Study on pulse shapes of germanium detectors with a point like contact geometry for the GERDA and MAJORANA experiments

Abstract

Point-like contact germanium detectors are one of the leading technologies in the neutrinoless double beta decay (0νββ) search and are used in the GERDA and MAJORANA experiments. These kinds of detectors provide a superb energy resolution as a result of their low capacitance. This low capacitance and the strong inhomogeneous electric field are due to the point-like geometry of the contact. The electric field is particularly strong close to the point contact and leads to distinctive pulse shape topologies for different event classes. 0νββ experiments take advantage of this feature by rejecting specific event types to maximize the signal to background ratio. The first part of this work describes the characterization procedure and corresponding results for the second batch of such point-like contact detectors used in the GERDA experiment. These detectors were manufactured from enriched 76Ge by Canberra Semiconductors N.V.. The focus of this study is to probe the homogeneity of the pulse shape behavior along the surface and bulk of the detectors. The results show that most of the detectors have small deviations from a homogeneous spatial distribution for the pulse shape parameters. Through simulations, which were also conducted in this work, it is shown that this behavior originates in the collection of free charges on the passivation layer close to the point contact. This effect can be neutralized by removing the passivation layer which is not needed if the detectors are operated in a noble liquid gas. The second part of this work reports the study of ultra pure and large prototype point contact detectors. These detectors were developed by ORTEC and are used in the MAJORANA experiment. Surface scans of these detectors with a collimated 241Am source and coincident measurements with a β+ source show that the low electric fields caused by the extremely high purity can lead to a degraded pulse shape performance. This effect was also investigated and reproduced in simulations by modeling the size of the charge cloud evolution during its drift. The resulting simulations enable the correct selection of the ingot impurity profiles for the manufacturing of large point-like contact detectors for the next generation of 0νββ experiments.