LArGe: active background suppression using argon scintillation for the Gerda $$0\nu \beta \beta $$ 0 ν β β -experiment

Abstract

LArGe is a Gerda low-background test facility to study novel background suppression methods in a lowbackground environment, for future application in the Gerda experiment. Similar to Gerda, LArGe operates bare germanium detectors submersed into liquid argon (1 m 3 , 1.4 tons), which in addition is instrumented with photomultipliers to detect argon scintillation light. The scintillation signals are used in anti-coincidence with the germanium detectors to effectively suppress background events that deposit energy in the liquid argon. The background suppression efficiency was studied in combination with a pulse shape discrimination (PSD) technique using a BEGe detector for various sources, which represent characteristic backgrounds to Gerda. Suppression factors of a few times 10 3 have been achieved. First background data of LArGe with a coaxial HPGe detector (without PSD) yield a background index of (0.12−4.6)×10 −2 cts/(keV kg year) (90 % C.L.), which is at the level of Gerda Phase I. Furthermore, for the first time we monitor the natural 42 Ar abundance (parallel to Gerda), and have indication for the 2νββ-decay in natural germanium. These results show the effectivity of an active liquid argon veto in an ultra-low background environment. As a consequence, the implementation of a liquid argon veto in Gerda Phase II is pursued. a

Upgrade for Phase II of the Gerda experiment

“…No signal was found for 0νββ decay, and a new 90% confidence level (CL) limit of T 0ν 1/2 > 2.1 × 10 25 year was derived (median sensitivity 2.4×10 25 year) that strongly disfavored a previous claim of observation [8,9]. Further Phase I results include a much improved half-life for 2νββ decay of 76 Ge and improved limits for Majoron ββ decay modes [10], as well as 2νββ decays of 76 Ge into excited states of 76 Se [11].…”

“…In order to increase the exposure (via increase of total detector mass) and to improve the background index (via an enhanced pulse shape performance), the Gerda collaboration opted for the production of 30 new detectors following the BEGe design of the company Canberra [15]. After extensive preparation, 30 detectors were delivered.…”

“…Note that these measurements were performed with 'passivated' detectors where the groove between the p + and n + electrode is covered by an insulating silicon monoxide ('passivation') layer [15]. The energy resolution of all detectors turned out to be excellent.…”

“…Great attention has been paid to minimize the activation of the newly produced BEGe detectors by cosmic rays [15]. As of January 1 2017, we expect from cosmic activation in the 30 BEGes detectors (20.02 kg) about 24 68 Ge nuclei and 300 60 Co nuclei; 2 for comparison, the saturation activity is ∼2300 68 Ge nuclei and ∼9200 60 Co nuclei per kg germanium, respectively.…”

“…Here, we assume 5.8 and 3.3 nuclei/(kg day) [15]. up that were highly correlated with the orientation of BEGe detectors such that detectors with the groove pointing upwards, the 'top' detectors in the mount, were much more affected than the 'bottom' detectors with the groove pointing downwards.…”