We describe here an efficient machine-learning based approach for the optimization of parameters used for extracting the arrival time of waveforms, in particular those generated by the detection of 511 keV annihilation gamma-rays by a 60 cm3 coaxial high purity germanium detector (HPGe). The method utilizes a type of artificial neural network (ANN) called a self-organizing map (SOM) to cluster the HPGe waveforms based on the shape of their rising edges. The optimal timing parameters for HPGe waveforms belonging to a particular cluster are found by minimizing the time difference between the HPGe signal and a signal produced by a BaF2 scintillation detector. Applying these variable timing parameters to the HPGe signals achieved a gamma-coincidence timing resolution of ~ 4.3 ns at the 511 keV photo peak (defined as 511 +- 50 keV) and a timing resolution of ~ 6.5 ns for the entire gamma spectrum--without rejecting any valid pulses. This timing resolution approaches the best obtained by analog nuclear electronics, without the corresponding complexities of analog optimization procedures. We further demonstrate the universality and efficacy of the machine learning approach by applying the method to the generation of secondary electron time-of-flight spectra following the implantation of energetic positrons on a sample.
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…
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 …
High-purity germanium (HPGe) crystals are required to be well-characterized before being fabricated into Ge detectors. The characterization of HPGe crystals is often performed with the Hall Effect system, which measures the carrier concentration, the Hall mobility, and the electrical resistivity. The reported values have a strong dependence on the size of the ohmic contacts and the geometry of the samples used in conducting the Hall Effect measurements. We conduct a systematic study using four samples cut from the same location in a HPGe crystal made into different sized ohmic contacts or diff…
High Purity germanium point-contact detectors have low energy thresholds and excellent energy resolution over a wide energy range, and are thus widely used in nuclear and particle physics. In rare event searches, such as neutrinoless double beta decay, the point-contact geometry is of particular importance since it allows for pulse-shape discrimination, and therefore for a significant background reduction. In this paper we investigate the pulse-shape discrimination performance of ultra-high purity germanium point contact detectors. It is demonstrated that a minimal net impurity concentration i…
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…
GALATEA is a test facility designed to investigate bulk and surface effects in high purity germanium detectors. A vacuum tank houses an infrared screened volume with a cooled detector inside. A system of three stages allows an almost complete scan of the detector. The main feature of GALATEA is that there is no material between source and detector. This allows the usage of alpha and beta sources as well as of a laser beam to study surface effects. A 19-fold segmented true-coaxial germanium detector was used for commissioning.
A high-purity co-axial germanium detector has been calibrated in efficiency to a precision of about 0.15% over a wide energy range. High-precision scans of the detector crystal and gamma-ray source measurements have been compared to Monte-Carlo simulations to adjust the dimensions of a detector model. For this purpose, standard calibration sources and short-lived on-line sources have been used. The resulting efficiency calibration reaches the precision needed e.g. for branching ratio measurements of super-allowed beta decays for tests of the weak-interaction standard model.
Following work done in the energy region above 100 keV, the high-precision calibration of a co-axial high-purity germanium detector has been continued in the energy region below 100 keV. Previous measurements or Monte-Carlo simulations have been repeated with higher statistics and new source measurements have been added. A precision as in the high-energy part, i.e. an absolute precision for the detection efficiency of 0.2%, has been reached. The low-energy behaviour of the germanium detector was further scrutinized by studying the germanium X-ray escape probability for the detection of low-ene…
The Gamma-Ray Infrastructure For Fundamental Investigations of Nuclei, GRIFFIN, is a new experimental facility for radioactive decay studies at the TRIUMF-ISAC laboratory. The performance of the 16 high-purity germanium (HPGe) clover detectors that will make up the GRIFFIN spectrometer is reported. The energy resolution, efficiency, timing resolution, crosstalk and preamplifier properties of each crystal were measured using a combination of analogue and digital data acquisition techniques. The absolute efficiency and add-back factors are determined for the energy range of 80 - 3450 keV. The de…
In order to suppress the background in rare event detection experiments such as 0ν\b{eta}\b{eta}, this paper developed a set of single/multi-site event pulse shape discrimination methods suitable for strip multi-electrode high-purity germanium detectors. In the simulation of 228Th, this method achieves 7.92 times suppression of SEP events at 2103 keV with a 57.43 % survival rate of DEP events at 1592 keV. The experimental study of 57Co and 137Cs sources was carried out by using the self-developed strip multi-electrode high-purity germanium detector prototype measurement system and compared wit…
