A search for rare and induced nuclear decays in hafnium

TL;DR

This study used a specialized ultra-low-background detector to search for rare nuclear decays and dark matter interactions in hafnium, setting new limits on decay half-lives and dark matter-nucleon cross sections.

Contribution

It provides the first limits on alpha decays, double electron capture, and dark matter-induced excitations in hafnium isotopes using a novel detection setup.

Findings

Limits on alpha decays of Hf isotopes to excited states ($10^{16}$-$10^{18}$ a).

Constraints on double electron capture and electron capture with positron emission ($10^{16}$-$10^{18}$ a).

New bounds on dark matter-induced nuclear excitations for 1 TeV/$c^2$ dark matter.

Abstract

A measurement of hafnium foil using a modified ultra-low-background high purity detector with optimized sample-to-detector geometry was performed at Laboratori Nazionale del Gran Sasso. Radiopurity of the stock Hf foil was studied in detail, in addition to an analysis of data collected over 310 days to search for rare processes that can occur in natural Hf isotopes. Firstly, limits on alpha decays of all natural Hf isotopes to the first excited state of the daughter nuclides were established in the range of $10^{16}$-$10^{18}$a (90% C.L.). Secondly, a search for modes of double electron capture and electron capture with positron emission in $^{174}$Hf was performed, yielding half-life limits $10^{16}$-$10^{18}$a (90% C.L.). Lastly, novel dark matter-induced nuclear excitations in hafnium isotopes were investigated. For dark matter with 1 TeV/$c^2$ mass, leading limits on the inelastic dark matter--nucleon cross section are set for mass splittings in the range 428 keV $< \delta <$ 473 keV.