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Register a new userPulse shapes from electron and photon induced events in segmented high-purity germanium detectors
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Experiments built to search for neutrinoless double beta-decay are limited in their sensitivity not only by the exposure but also by the amount of background encountered. Radioactive isotopes in the surrounding of the detectors which emit gamma-radiation are expected to be a significant source of background in the GERmanium Detector Array, GERDA. Methods to select electron induced events and discriminate against photon induced events inside a germanium detector are presented in this paper. The methods are based on the analysis of the time structure of the detector response. Data were taken with a segmented GERDA prototype detector. It is shown that the analysis of the time response of the detector can be used to distinguish multiply scattered photons from electrons.
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Large, high-purity, germanium (HPGe) detectors are needed for neutrinoless double-beta decay and dark matter experiments. Currently, large (> 4 inches in diameter) HPGe crystals can be grown at the University of South Dakota (USD). We verify that the quality of the grown crystals is sufficient for use in large detectors by fabricating and characterizing smaller HPGe detectors made from those crystals. We report the results from eight detectors fabricated over six months using crystals grown at USD. Amorphous germanium (a-Ge) contacts are used for blocking both electrons and holes. Two types of geometry were used to fabricate HPGe detectors. As a result, the fabrication process of small planar detectors at USD is discussed in great detail. The impact of the procedure and geometry on the detector performance was analyzed for eight detectors. We characterized the detectors by measuring the leakage current, capacitance, and energy resolution at 662 keV with a Cs-137 source. Four detectors show good performance, which indicates that crystals grown at USD are suitable for making HPGe detectors.
For the first time, planar high-purity germanium detectors with thin amorphous germanium contacts were successfully operated directly in liquid nitrogen and liquid argon in a cryostat at the Max-Planck-Institut fuer Physics in Munich. The detectors were fabricated at the Lawrence Berkeley National Laboratory and the University of South Dakota, using crystals grown at the University of South Dakota. They survived long-distance transportation and multiple thermal cycles in both cryogenic liquids and showed reasonable leakage currents and spectroscopic performance. Also discussed are the pros and cons of using thin amorphous semiconductor materials as an alternative contact technology in large-scale germanium experiments searching for physics beyond the Standard Model.
Events near the cathode and anode surfaces of a coplanar grid CdZnTe detector are identifiable by means of the interaction depth information encoded in the signal amplitudes. However, the amplitudes cannot be used to identify events near the lateral surfaces. In this paper a method is described to identify lateral surface events by means of their pulse shapes. Such identification allows for discrimination of surface alpha particle interactions from more penetrating forms of radiation, which is particularly important for rare event searches. The effectiveness of the presented technique in suppressing backgrounds due to alpha contamination in the search for neutrinoless double beta decay with the COBRA experiment is demonstrated.
The next generation of radioactive ion beam facilities, which will give experimental access to many exotic nuclei, are presently being developed. At the same time the next generation of high resolution gamma-ray spectrometers, based on gamma-ray tracking, for studying the structure of these exotic nuclei are being developed. One of the main differences in tracking of $gamma$ rays versus charged particles is that the gamma rays do not deposit their energy continuously in the detector, but in a few discrete steps. Also, in the field of nuclear spectroscopy, the location of the source is mostly well known while the exact interaction position in the detector is the unknown quantity. This makes the challenges of gamma-ray tracking in germanium somewhat different compared to vertexing in silicon detectors. In these proceedings we present the methods for determining the 3D interaction positions in the detector and how these are used to reconstruct the gamma-ray tracks in the AGATA detector array. We also present preliminary simulation results of a proposed in-beam method to measure the interaction position resolution in the germanium detectors.
The GERmanium Detector Array, GERDA, will search for neutrinoless double beta decay in 76Ge at the National Gran Sasso Laboratory of the INFN. Bare high-purity germanium detectors enriched in 76Ge will be submerged in liquid argon serving simultaneously as a shield against external radioactivity and as a cooling medium. In GERDA Phase-I, reprocessed enriched-Ge detectors, which were previously operated by the Heidelberg-Moscow and IGEX collaborations, will be redeployed. Before operating the enriched detectors, tests are performed with non-enriched bare HPGe detectors in the GERDA underground Detector Laboratory to test the Phase-I detector assembly, the detector handling protocols, the refurbishment technology and to study the long-term stability in liquid argon. The leakage currents in liquid argon and liquid nitrogen have been extensively studied under varying gamma irradiation conditions. In total three non-enriched high-purity p-type prototype germanium detectors have been operated successfully. The detector performance is stable over the long-term measurements. For the first time, performance of bare high-purity germanium detectors in liquid argon is reported.
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