Crystal scintillators for high-precision beta spectroscopy: the GAIAS experiment

The effective value of the axial-vector coupling constant g_A​ in nuclear media represents one of the dominant sources of uncertainty in the interpretation of neutrinoless double-beta decay (0νββ) experiments. Its quenching relative to the free-nucleon value significantly impacts nuclear matrix element calculations and, consequently, the inferred values (limits) of the Majorana neutrino mass.

The GAIAS (GAxIal Analysis with Scintillators) project proposes a crystal-scintillator-based strategy to probe the g_A​ through high-precision measurements of forbidden non-unique beta decays. These transitions exhibit strong sensitivity of the beta spectral shape to the axial coupling constant, particularly in the low-energy region.

The method exploits key properties of scintillating crystals: a high light yield, low energy threshold (<5–20 keV), good energy resolution, high radiopurity, stable operation over a long time to accumulate a large enough statistic and the possibility of incorporating beta-emitting isotopes directly into the crystal bulk. Materials under investigation include CdWO₄, ¹⁰⁶CdWO₄​, CsI(Na,Rb), NaI(Tl,Tc), and CeCl₃​, hosting isotopes such as ¹¹³Cd, ^113mCd, ⁸⁷Rb, and ⁹⁹Tc either intrinsically or as controlled dopants.

The experiment is being installed in the low-background environment of the INFN Gran Sasso National Laboratory (LNGS), enabling high-statistics beta spectroscopy under stable and well-controlled conditions. Particular emphasis is placed on crystal growth strategies, isotope incorporation, control of scintillation non-proportionality below 100 keV, and long-term energy-scale stability.

By combining optimized crystal engineering, optimized electronic read-outs, precision spectroscopy, Monte Carlo simulations, and nuclear-structure modeling, GAIAS aims to extract g_A​ with high accuracy, strengthening the role of functional crystalline materials in fundamental physics.