Characterization of inverted coaxial $$^{76}$$Ge detectors in GERDA for future double-$$\beta $$ decay experiments

Abstract

Neutrinoless double-$$\beta $$ β decay of $$^{76}$$ 76 Ge is searched for with germanium detectors where source and detector of the decay are identical. For the success of future experiments it is important to increase the mass of the detectors. We report here on the characterization and testing of five prototype detectors manufactured in inverted coaxial (IC) geometry from material enriched to 88% in $$^{76}$$ 76 Ge. IC detectors combine the large mass of the traditional semi-coaxial Ge detectors with the superior resolution and pulse shape discrimination power of point contact detectors which exhibited so far much lower mass. Their performance has been found to be satisfactory both when operated in vacuum cryostat and bare in liquid argon within the Gerda setup. The measured resolutions at the Q-value for double-$$\beta $$ β decay of $$^{76}$$ 76 Ge ($$Q_{\beta \beta }$$ Q β β = 2039 keV) are about 2.1 keV full width at half maximum in vacuum cryostat. After 18 months of operation within the ultra-low background environment of the GERmanium Detector Array (Gerda) experiment and an accumulated exposure of 8.5 kg$$\cdot $$ · year, the background index after analysis cuts is measured to be $$4.9^{+7.3}_{-3.4}\times 10^{-4} \ \text {counts}/(\text {keV} \cdot \text {kg} \cdot \text {year})$$ 4 . 9 - 3.4 + 7.3 × 10 - 4 counts / ( keV · kg · year ) around $$Q_{\beta \beta }$$ Q β β . This work confirms the feasibility of IC detectors for the next-generation experiment Legend.

GERDA and LEGEND: Probing the Neutrino Nature and Mass at 100 meV and beyond

“…• Development of the knowledge, procedures and ability to safely handle, cool down and warm up the bare HP-Ge detectors; • Findings on the role of the passivation layer for the stable operation of the bare HP-Ge detectors in LAr [31] and definition of the detector contacts (re)processing for the stable operation in LAr; • Mitigation of the 42 Ar-related 2 background: since the first commissioning, the 42 K γlines 3 intensity has been found to be higher when compared to expectations. 42 K is the decay product of 42 Ar, an isotope produced in the upper atmosphere by cosmic rays via the 40 Ar(α,2p) 42 Ar with the expected ratio N(42)/N(Ar) ∼10 −20 [32]; at the time of commissioning, the measured limit for N(42)/N(Ar) was <4.3•10 −21 [33]. GERDA found that the BI at Q ββ scaled with the 42 K γ-lines intensity.…”

“…GERDA found that the BI at Q ββ scaled with the 42 K γ-lines intensity. The electrostatic field dispersed in LAr from the Ge detector HV-biased surfaces was driving 42 [25].…”

“…Concerning 39 Ar, its β endpoint at 565 keV is harmless for 0νββ decay searches, but its significant activity O(1) Bq/kg of LAr greatly reduces the GERDA potential for WIMP dark matter searches. By the spectral decomposition and fit of the physics data with the known background sources, GERDA found the 42 Ar activity to range from 72-111 µBq/kg [34] in fresh LAr distilled from the atmosphere, corresponding to a concentration N(42)/N(Ar) = 7-12• 10 −21 , exceeding the available limit [33].…”

“…Before the unveiling, a model [34] of the radiation sources and their location was worked out. The source components, their intensity and location are identified and constrained by the characteristic γ and α-lines identified in the spectra: the model (in the following BM) reproduces the energy spectra well over almost two energy decades from 100 keV (the analysis threshold) up to 7 MeV: the minimal BM includes 2νββ in enr Ge, 39 Ar, 42 Ar and 42 K in LAr, 40 K in holders (H), responsible for both the continuum and the few visible γ lines up to energies of ∼1600 keV. 214 Bi, 228 Th, 228 Ac in detector holders, 214 Bi and 42 K βs at p + contact, 60 Co both in H and in detectors plus degraded α are the relevant components in the energy range around Q ββ and up to ∼3 MeV (Figure 13 top panel).…”

“…The source components, their intensity and location are identified and constrained by the characteristic γ and α-lines identified in the spectra: the model (in the following BM) reproduces the energy spectra well over almost two energy decades from 100 keV (the analysis threshold) up to 7 MeV: the minimal BM includes 2νββ in enr Ge, 39 Ar, 42 Ar and 42 K in LAr, 40 K in holders (H), responsible for both the continuum and the few visible γ lines up to energies of ∼1600 keV. 214 Bi, 228 Th, 228 Ac in detector holders, 214 Bi and 42 K βs at p + contact, 60 Co both in H and in detectors plus degraded α are the relevant components in the energy range around Q ββ and up to ∼3 MeV (Figure 13 top panel). The α region above 3 MeV is also shown in the Figure 13 bottom panel for the GOLD COAX: the data show that COAX are on average ∼10 times more contaminated in 210 Po on the p + surface and this most probably happened at the time of the COAX detector refurbishment, when the p + contact was newly implanted.…”