The electric quadrupole channel of the 7.8 eV $~^{229}\mathrm{Th}$ transition

TL;DR

This paper investigates the electric quadrupole (E2) decay channel of the 7.8 eV isomeric transition in thorium-229, revealing its significant role in nuclear-electronic interactions and challenging previous assumptions that neglected E2 contributions.

Contribution

It provides the first detailed analysis of the E2 component's impact on nuclear coupling to atomic shells in $^{229}$Th, highlighting its importance in specific electronic transitions.

Findings

E2 contribution is dominant or comparable to M1 in certain atomic orbitals.

E2 dominates in transitions involving 7p, 6d, and 5f orbitals.

M1 remains dominant in internal conversion of 7s and in 7s-7p electronic bridge.

Abstract

The unique isomeric transition at 7.8 eV in $~^{229}\mathrm{Th}$ has a magnetic dipole ($M1$) and an electric quadrupole ($E2$) multipole mixing. So far, the $E2$ component has been widely disregarded. Here, we investigate the nuclear physics nature and the impact of the $E2$ decay channel for the nuclear coupling to the atomic shell based on the newest theoretical predictions for the corresponding reduced nuclear transition probabilities. Our results show that the contribution of the $E2$ channel is dominant or at least of the same order of magnitude for internal conversion or electronic bridge transitions involving the atomic orbitals $7p$, $6d$ and $5f$. Notable exceptions are the internal conversion of the $7s$ electron and the electronic bridge between the electronic states $7s$ and $7p$, for which the $M1$ component dominates by two to three orders of magnitude. Caution is therefore advised when considering isomeric excitation or decay via nuclear coupling to the atomic shell, as the involved orbitals determine which multipole transition component dominates.