Prospects for measuring the 229Th isomer energy using a metallic magnetic microcalorimeter

TL;DR

This paper explores the potential of advanced metallic microcalorimeters to precisely measure the energy of the Thorium-229 nuclear isomer, aiming to improve accuracy over previous indirect methods.

Contribution

It proposes using state-of-the-art magnetic microcalorimeters to directly resolve the isomer energy through gamma-spectrum analysis, enhancing measurement precision.

Findings

Potential to resolve the 29.18 keV doublet as a doublet line

Analysis of how resolution and count rates affect measurement accuracy

Feasibility of direct isomer energy measurement with improved precision

Abstract

The Thorium-229 isotope features a nuclear isomer state with an extremely low energy. The currently most accepted energy value, 7.8 +- 0.5 eV, was obtained from an indirect measurement using a NASA x-ray microcalorimeter with an instrumental resolution 26 eV. We study, how state-of-the-art magnetic metallic microcalorimeters with an energy resolution down to a few eV can be used to measure the isomer energy. In particular, resolving the 29.18 keV doublet in the \gamma-spectrum following the \alpha-decay of Uranium-233, corresponding to the decay into the ground and isomer state, allows to measure the isomer transition energy without additional theoretical input parameters, and increase the energy accuracy. We study the possibility of resolving the 29.18 keV line as a doublet and the dependence of the attainable precision of the energy measurement on the signal and background count rates and the instrumental resolution.