$Pb revealed not only a strong coupling to the single phonon, but also indicated a component of coupling to the proposed 6$^+$ double-phonon state in the $^{208}$Pb core. Measuring the lifetime of the 17/2$^+$ state in $^{207}$Tl will contribute to a better understanding of the interplay between single-particle and collective nuclear excitations in this region of the chart of nuclides.
A Miniball plunger experiment (IS774) was performed at HIE-ISOLDE in July 2025, enabling lifetime analysis of excited states in $^{206}$Tl and $^{207}$Tl. We used a re-accelerated $^{206}$Hg beam impinging on a $^{7}$Li target, inducing the transfer reaction $^{7}$Li($^{206}$Hg,αxn). A Au degrader was employed as the plunger, with a CD+PAD Si detector telescope and the Miniball HPGe array used for particle and gamma-ray detection, respectively.
At the HIE-ISOLDE meeting we will present a report on the ongoing gamma-ray spectroscopy analysis of IS774.
[1] Wilson, E., et al. "Core excitations across the neutron shell gap in 207Tl." Physics Letters B 747 (2015): 88-92.
[2] Rejmund, M., et al. "Particle octupole-vibration coupling near 208Pb." The European Physical Journal A 8.2 (2000): 161-164.