Eur. Phys. J. A (2026) 62: 16
https://doi.org/10.1140/epja/s10050-025-01780-6

Regular Article - Experimental Physics

Electron spectral shape of the third-forbidden $\beta$-decay of $_{}^{87}$Rb measured using a ${\text{Rb}}_2{\text{ZrCl}}_6$ crystal scintillator

¹ Dipartimento di Fisica, Università di Roma “Tor Vergata”, 00133, Rome, Italy
² INFN, sezione di Roma “Tor Vergata”, 00133, Rome, Italy
³ INFN, sezione di Roma, 00185, Rome, Italy
⁴ Dipartimento di Fisica, Università di Roma “La Sapienza”, 00185, Rome, Italy
⁵ Gran Sasso Science Institute, 67100, L’Aquila, Italy
⁶ Department of Chemistry, Queen’s University, K7L 3N6, Kingston, ON, Canada
⁷ INFN Laboratori Nazionali del Gran Sasso, 67100, Assergi, AQ, Italy
⁸ Department of Physics, University of Jyväskylä, P.O. Box 35, 40014, Jyväskylä, Finland
⁹ International Centre for Advanced Training and Research in Physics (CIFRA), P.O. Box MG12, 077125, Bucharest-Măgurele, Romania
¹⁰ School of Physics, Engineering and Technology, University of York, Heslington, YO10 5DD, York, UK
¹¹ Natural Resources Institute Finland, Yliopistokatu 6B, 80100, Joensuu, Finland

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Received: 24 June 2025
Accepted: 20 December 2025
Published online: 22 January 2026

Abstract

In recent years, interest in experimental studies of $\beta$-decay electron spectra – often referred to as $\beta$ spectra – has been growing. This is particularly true for $\beta$ transitions where the electron spectra are sensitive to the effective value of the weak axial coupling, $g_{\text{A}}$. Such measurements serve as important benchmarks for nuclear physics calculations and can also be used to characterize background in astroparticle physics experiments. In this work, a dedicated experiment has been carried out to investigate the spectral shape of the third-forbidden $_{}^{87}$Rb $\beta$-decays, with the goal of estimating the effective $g_{\text{A}}$ value for this transition and of deriving the ${\text{T}}_{1/2}$ value. This was done by comparing the experimental spectral shape with the estimates from various phenomenological models. The $_{}^{87}$Rb source was embedded directly within the detector material of a new ${\text{Rb}}_2{\text{ZrCl}}_6$ crystal scintillator; the data taking was performed deep underground at Gran Sasso National Laboratory. The obtained experimental half-life value for the studied process is ${\text{T}}_{1/2}=5.08(13)\times {10}^{10}$ yr; while a $g_{\text{A}}$ value in the range 0.4–0.6 is obtained when accounting for uncertainties and depending on the model adopted as discussed in detail in the text.