# Nuclear resonant scattering experiment with fast time response: new scheme for observation of $^{229\rm m}$Th radiative decay

## TL;DR

This paper introduces a new nuclear resonant scattering scheme with a fast detector system to observe the radiative decay of $^{229}$Th's low-energy isomeric state, overcoming previous limitations in time resolution.

## Contribution

The authors developed a detector with 56 ps resolution and demonstrated its effectiveness using $^{201}$Hg, enabling more precise measurements relevant to $^{229}$Th research.

## Key findings

- Successfully fabricated a detector with 56 ps time resolution.
- Demonstrated clear NRS signals distinct from electronic scattering.
- Achieved threefold improvement in half-life measurement precision for $^{201}$Hg.

## Abstract

Nuclear resonant excitation of the 29.19-keV level in $^{229}$Th with high-brilliance synchrotron- radiation and detection of its decay signal, are proposed with the aim of populating the extremely low-energy isomeric state of $^{229}$Th.The proposed experiment, known as nuclear resonant scattering (NRS), has the merit of being free from uncertainties about the isomer level energy. However, it requires higher time resolution and shorter tail in the response function of the detector than that of conventional NRS experiments because of the short lifetime of the 29.19-keV state. We have fabricated an X-ray detector system which has a time resolution of 56 ps and a shorter tail function than the previously reported one. We have demonstrated an NRS experiment with the 26.27-keV nuclear level of $^{201}$Hg for feasibility assessment of the $^{229}$Th experiment. The NRS signal is clearly distinct from the prompt electronic scattering signal by the implemented detector system. The half-life of the 26.27-keV state of $^{201}$Hg is determined as 629 $\pm$ 18 ps which is better precision by a factor three than that reported to date.