Dear Geant4 community,
In the ANAIS-112 experiment, using NaI(Tl) detectors for the search of dark matter, 210Pb is one of the main components of the background. Within my PhD, a thorough revision of the background model of the experiment was carried out (all the information can be accessed in Section 5.2.3.2 of https://arxiv.org/abs/2510.19469
In the high energy region, the beta decay of 210Bi, a first-forbidden non-unique transition, is a relevant contribution. Our Geant4 based background model systematically overestimated the energy region around 1 MeV and our recent work suggests that the origin of such an excess could be the spectral shape used in Geant4 for the 210Bi beta.
In Geant4 v10.7.0, the decay of 210Bi follows the analytical calculation; however, a noticeable disagreement exists between the experimentally measured and analytically calculated β⁻ spectra reported in the BetaShape database for this transition, reaching approximately 60% in the energy range from 600 keV up to the endpoint. This value represents the relative difference in the predicted event rate between the analytical spectrum implemented in Geant4 and the experimental spectrum over this energy window, which extends up to the decay endpoint at approximately 1.16 MeV.These discrepancies could partially account for the observed excess in our simulated background. When using instead the experimental β⁻ spectral shape for the 210Bi transition, a noticeably better agreement with the measured ANAIS-112 data in the [600-1000] keV energy region is obtained, mitigating the longstanding overestimation. Although these results were obtained using Geant4 version 10.7, the underlying data libraries are the same in more recent versions; therefore, this issue is expected to persist.
Given the importance of accurately modelling forbidden β transitions in low-background experiments, we would appreciate any clarification from the Geant4 Collaboration regarding the current implementation of the 210Bi β⁻ spectrum. In particular, we would be interested to know whether the collaboration was previously aware of the discrepancy between the analytical and experimental spectral shapes, and whether there are any plans or ongoing discussions about adopting the experimental spectral shape for the 210Bi transition in future Geant4 releases. Alternatively, any guidance on recommended approaches to incorporate alternative experimental spectral shapes for background-modelling purposes would be helpful. Finally, should it be of interest to the collaboration, we would be willing to share relevant data or quantitative studies to help further investigate the observed disagreement and to inform the choice of an appropriate experimental spectral shape for this transition.
Thank you in advance and best regards,
Tamara Pardo on behalf of the ANAIS research team
CAPA, Universidad de Zaragoza
