Identification of new isomers in ^{228}Ac: impact on dark matter searches

K.w Kim ,N Carlin,W K Kim,R Maruyama ,E J Jeon,D S Leonard,H Kim,R L C Pitta,S Choi,A C Ezeribe,Y H Kim,E K Lee,K S Park,S H Lee,J Y Lee,H Prihtiadi,W G Thompson,S J ,N J C Spooner,C Rott,W G Kang,L E França,E Barbosa De Souza,B J Park,Y D Kim,J J Choi,H S Park,C Ha,L Yang,I S Lee,S L Olsen,I S Hahn,H K Park,J H Jo,H J Kim,Prabal Adhikari ,S M Lee,Keon Ah Shin ,Hyun-Woo Joo ,H Y Lee,Mitra Djamal ,M Kauer,S K Kim,H S Lee,H Lee,B B Manzato,Robert J Neal ,A Scarff,S H Kim,Y J Ko,M H Lee,J Lee,G H Yu

doi:10.1140/epjc/s10052-021-09473-2

Abstract

We report the identification of metastable isomeric states of ^{228}Ac at 6.28 keV, 6.67 keV and 20.19 keV, with lifetimes of an order of 100 ns. These states are produced by the beta -decay of ^{228}Ra, a component of the ^{232}Th decay chain, with beta Q-values of 39.52 keV, 39.13 keV and 25.61 keV, respectively. Due to the low Q-value of ^{228}Ra as well as the relative abundance of ^{232}Th and their progeny in low background experiments, these observations potentially impact the low-energy background modeling of dark matter search experiments.

Highlights

Background modelingC2 0.034 ± 0.010 0.032 ± 0.011 C3 0.017 ± 0.005 0.029 ± 0.010 C4 0.024 ± 0.007 0.012 ± 0.004 C6 0.008 ± 0.002
We report the identification of the isomeric states in 228 Ac using the COSINE-100 dark matter search detector [21]
We have developed an offline selection algorithm to identify the two-pulse events using the summed waveforms from the two photomultiplier tubes (PMTs) of each crystal

Summary

Introduction

Numerous astronomical observations support the conclusion that most of the matter in the universe is invisible dark matter, an understanding of its nature and interactions remains elusive [1,2]. The recently observed event excess seen by XENON1T [11] can be explained by new physics interactions and by tritium (3 H) contamination, which undergoes a low-energy β-decay (Q-value 18.6 keV, half-life t1/2 = 12.3 years) Because of their long half-lives and large natural abundances, contamination from 238 U and 232 Th as well as their progenies are significant issues for low-background dark matter search experiments [14,15]. The data are consistent with the identification of both the 6.28 keV and 20.19 keV excited states of 228 Ac as metastable isomers with lifetimes of O(100 ns) To confirm this hypothesis, we simulate the β spectra of 228 Ra using a Geant4-based simulation that is the same as the one used for background modeling of the COSINE-100 detectors [22,30].

Three-pulse events

Consistency check of 228 Ra decays

Conclusion