The background in the $$0\nu \beta \beta $$ 0 ν β β experiment Gerda

Abstract

The GERmanium Detector Array (Gerda) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double beta (0νββ) decay of 76 Ge. The signature of the signal is a monoenergetic peak at 2039 keV, the Q ββ value of the decay. To avoid bias in the signal search, the present analysis does not consider all those events, that fall in a 40 keV wide region centered around Q ββ . The main parameters needed for the 0νββ analysis are described. A background model was devel- oped to describe the observed energy spectrum. The model contains several contributions, that are expected on the basis of material screening or that are established by the observation of characteristic structures in the energy spectrum. The model predicts a flat energy spectrum for the blinding window around Q ββ with a background index ranging from 17.6 to 23.8 × 10 −3 cts/(keV kg yr). A part of the data not considered before has been used to test if the predictions of the background model are consistent. The observed number of events in this energy region is consistent with the background model. The background at Q ββ is dominated by close sources, mainly due to 42 K, 214 Bi, 228 Th, 60 Co and α emitting isotopes from the 226 Ra decay chain. The individual fractions depend on the assumed locations of the contaminants. It is shown, that after removal of the known γ peaks, the energy spectrum can be fitted in an energy range of 200 keV around Q ββ with a constant background. This gives a background index consistent with the full model and uncertainties of the same size.

Cosmogenic activation of materials

“…A sample with a mass of 18 kg, exposed at 250 m above sea level for 1 year after casting according to company records, as well as bricks exposed to cosmic rays for 41 days, were analyzed. Activities found for cobalt isotopes and 54 Mn agree with the expectations from previously measured production rates.…”

“…50 and cosmogenic products are usually considered in the background models of experiments. 54 In this section, the main results obtained for quantifying the cosmogenic yields in both natural and enriched a germanium are summarized.…”

On concentration of 42Ar in the Earth's atmosphere

“…Using the above parameters and accounting for the total fiducial Ar mass of 52 kg, the concentration of 42 Ar in the Earth's atmosphere can be estimated as 6.8 +1.7 −3.2 · 10 −21 atoms of 42 Ar/atom of 40 Ar, which corresponds to the 42 Ar activity of 1.2 +0.3 −0.5 µBq/m 3 of air. This leads to a specific activity of freshly produced liquid argon of 68 +17 −32 µBq/kg, in agreement with the earlier GERDA-I result [11]. We also note that the presented result is consistent with the mechanism of 42 Ar production suggested in [6].…”

“…More recently, the GERDA-I experiment carried out an independent measurement of the 42 Ar concentration [11]. The aim of the experiment is to search for the neutrinoless double beta decay (0νββ) of 76 Ge.…”

The GALATEA test-facility for high purity germanium detectors

“…To enhance the sensitivity of such searches, future large-scale HPGe detector based experiments are foreseen [7]. One of the limiting sources of background in such experiments is surface contamination [8]. This requires complete understanding and characterization of the detector response to surface events.…”