Neutrinoless double-beta decay (0νββ) experiments can help determine the mechanism by which neutrinos acquire mass – whether it is the Higgs mechanism, like other fermions, or a different mechanism due to their neutrality, which introduces a Majorana mass term in the Lagrangian of the Standard Model. Bolometers, particle detectors that measure the heat deposited by a particle, are promising for next-generation 0νββ experiments. However, the high sensitivity of these crystal bolometers also creates the challenge of having to properly reject events caused by radioactive impurities near the detector surface contaminating the data.

Bandac et al. showed that a bilayer coating of an aluminum-palladium grid atop a Li2MoO4 bolometer can be used to tag and reject near-surface α and β events that arise from radioactive impurities.

These improvements can be crucial in experiments that hope to uncover the nature of neutrino mass.

“This would actually be a big discovery and a clear sign of new physics, and also of a new energy scale,” said author Andrea Giuliani. “This can be done only through the study of 0νββ, and this is the main motivation that pushes my group to dedicate such big efforts to this topic.”

The coating acts to modify the pulse shape of surface events, reducing the rise time and altering the time evolution of their signals. These changes allow events induced by radioactive impurities occurring up to about 4 millimeters from the detector, well beyond its thickness, to be easily discarded with efficiencies up to 90%.

“The final aim is a ton-scale bolometric experiment with zero background capable of providing a high discovery potential for 0νββ, even if the [mass] hierarchy is normal,” Giuliani said.

Source: “Phonon-mediated crystal detectors with metallic film coating capable of rejecting α and β events induced by surface radioactivity,” by I. C. Bandac, A. S. Barabash, L. Bergé, Ch. Bourgeois, J. M. Calvo-Mozota, P. Carniti, M. Chapellier, M. de Combarieu, I. Dafinei, F. A. Danevich, L. Dumoulin, F. Ferri, A. Giuliani, C. Gotti, Ph. Gras, E. Guerard, A. Ianni, H. Khalife, S. I. Konovalov, P. Loaiza, M. Madhukuttan, P. deMarcillac, R. Mariam, S. Marnieros, C. A. Marrache-Kikuchi, M. Martinez, C. Nones, E. Olivieri, G. Pessina, D. V. Poda, Th. Redon, J.-A. Scarpaci, V. I. Tretyak, V. I. Umatov, M. M. Zarytskyy, and A. S. Zolotarova, Applied Physics Letters (2021). The article can be accessed at