Low-lying energy levels of 229Th35+ and the electronic bridge process

S G Porsev,M S Safronova,C Cheung

doi:10.1088/2058-9565/ac08f1

Abstract

The nuclear transition between the ground and the low-energy isomeric state in the 229Th nucleus is of interest due to its high sensitivity to a hypothetical temporal variation of the fundamental constants and a possibility to build a very precise nuclear clock, but precise knowledge of the nuclear clock transition frequency is required. In this work, we estimate the probability of an electronic bridge (EB) process in 229Th35+, allowing to determine the nuclear transition frequency and reduce its uncertainty. Using configuration interaction methods, we calculated energies of the low-lying states of Th35+ and determined their uncertainties. Our calculations showed that the transition energy from the J = 15/2 state to the ground state, 8.31 eV, is close to the central value of the experimentally determined nuclear isomer energy, 8.19 eV, and practically coincides with the upper edge value, 8.31 eV. It opens new possibilities for a more precise measurement of the nuclear isomer energy using an EB process.

Highlights

A specific feature of the 229Th nucleus is that the energy difference between the ground state and the first excited state is only several eV
The current most precise value differs from the result of 1994 by more than two times, but is in a good agreement with the result obtained in Ref. [5]
We consider the process of the excitation of the nucleus from the ground (g) to the isomeric state (m) by an electronic bridge process driven by a one-photon excitation of the electron shell

Summary

INTRODUCTION

A specific feature of the 229Th nucleus is that the energy difference between the ground state and the first excited state is only several eV. In such a way we were able to estimate the role of core-valence correlations, what allowed us to determine the uncertainties of the energies To carry out these calculations for such a complicated open-shell system as Th35+ we used a new parallel atomic structure code package developed and described in Ref. Using the energies obtained and assuming a resonant character of the induced EB process, we estimated its rate for several possible values of the nuclear isomeric energy Based on these calculations we conclude that modern facilities of EBITs and available ultra-violet lasers allow us to determine more precisely the nuclear isomeric energy and reduce its uncertainty

ELECTRONIC BRIDGE EXCITATION

Core-valence correlations

EB process excitation rate

Findings

CONCLUSION