Characterization of 30 76 Ge enriched Broad Energy Ge detectors for GERDA Phase II

Abstract

The GERmanium Detector Array (Gerda) is a low background experiment located at the Laboratori Nazionali del Gran Sasso in Italy, which searches for neutrinoless double-beta decay of ${}^{76}$ Ge into ${}^{76}$ Se+2e ${}^{-}$ . Gerda has been conceived in two phases. Phase II, which started in December 2015, features several novelties including 30 new ⁷⁶Ge enriched detectors. These were manufactured according to the Broad Energy Germanium (BEGe) detector design that has a better background discrimination capability and energy resolution compared to formerly widely-used types. Prior to their installation, the new BEGe detectors were mounted in vacuum cryostats and characterized in detail in the Hades underground laboratory in Belgium. This paper describes the properties and the overall performance of these detectors during operation in vacuum. The characterization campaign provided not only direct input for Gerda Phase II data collection and analyses, but also allowed to study detector phenomena, detector correlations as well as to test the accuracy of pulse shape simulation codes.

Fig. 1

Full inventory of crystal slices/diodes belonging to the Gerda Phase II BEGe detector production. Crystals/diodes obtained from the same ingot are framed in blue. The GD32 and GD35 detector series belonging to the 1st batch are depicted in their final diode form (row 1), while the other 7 series from the 2nd batch are shown as crystal slices prior to diode conversion (rows 2–4)

Fig. 2

Gerda Phase II BEGe diodes are either (1) cylindrical, (2) single-conical, or (3) double-conical. Overall heights are denoted with H1, while corner heights with H2 and H3. The overall diameter is distinguished from corner diameters by D1 vs. D2, D3

Fig. 3

Characteristic HV scan curves of detector GD00B based on the evaluation of the 1333  keV $\gamma$ line from a ${}^{60}$Co calibration source. The three curves of the peak position, energy resolution and peak integral are used for the definition of the full depletion and operational voltage in Gerda. Additionally, the peak asymmetry curve in the bottom canvas demonstrates that the Gaussian peak form is conserved over a large voltage interval. More explanations are included in the text

Fig. 4

Characteristic HV scan curves of detector GD00D based on the evaluation of the 1333  keV $\gamma$ line from a ${}^{60}$Co calibration source. For a given curve, the distance between two values are 50 V. In all three curves of the peak position, energy resolution and peak integral the presence of two ‘bubbles’ becomes visible

Fig. 5

ADL3-simulation of the electrical depletion process of the detectors GD00B, GD00C and GD00D: the voltage along the central axis starting at the p+ electrode read-out is depicted as function of the voltage applied externally to the n+ contact. The curves in orange represent the voltage when the full depletion is reached. Curves with an intermediate constant interval are marked with a dotted line and correspond to voltages, at which a ‘bubble’ persists. For experimental data see Figs. 3 and 4

Fig. 6

Correlation between the full depletion voltage $V_d(95\%$ $\Delta E)$ and the product of the net impurity concentration and the squared height for all 30 Gerda Phase II BEGe detectors except for GD02D. Open symbols pertain to detectors with no appearance of the ‘bubble depletion’ effect or which do not belong to the GD91 series. The dashed line results from the fit to these 25 data points

Fig. 7

Detector energy resolution dependence on the detector mass of all 30 Gerda Phase II BEGe detectors but GD02D. Open symbols are used as in Fig. 6

Fig. 8

Energy resolution in terms of FWHM as function of energy for detector GD89A. For the fit function only the two energy resolution terms $\Delta E_{\mathit{sf}}$ and $\Delta E_{\mathit{el}}$ were considered, while $\Delta E_{\mathit{cc}}$ was neglected

Fig. 9

Conceptual representation of the full charge collection depth (FCCD) and active volume (AV) of the Gerda Phase II BEGe detectors. The charge collection efficiency in the dead layer (DL), transition layer (TL) and AV is denoted with $ϵ$. Moreover, the boron (B) implanted p+ contact is depicted in red, while the inactive wrapped around lithium (Li) diffused n+ contact is drawn in blue. The insulating ring between the two electrodes is shown in pink. For further details see the text

Fig. 10

FCCD values of 29 Gerda Phase II BEGe detectors excluding GD02D. The FCCD values in plot (a) contain only the uncorrelated uncertainties, while in plot (b) they contain the combined correlated and uncorrelated uncertainties

Fig. 11

Energy spectra of the ${}^{60}$Co source measurements performed with the detectors GD02D and GD91B

Fig. 12

Detector GD89B: measured rise time curves of circular scans on the top diode surface, with a ${\tau }_r$ definition in the [2,70] % interval

Fig. 13

Detector GD89B: ADL3-based simulation of rise time curves of circular scans on the top diode surface using the ${\tau }_r$([2,70] %) definition

Fig. 14

Detector GD89C: measurement vs. siggen-based simulation of the A / E ratio distribution of DEP events. In both cases, the ${}^{228}$Th source placed in 20 cm distance from the detector was not collimated

Fig. 15

Illustration of the PSD survival fractions of different ${}^{228}$Th-induced $\gamma$-ray background in all 30 Gerda Phase II BEGe detectors

Fig. 16

PSD survival fraction of ${}^{228}$Th SEP vs. the DEP A / E resolution for all 30 Gerda Phase II BEGe detectors operated under vacuum conditions. Open symbols are used like in Fig. 6