Dušan Budjáš

Pulse shape discrimination studies with a Broad-Energy Germanium detector for signal identification and background suppression in the GERDA double beta decay experiment

First studies of event discrimination with a Broad-Energy Germanium (BEGe) detector are presented. A novel pulse shape method, exploiting the characteristic electrical field distribution inside BEGe detectors, allows to identify efficiently single-site events and to reject multi-site events. The first are typical for neutrinoless double beta decays (0νββ) and the latter for backgrounds from gamma-ray interactions. The obtained survival probabilities of backgrounds at energies close to Qββ(76Ge) = 2039 keV are (0.93 ± 0.08)% for events from 60Co, (21 ± 3)% from 226Ra and (40 ± 2)% from 228Th. This background suppression is achieved with (89 ± 1)% acceptance of 228Th double escape events, which are dominated by single site interactions. Approximately equal acceptance is expected for 0νββ-decay events. Collimated beam and Compton coincidence measurements demonstrate that the discrimination is largely independent of the interaction location inside the crystal and validate the pulse-shape cut in the energy range of Qββ. The application of BEGe detectors in the GERDA and the Majorana double beta decay experiments is under study.

Optimisation of the MC-model of a p-type Ge-spectrometer for the purpose of efficiency determination

An optimisation of the geometrical model of a p-type detector used for material screening was carried out to improve the accuracy of Monte Carlo simulations in reproducing spectrometric measurements. Gamma-ray sources were measured to determine the dimensions of the detector dead layer and borehole. An agreement between simulations and measurement within 3% was achieved at energies above 100 keV. In contrast, discrepancies on the order of 23% were encountered using the nominal parameters from the detector manufacturer.

Gamma-ray spectrometry of ultra low levels of radioactivity within the material screening program for the GERDA experiment

In present and future experiments in the field of rare events physics a background index of 10(-3) counts/(keV kg a) or better in the region of interest is envisaged. A thorough material screening is mandatory in order to achieve this goal. The results of a systematic study of radioactive trace impurities in selected materials using ultra low-level gamma-ray spectrometry in the framework of the GERDA experiment are reported.

Signal modeling of high-purity Ge detectors with a small read-out electrode and application to neutrinoless double beta decay search in Ge-76

The GERDA experiment searches for the neutrinoless double beta decay of 76Ge using high-purity germanium detectors enriched in 76Ge. The analysis of the signal time structure provides a powerful tool to identify neutrinoless double beta decay events and to discriminate them from gamma-ray induced backgrounds. Enhanced pulse shape discrimination capabilities of Broad Energy Germanium detectors with a small read-out electrode have been recently reported. This paper describes the full simulation of the response of such a detector, including the Monte Carlo modeling of radiation interaction and subsequent signal shape calculation. A pulse shape discrimination method based on the ratio between the maximum current signal amplitude and the event energy applied to the simulated data shows quantitative agreement with the experimental data acquired with calibration sources. The simulation has been used to study the survival probabilities of the decays which occur inside the detector volume and are difficult to assess experimentally. Such internal decay events are produced by the cosmogenic radio-isotopes 68Ge and 60Co and the neutrinoless double beta decay of 76Ge. Fixing the experimental acceptance of the double escape peak of the 2.614 MeV photon to 90%, the estimated survival probabilities at Qββ = 2.039 MeV are (86±3)% for 76Ge neutrinoless double beta decays, (4.5±0.3)% for the 68Ge daughter 68Ga, and (0.9+0.4−0.2)% for 60Co decays.