The Advanced Molybdenum-based Rare process Experiment in its second phase (AMoRE-II) will search for neutrinoless double-beta (0ν\b{eta}\b{eta}) decay of 100Mo in 200 kg of molybdate crystals. To achieve the zero-background level in the energy range of the double-beta decay Q-value of 100Mo, the radioactive contamination levels in AMoRE crystals should be low. 100EnrMoO3 powder, which is enriched in the 100Mo isotope, is used to grow the AMoRE crystals. A shielded array of fourteen high-purity germanium detectors with 70% relative efficiency each was used for the measurement of background acti…
Pulse shape discrimination for $α$ event rejection in BEGe-type high-purity germanium detectors
M. Smoluchowski Institute of Physics, Jagiellonian University（马泰乌什·斯莫卢霍夫斯基物理研究所，雅盖隆大学）；Jagiellonian University, Doctoral School of Exact and Natural Sciences（雅盖隆大学，精确与自然科学学院博士学校）
A study of signals originating near the lithium-diffused n+ contact of p-type point contact (PPC) high purity germanium detectors (HPGe) is presented. The transition region between the active germanium and the fully dead layer of the n+ contact is examined. Energy depositions in this transition region are shown to result in partial charge collection. This provides a mechanism for events with a well defined energy to contribute to the continuum of the energy spectrum at lower energies. A novel technique to quantify the contribution from this source of background is introduced. Experiments that …
The Advanced Molybdenum-based Rare process Experiment (AMoRE) searches for neutrino-less double-beta (0ν\b{eta}\b{eta}) decay of 100Mo in enriched molybdate crystals. The AMoRE crystals must have low levels of radioactive contamination to achieve low background signals with energies near the Q-value of the 100Mo 0ν\b{eta}\b{eta} decay. To produce low-activity crystals, radioactive contaminants in the raw materials used to form the crystals must be controlled and quantified. 100EnrMoO3 powder, which is enriched in the 100Mo isotope, is of particular interest as it is the source of 100Mo in the …
High purity Ge (HPGe) is the key material for gamma ray detector production. Its high purity level (< 2x10^(-4) ppb of doping impurity) has to be preserved in the bulk during the processes needed to form the detector junctions. With the goal of improving the device performance and expanding the application fields, in this paper many alternative doping processes are evaluated, in order to verify their effect on the purity of the material. In more detail, we investigated the electrical activation of contaminating doping defects or impurities inside the bulk HPGe, induced by both conventional and…
Semiconductor detector fabrication technologies developed decades ago are widely employed today to produce gamma-ray detectors from large volume, single crystals of high purity Ge (HPGe). Most all of these detectors are used exclusively for spectroscopy measurements and are of simple designs with only two impurity based electrical contacts produced with B implantation and Li diffusion. Though these technologies work well for the simple spectroscopy detectors, the Li contact in particular is thick and lacks room temperature stability in a manner that makes it inappropriate for many of the more …
Electrical conduction mechanisms in the disordered material system is experimentally studied for p-type amorphous germanium (a-Ge) used for high-purity Ge detector contacts. The localization length and the hopping parameters in a-Ge are determined using the surface leakage current measured from three high-purity planar Ge detectors. The temperature-dependent hopping distance and hopping energy are obtained for a-Ge fabricated as the electrical contact materials for high-purity Ge planar detectors. As a result, we find that the hopping energy in a-Ge increases as temperature increases while the…
Bayesian inference of high-purity germanium detector impurities based on capacitance measurements and machine-learning accelerated capacitance calculations
Radiation detectors constructed from large volume high purity Ge (HPGe) single crystals are widely used for gamma-ray spectroscopy. The detectors for this application can be simple in that they need only have two electrical contacts for voltage application and signal readout. Such HPGe based detectors have been commercially produced for many decades using standard semiconductor fabrication processes. For the applications of gamma-ray imaging and particle tracking, however, interaction position measurement within the detector as well as the measurement of the deposited energy is required. This …
The time analysis of the signal induced by the drift of charge carriers in high purity germanium detectors provides information on the event topology. Millions of charge carriers are produced in a typical event. Their initial distribution, stochastic diffusion and Coulomb self-repulsion affect the time structure of the signal. We present a comprehensive study of these effects and evaluate their impact on the event discrimination capabilities for the three geometries which will be used in the Legend experiment for neutrinoless double-beta decay.
