α -event characterization and rejection in point-contact HPGe detectors

Author(s): Collaboration, The MAJORANA; Arnquist, IJ; III, FT Avignone; Barabash, AS; Barton, CJ; Bertrand, FE; Blalock, E; Bos, B; Busch, M; Buuck, M; Caldwell, TS; Chan, Y-D; Christofferson, CD; Chu, P-H; Clark, ML; Cuesta, C; Detwiler, JA; Drobizhev, A; Edwards, TR; Edwins, DW; Efremenko, Yu; Elliott, SR; Gilliss, T; Giovanetti, GK; Green, MP; Gruszko, J; Guinn, IS; Guiseppe, VE; Haufe, CR; Hegedus, RJ; Henning, R; Aguilar, D Hervas; Hoppe, EW; Hostiuc, A; Kim, I; Kouzes, RT; Lopez, AM; Lopez-Castano, JM; Martin, EL; Martin, RD; Massarczyk, R; Meijer, SJ; Mertens, S; Myslik, J; Oli, TK; Othman, G; Pettus, W; Poon, AWP; Radford, DC; Rager, J; Reine, AL; Rielage, K; Ruof, NW; Sayki, B; onert, S Sch; Stortini, MJ; Tedeschi, D; Varner, RL; Vasilyev, S; Wilkerson, JF; Willers, M; Wiseman, C; Xu, W; Yu, C-H; Zhu, BX | Abstract: P-type point contact (PPC) HPGe detectors are a leading technology for rare event searches due to their excellent energy resolution, low thresholds, and multi-site event rejection capabilities. We have characterized a PPC detector's response to $\alpha$ particles incident on the sensitive passivated and p+ surfaces, a previously poorly-understood source of background. The detector studied is identical to those in the MAJORANA DEMONSTRATOR experiment, a search for neutrinoless double-beta decay ($0\nu\beta\beta$) in $^{76}$Ge. $\alpha$ decays on most of the passivated surface exhibit significant energy loss due to charge trapping, with waveforms exhibiting a delayed charge recovery (DCR) signature caused by the slow collection of a fraction of the trapped charge. The DCR is found to be complementary to existing methods of $\alpha$ identification, reliably identifying $\alpha$ background events on the passivated surface of the detector. We demonstrate effective rejection of all surface $\alpha$ events (to within statistical uncertainty) with a loss of only 0.2% of bulk events by combining the DCR discriminator with previously-used methods. The DCR discriminator has been used to reduce the background rate in the $0\nu\beta\beta$ region of interest window by an order of magnitude in the MAJORANA DEMONSTRATOR, and will be used in the upcoming LEGEND-200 experiment.