The Advanced Mo-based Rare process Experiment (AMoRE)-Pilot experiment is an initial phase of the AMoRE search for neutrinoless double beta decay of 100Mo, with the purpose of investigating the level and sources of backgrounds. Searches for neutrinoless double beta decay generally require ultimately low backgrounds. Surface α decays on the crystals themselves or nearby materials can deposit a continuum of energies that can be as high as the Q-value of the decay itself and may fall in the region of interest (ROI). To understand these background events, we studied backgrounds from radioactive contaminations internal to and on the surface of the crystals or nearby materials with Geant4-based Monte Carlo simulations. In this study, we report on the measured α energy spectra fitted with the corresponding simulated spectra for six crystal detectors, where sources of background contributions could be identified through high energy α peaks and continuum parts in the energy spectrum for both internal and surface contaminations. We determine the low-energy contributions from internal and surface α contaminations by extrapolating from the α background fitting model.
We studied the surface background of CMO crystal detectors in AMoRE-Pilot using simulations with the Geant4 toolkit. To understand background contributions from both internal and surface radioactive contaminations in crystals and nearby materials, we simulated the decay chains of 238U, 232Th, and 235U for both internal and surface contaminations. The α background modeling was carried out by fitting the measured α spectra with the simulated data. We found that the overall simulated background spectrum describes well the data for all six detectors in the 2.5–7.0 MeV energy region. Some features of the experimental spectrum can be reproduced by MC simulations for the surface contaminations of crystals and nearby materials. We found that Crystal 2, which was grown with single crystallization, has an exceptionally large internal contamination. The averaged background level, estimated at (1.95±0.17)×10−2 ckky in the ROI, for the surface contaminations of the five crystals, is higher than that of the AMoRE-II goal. Special efforts are needed to reduce these backgrounds in the future.
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