Pile-up rejection for AMoRE-II Yoomin Oh References: [1] K.S. Park et al, DOI:10.3389/fphy.2024.1323991. [2] G.B. Kim et al, DOI:10.1016/j.astropartphys.2017.02.009. [3] S.C. Kim et al, DOI:10.1088/1748-0221/17/07/P07034. [4] C. Augier et al, DOI:10.1103/PhysRevLett.131.162501. [5] T. Chen and C. Guestrin, DOI:10.1145/2939672.2939785. [6] V. Alenkov et al, DOI:10.1140/epjc/s10052-022-11104-3. AMoRE: 0νDBD search • Mo-100 enriched scintillation crystal. • Cryogenic detector technique (MMC). • AMoRE-II using 157 kg of Li2MoO4. • Being prepared at 1000 m underground in Yemilab [1]. • 5+ years running: mass·time exposure > 500 kg·year. • Energy resolution ~ 10 keV FWHM at Qββ=3034 keV. • Background level ≪ 2×10-4 count/keV/kg/year (ckky). • Half-life sensitivity ~ 4×1026 years at 90% CL. Pile-up background • Preference for a larger-size crystal detector to reduce the number of detector channels (~€$₩). • Random coincidence of two signals in a detector crystal volume. • Expecting the largest contribution from two 2νDBD signals. • Rate scales with the crystal size (internal~volume, external~area) and coincidence t window. • For Li2MoO4 with Φ/H=6 cm and a 500 µs coincidence time window (Δt): pile-up background rate at ROI ~ 2.2×10-4 ckky [4]. • Rejection at the analysis level is required with εrejection≳90%. 2νββ random coincidence Each total Pile-up simulation and analysis • Signal templates of Li2MoO4 detectors from R&D data: 1. Fast signal: rise-time ~ 1.9 ms, 10-90%, (Diffused surface / 20 mK / MMC thermal link to Cu-frame ❍) 2. Slow signal: rise-time ~ 5.8 ms, 10-90%, (Diffused surface / 10 mK / MMC thermal link to Cu-frame ✕) • Noise (baseline): randomly sampled from real data. • Analysis utilizing a machine learning method: gradient boosting [5]. Selected parameters for discrimination and training/evaluation data samples Pileup Single Single Pileup Pileup Single Pileup Pileup Single Single Pileup Single Pileup Pileup Single Pileup Pileup Single Pileup Pileup Single Pileup Pileup Single Pileup Pileup Single Pileup samples Butterworth bandpass filters with different cutoff frequencies Raw heat Raw light Raw heat Raw light Validation using real data: Bi-Po β-α decay • A CaMoO4 crystal detector with a high U/Th contamination in AMoRE-I [6]. 214Bi β−decay −−−−−−−−! Q=3.27 MeV 214Po (T1/2 = 164 μs) α−decay −−−−−−−−! Q=7.83 MeV ··· 212Bi β−decay −−−−−−−−! Q=1.88 MeV 212Po (T1/2 = 299 ns) α−decay −−−−−−−−! Q=8.95 MeV ··· Selection efficiency ~97% for single α events Some can be discriminated. Rejection efficiency~60% Expecting no rejection at all! Signal and noise characteristics affects pile-up rate • A faster signal and higher signal/noise preferred for less pile-up rate [2]. • Signal parameters such as size and speed (rise-/decay-time) can be tuned with: 1. temperature, 2. crystal surface condition, 3. thermal link between sensor (MMC) and Cu frame [3]. • Baseline r.m.s. can be controlled to be as small as a few keV level. Result and Discussion • Rejection efficiency for the pile-up of 2νDBD signals at ROI in 500 µs: • Better than 90% with faster signals on lower noise baselines. • Close to 80% with slower signals on higher noise baselines. • Selection efficiency for the single signal event > 95%. • Pile-up rate by 2νDBD at ROI can be suppressed down to (2–4)×10-5 ckky. • Studying the possibility for further improvements, not only for pile-up discrimination but also for more general event type classification. yoomin@ibs.re.kr Fast signal, Lower noise case