Pulse shape simulation for the reduced charge collection layer in p-type high-purity germanium detectors
School of Physics and Astronomy, Beijing Normal University（北京师范大学物理与天文学院）；Department of Engineering Physics, Tsinghua University（清华大学工程物理系）；Max-Planck-Institut für Physik（马克斯·普朗克物理研究所）
We report a new method of using the plasma time difference, which results from the plasma effect, between the nuclear and electronic recoil events in high-purity germanium detectors to distinguish these two types of events in the search for rare physics processes. The physics mechanism of the plasma effect is discussed in detail. A numerical model is developed to calculate the plasma time for nuclear and electronic recoils at various energies in germanium detectors. It can be shown that under certain conditions the plasma time difference is large enough to be observable. The experimental aspec…
The characterization of detectors fabricated from home-grown crystals is the most direct way to study crystal properties. We fabricated planar detectors from high-purity germanium (HPGe) crystals grown at the University of South Dakota (USD). In the fabrication process, a HPGe crystal slice cut from a USD-grown crystal was coated with a high resistivity thin film of amorphous Ge (a-Ge) followed by depositing a thin layer of aluminum on top of the a-Ge film to define the physical area of the contacts. We investigated the detector performance including the $I$-$V$ characteristics, $C$-$V$ charac…
We report on the development and construction of the high-purity germanium spectrometer setup GIOVE (Germanium Inner Outer Veto), recently built and now operated at the shallow underground laboratory of the Max-Planck-Institut für Kernphysik, Heidelberg. Particular attention was paid to the design of a novel passive and active shield, aiming at efficient rejection of environmental and muon induced radiation backgrounds. The achieved sensitivity level of <100 μBq/kg for primordial radionuclides from U and Th in typical γ ray sample screening measurements is unique among instruments located at c…
This article reports the measurement of the ionization quenching factor in germanium for nuclear recoil energies between 0.4 and 6.3 keV$_{nr}$. Precise knowledge of this factor in this energy range is relevant for coherent elastic neutrino-nucleus scattering and low mass dark matter searches with germanium-based detectors. Nuclear recoils were produced in a thin high-purity germanium target with a very low energy threshold via irradiation with monoenergetic neutron beams. The energy dependence of the ionization quenching factor was directly measured via kinematically constrained coincidences …
We present results on quasidiffusion studies in large, 3 inch diameter, 1 inch thick [100] high purity germanium crystals, cooled to 50 mK in the vacuum of a dilution refrigerator, and exposed with 59.5 keV gamma-rays from an Am-241 calibration source. We compare data obtained in two different detector types, with different phonon sensor area coverage, with results from a Monte Carlo. The Monte Carlo includes phonon quasidiffusion and the generation of phonons created by charge carriers as they are drifted across the detector by ionization readout channels.
Low capacitance, large volume, high purity germanium (HPGe) radiation detectors have been successfully employed in low-background physics experiments. However, some physical processes may not be detectable with existing detectors whose energy thresholds are limited by electronic noise. In this paper, methods are presented which can lower the electronic noise of these detectors. Through ultra-low vibration mechanical cooling and wire bonding of a CMOS charge sensitive preamplifier to a sub-pF p-type point contact HPGe detector, we demonstrate electronic noise levels below 40 eV-FWHM.
