The calorimetric spectrum of the electron-capture decay of $^{163}$Ho. The spectral endpoint region

TL;DR

This paper explores the calorimetric spectrum of $^{163}$Ho decay, revealing a previously neglected process that could enhance detection rates and reduce pile-up issues in neutrino mass measurements.

Contribution

It introduces the significance of electron-capture accompanied by shake-off of a second electron in $^{163}$Ho decay, impacting experimental detection strategies.

Findings

Counting rate near the endpoint may be over ten times higher than expected

The shake-off process can significantly reduce pile-up problems

Implications for improved neutrino mass measurements

Abstract

The electron-neutrino mass (or masses and mixing angles) may be directly measurable in weak electron-capture decays. The favoured experimental technique is "calorimetric". The optimal nuclide is $^{163}$Ho, and several experiments (ECHo, HOLMES and NuMECS) are currently studying its decay. The most relevant range of the calorimetric-energy spectrum extends for the last few hundred eV below its endpoint. It has not yet been well measured. We explore the theory, mainly in the cited range, of electron capture in $^{163}$Ho decay. A so far neglected process turns out to be most relevant: electron-capture accompanied by the shake-off of a second electron. Our two main conclusions are very encouraging: the counting rate close to the endpoint may be more than an order of magnitude larger than previously expected; the "pile-up" problem may be significantly reduced.