Emergence of an island of extreme nuclear isomerism at high excitation near $^{208}$Pb

TL;DR

This paper reports the discovery of exceptionally long-lived metastable nuclear states at high excitation energies near $^{208}$Pb, revealing an island of extreme nuclear isomerism that challenges existing nuclear models.

Contribution

It identifies and characterizes the longest-lived high-energy nuclear isomers near $^{208}$Pb, highlighting a new region of extreme nuclear isomerism.

Findings

Metastable states with ms-range half-lives at ~8 MeV excitation energy.

Longest-lived states above 7 MeV identified in the nuclear chart.

Provides data to test and refine nuclear shell-model interactions.

Abstract

Metastable states with T$_{1/2}$ = 8(2) ms in $^{205}$Bi and T$_{1/2}$ = 0.22(2) ms in $^{204}$Pb, with $\approx $ 8 MeV excitation energy and angular momentum $\ge $ 22 $\hbar $, have been established. These represent, by up to two orders of magnitude, the longest-lived nuclear states above an excitation energy of 7 MeV, ever identified in the nuclear chart. Additionally, the half-life of the 10.17 MeV state in $^{206}$Bi has been determined to be 0.027(2) ms, the next highest value in this highly excited regime. These observations indicate the emergence of an island of extreme nuclear isomerism arising from core-excited configurations at high excitation in the vicinity of the doubly closed-shell nucleus $^{208}$Pb. These results are expected to provide discriminating tests of the effective interactions used in current large-scale shell-model calculations.