Background model of the CUPID-0 experiment

CUPID-0 is the first large mass array of enriched Zn$$^{82}$$Se scintillating low temperature calorimeters, operated at LNGS since 2017. During its first scientific runs, CUPID-0 collected an exposure of 9.95 kg year. Thanks to the excellent rejection of $$\alpha $$ particles, we attained the lowest background ever measured with thermal detectors in the energy region where we search for the signature of $$^{82}\hbox {Se}$$ neutrinoless double beta decay. In this work we develop a model to reconstruct the CUPID-0 background over the whole energy range of experimental data. We identify the background sources exploiting their distinctive signatures and we assess their extremely low contribution [down to $$\sim 10^{-4}$$ counts/(keV kg year)] in the region of interest for $$^{82}\hbox {Se}$$ neutrinoless double beta decay search. This result represents a crucial step towards the comprehension of the background in experiments based on scintillating calorimeters and in next generation projects such as CUPID