As mentioned in the previous Sec. 2.2, the germanium detectors setting up the array can
be distinguished by two different types: the semi-coaxial and theenrBEGEdetectors. Con-
sidering the different detector types, the electric field strength across the detectors active volume can vary markedly for different detector types, since it determines the drift velocity of the charge carriers. Consequently, the properties of pulse shape, timing behavior, and completeness of the charge collection process are effected. A sketch of both used detector types reporting their typical geometrical dimensions, electrodes and grooves is shown in Fig. 2.4a.
(a) (b)
Fig. 2.4(a) Cross section of a semi-coaxial detector (top) and aenrBEGEdetector (bottom). The p+
and n+ electrodes are indicated in Grey and black, respectively. The electrodes are separated by an
insulating groove. (b) Weighting potential of a semi-coaxial (top) and aenrBEGEdetector (bottom)
shown with color profiles.[80]
Semi-coaxial detectors The semi-coaxial detectors are characterized by a cylindrical
shape with a bore hole on one of the flat surfaces. The core of the crystal is either removed
extends across the outer mantle and top surface. It is separated by a circular groove from the
boron-implanted p+ read-out electrode. The p+ electrode covers the bore hole surface with
a thickness of only about 1 µm and connects to a charge sensitive amplifier (see Fig. 2.4a, top left). Generally limited by the maximum depletion depth of only several centimeters,
compared toenrBEGEdetectors much larger active volumes can be achieved for semi-coaxial
detectors with masses exceeding 2 kg. Due to the core position of the electrode, the detectors height can be extended to about 10 cm in axial direction (compared to typical height of 2 to
4 cm forenrBEGEdetectors). The detectors were operated by biasing the n+ electrode with
high voltages reaching from 3 to 4.6 kV.
The electric field configuration of the coaxial configuration varies inversely with the radial distance from the detector axis, resulting in a strongly inhomogeneous weighting potential. Because of the radial variation the drift velocities change as the carriers are collected. The effect of the different mobilities of the electron and hole is clearly reflected in the resulting pulse. This generally brings the ability to resolve the radial dependence of the interaction position within the detector volume, but involves an existing library of position dependent pulse shapes or further analysis based on multi-parameters. In Sec. 3.2.5, a compilation of
the pulse shape strategy applied by GERDAwill be presented.
Broad energy germanium detectors The enrBEGEdetectors are likewise cylindrically
shaped, but enclose a small-sized boron-implanted p+ electrode on one of the flat surfaces
(see Fig. 2.4a, bottom left). Along with the minimized capacitance of the detector it reduces the electric noise. Thereby, a superior energy resolution and a lower energy threshold can be achieved with respect to the standard semi-coaxial detectors. But the small area of
the p+ electrode disables the realization of large volume diodes with a feasible depletion
voltage. Hence, the crystals are on average 2-3 times smaller than the semi-coaxial detectors, with diameters from 58.3(1) mm to 79.0(1) mm, heights from 22.9(3) to 35.3(1) mm and masses below 900 g. To maximize the number of detectors to get out of the Ge ingots, conical tail slices were used leading to 9 detectors with a conical shape. The average active
volume (AV) fraction favand the total active mass Mact of allenrBEGEdetectors have been
determined. The values include a correction which considers a growth of the full charge collection depth (FCCD) by 0.2 to 0.3 mm due to storage at room temperature over a period
of three years before deployment in GERDA [87], as specified in Tab. B.2. All crystals
feature different doping and impurity properties and, consequently, exhibit slightly different
electrical properties. Prior to installation, allenrBEGE detectors have been characterized
by their leakage currents, their detection efficiencies, as well as their spectroscopic and pulse-shape discrimination (PSD) performance [85],[87].
2.3 Germanium detectors 39
Arising from the small area of the p+electrode, theenrBEGEdetectors possess a charac-
teristic electric field distribution. The weighting potential has a strong variation concentrated
around the small p+ electrode. Elsewhere throughout the detector volume, it is relatively
weak (see Fig. 2.4b). Consequently, the length of the path to reach the strong weighting potential, thus the drift times of the charge carriers, depend on the site of the interaction and cause differences in the rising part of the induced pulse. Besides, the majority of the charge is collected only at the very end of the trajectory of the charge drift. Moreover, the contribution by the electron is expected to have only an influence at the beginning of the pulse with a negligible amplitude for the most of the interaction points within the detector volume. The induced signal remains small until the charge arrives the electrode and then increases strongly until the holes are collected. Compared to the usual coaxial detectors, the pulse shape analysis enables a superior event classification used for background rejection.
How GERDAtakes advantage of the capability to discriminate background events from signal
Chapter 3
G
ERDA
Phase II background reduction
strategies
In Phase II, GERDAaimed to increase the sensitivity by reducing the background by one order
of magnitude. To achieve such a low background level, the second phase of GERDArequired
supplementary background suppression systems with respect to Phase I. Besides, a more strict radio-purity criteria has been pursued when selecting construction materials. In order to
further optimize the signal to background ratio at Qβ β, a multitude of background reduction
techniques were applied. However, such techniques can only be built on the knowledge and estimation of the background sources observed in the energy spectrum. This chapter will start with the background composition of the observed energy spectrum and pass then to the description of the developed strategies.