Measurement of the $^{229}$Th isomer energy with a magnetic micro-calorimeter

TL;DR

This paper reports a highly precise measurement of the $^{229}$Th isomer energy using a cryogenic magnetic micro-calorimeter, improving accuracy over previous gamma spectroscopy methods.

Contribution

The study introduces a novel application of a cryogenic magnetic micro-calorimeter to measure the $^{229}$Th isomer energy with unprecedented precision.

Findings

Isomer energy measured as 8.10(17) eV

Branching ratios of excited states determined as 9.3(6)% and 0.3(3)%

Measurement agrees with recent electron-based results.

Abstract

We present a measurement of the low-energy (0--60$\,$keV) $\gamma$ ray spectrum produced in the $\alpha$-decay of $^{233}$U using a dedicated cryogenic magnetic micro-calorimeter. The energy resolution of $\sim$$10\,$eV, together with exceptional gain linearity, allow us to measure the energy of the low-lying isomeric state in $^{229}$Th using four complementary evaluation schemes. The most accurate scheme determines the $^{229}$Th isomer energy to be $8.10(17)\,$eV, corresponding to 153.1(37)$\,$nm, superseding in precision previous values based on $\gamma$ spectroscopy, and agreeing with a recent measurement based on internal conversion electrons. We also measure branching ratios of the relevant excited states to be $b_{29}=9.3(6)\%$ and $b_{42}=0.3(3)\%$.