Cryogenic Performance of a Low-Noise JFET-CMOS Preamplifier for HPGe Detectors

Cryogenic low-noise charge sensitive preamplifiers have been developed and realized for the GERmanium Detector Array (GERDA). An integrated JFET-CMOS preamplifier, which is fully functional at cryogenic temperatures, has been tested in conjunction with an unsegmented p-type HPGe detector. Both the crystal and the preamplifier were operated inside a liquid nitrogen dewar at 77 K. The detector capacitance was ~60 pF. An optimum resolution of 1.6 keV FWHM has been obtained for the pulser line at 6 ¿s shaping time. A resolution of 2.1 keV FWHM has been achieved for the 1.332 MeV line from a 60Co source. A wide bandwidth (rise time of ~16 ns) permits use of pulse-shape analysis techniques to localize the position of the photon interactions inside the crystal. A low power consumption (~23 mW) makes the preamplifier suitable for a multi-channel array of germanium detectors.

Operation and performance of a bare broad-energy germanium detector in liquid argon

The GERmanium Detector Array, GERDA, will search for neutrinoless double beta decay of 76Ge by operating bare high-purity germanium detectors, enriched in 76Ge, in liquid argon. To reduce the background to the required level below 10−3 cts/(keVkgy), it is necessary to employ active background-suppression techniques. Detectors based on the design of commercially available Broad-Energy Germanium (BEGe) detector are one of the two technologies included in research and development for the second phase of GERDA. BEGe detectors feature an enhanced capability to distinguish between an interaction of an electron from beta-decay and an interaction of a multiple-scattered photon inside the detector, via pulse-shape analysis. A GERDA Phase II prototype BEGe detector mounted in a low-mass holder was operated bare for the first time in liquid argon. The detector showed stable performance over more than one month, with an energy resolution of 1.9 keV (FWHM) at 1.3 MeV, and a low leakage current of ≤ 20 pA. Periodic pulse-shape analysis checks were performed and the results are equal to those obtained with the same detector in a vacuum cryostat.

Pulse shape analysis with a broad-energy germanium detector for the GERDA experiment

To reduce background in experiments looking for rare events, such as the GERDA double beta decay experiment, it is necessary to employ active background-suppression techniques. One of such techniques is the pulse shape analysis of signals induced by the interaction of radiation with the detector. Analysis of the time-development of the impulses can distinguish between an interaction of an electron and an interaction of a multiple-scattered photon inside the detector. This information can be used to eliminate background events from the recorded data. Results of pulse-shape analysis of signals from a commercially available broad-energy germanium detector are presented and the pulse-shape discrimination capability of such detector configuration for use in low-background experiments is discussed.

A Comparison of Low-level Gamma-spectrometers within the GERDA Collaboration

In low‐level gamma spectrometry, one of the main goals is the improvement of detection sensitivity. However, ensuring the accuracy and compatibility of the measurement results in different laboratories is also very important. This has been particularly highlighted within the GERDA collaboration, which incorporates several low‐level material screening laboratories. For this reason a comparison of gamma‐spectrometers situated at MPIK Heidelberg, IRMM Geel, LNGS Assergi and JINR Dubna was performed. This comparison was based on the National Physical Laboratory’s “Environmental Radioactivity Comparison 2005” exercise, in which MPIK Heidelberg and IRMM Geel participated. The exercise allowed comparing the accuracy of the measurements and the different evaluation techniques between many low‐level laboratories, using the same standard mixture of low‐radioactivity nuclides. The results of the internal comparison are presented and discussed, as well as the implementation of the Monte‐Carlo simulations in the evalu...

Background Suppression Using Pulse Shape Analysis with a BEGe Detector for Neutrinoless Double Beta Decay Search with GERDA

A pulse shape analysis for distinguishing between double beta decay‐like interactions and multiple‐scattered photons was performed for the first time using a BEGe‐type detector. This discrimination method is included in the research and development for the second phase of the GERDA experiment, since active background suppression techniques are necessary to reach sensitivity for the 76Ge neutrinoless double beta decay half life of >1026 years. A suppression of backgrounds in the energy region of interest around the 76Ge Qββ = 2039 keV is demonstrated, with (0.93±0.08)% survival probability for events from 60Co, (21±3)% for 226Ra, and (40±2)% for 228Th. This performance is achieved with (89±1)% acceptance of 228Th double escape events, which are analogous to double beta decay.