  Localization and modeling of radioactive contaminations is a challenge that
ultra-low background experiments are constantly facing. These are fundamental
steps both to extract scientific results and to further reduce the background
of the detectors. Here we present an innovative technique based on the analysis
of $\alpha-\alpha$ delayed coincidences in $^{232}$Th and $^{238}$U decay
chains, developed to investigate the contaminations of the ZnSe crystals in the
CUPID-0 experiment. This method allows to disentangle surface and bulk
contaminations of the detectors relying on the different probability to tag
delayed coincidences as function of the $\alpha$ decay position.
