Characterization of a broad energy germanium detector and application to neutrinoless double beta decay search in 76Ge

The performance of a 630 g commercial broad energy germanium (BEGe) detector has been systematically investigated. Energy resolution, linearity, stability vs. high-voltage (HV) bias, thickness and uniformity of dead layers have been measured and found to be excellent. Special attention has been dedicated to the study of the detector response as a function of bias HV. The recommended bias voltage being 3500 V, the detector shows a peculiar behavior for biases around 2000 V: in a narrow range of about 100 V the charge collection is strongly reduced. The detector seems to be composed by two parts: a small volume around the HV contact where charges are efficiently collected as at higher voltage, and a large volume where charges are poorly collected. A qualitative explanation of this behavior is presented. However such a behavior does not affect the expected performance in the normal working conditions, being the detector operated at much higher bias HV than the anomalous region. An event-by-event pulse shape analysis based on A/E (maximum amplitude of the current pulse over the total energy released in the detector) has been applied to events in different energy regions and found very effective in rejecting non localized events. In conclusion, BEGe detectors are excellent candidates for the second phase of GERDA, an experiment devoted to neutrinoless double beta decay of Ge-76.