GERDA is an experiment designed to look for the neutrinoless double beta decay of Ge-76. The experiment uses an array of high-purity germanium detectors, enriched in Ge-76, directly immersed in liquid argon. GERDA is presently operating eight enriched coaxial detectors (approximately 15 kg of Ge-76) and about 30 new custom-made enriched BEGe detectors will be deployed in the next phase (additional 20 kg of Ge-76). The paper describes the GERDA off-line analysis of the high-purity germanium detector data. Firstly we present the signal processing flow, focusing on the digital filters and on the …
This article reports the characterization of two High Purity Germanium detectors performed by extracting and comparing their efficiencies using experimental data and Monte Carlo simulations. The efficiencies were calculated for pointlike $γ$-ray sources as well as for extended calibration sources. Characteristics of the detectors such as energy linearity, energy resolution, and full energy peak efficiencies are reported from measurements performed on surface laboratories. The detectors will be deployed in a $γ$-ray assay facility that will be located in the first underground laboratory in Mexi…
A standard high purity germanium (HPGe) detector was used to determine the previously unknown neutron activity of a weak americium-beryllium (AmBe) neutron source. Gamma rays were created through 27Al(n,n'), 27Al(n,gamma) and 1H(n,gamma) reactions induced by the neutrons on aluminum and acrylic disks, respectively. These gamma rays were measured using the HPGe detector. Given the unorthodox experimental arrangement, a Monte Carlo simulation was developed to model the efficiency of the detector system to determine the neutron activity from the measured gamma rays. The activity of our neutron so…
A model of the Li-diffused inactive layer in P-type high purity germanium detectors is built to describe the transportation of charge carriers and calculate the charge collection efficiency therein. The model is applied to calculate charge collection efficiency of a P-type point-contact germanium detector used in rare event physics experiments and validated in another P-type semi-planar germanium detector. The calculated charge collection efficiency curves are well consistent with measurements for both detectors. Effects of the Li doping processes on the charge collection efficiency are discus…
High-purity germanium crystals approximately 12 cm in diameter were grown in a hydrogen atmosphere using the Czochralski method. The dislocation density of the crystals was determined to be in the range of 2000 - 4200 cm-2, which meets a requirement for use as a radiation detector. The axial and radial distributions of impurities in the crystals were measured and are discussed. A planar detector was also fabricated from one of the crystals and then evaluated for electrical and spectral performance. Measurements of gamma-ray spectra from Cs-137 and Am-241 sources demonstrate that the detector h…
A study on cosmogenic activation in germanium was carried out to evaluate the cosmogenic background level of natural and $^{70}$Ge depleted germanium detectors. The production rates of long-lived radionuclides were calculated with Geant4 and CRY. Results were validated by comparing the simulated and experimental spectra of CDEX-1B detector. Based on the validated codes, the cosmogenic background level was predicted for further tonne-scale CDEX experiment. The suppression of cosmogenic background level could be achieved by underground germanium crystal growth and high-purity germanium detector …
In neutron transmutation doped germanium, the thermal neutron fluence of reactor irradiation is as high as 10$^{18}$~n$\cdot$cm$^{-2}$. For radiological safety reasons, general Co or Au neutron flux monitors cannot be easily used. We have experimentally demonstrated the feasibility of measuring the X-rays emitted by the NTD-Ge itself to determine the absolute thermal neutron fluence for the first time. A Micro-Megas Detector (MMD) and a Silicon Drift Detector (SDD) are set up to detect the tagging KX-rays with 9.2 keV and 10.3 keV cascading from the decays of $^{71}$Ge. Combined the detection …
Recent progress on the development of very low noise high purity germanium ionization spectrometers has produced an instrument of 1.2 kg mass and excellent noise performance. The detector was installed in a low-background cryostat intended for use in a direct detection search for low mass, WIMP dark matter. This Transaction reports the thermal characterization of the cryostat, specifications of the newly prepared 1.2 kg p-type point contact germanium detector, and the spectroscopic performance of the integrated system. The integrated detector and low background cryostat achieved full-width-at-…
Measurements with a bare p-type high purity germanium diode (HPGe) submerged in a 19 kg liquid argon (LAr) scintillation detector at MPIK Heidelberg are reported. The liquid argon--germanium system (LArGe) is operated as a 4$π$ anti-Compton spectrometer to suppress backgrounds in the HPGe. This R&D is carried out in the framework of the GERDA experiment which searches for 0$νββ$ decays with HPGe detectors enriched in $^{76}$Ge. The goal of this work is to develop a novel method to discriminate backgrounds in 0$νββ$ search which would ultimately allow to investigate the effective neutrino mass …
The GERDA experiment searches for the neutrinoless double beta decay of Ge-76 using high-purity germanium detectors enriched in Ge-76. 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 an…
The LEGEND experiment, now under construction, will operate a large array of Ge detectors for the search of neutrinoless double-beta decay of $^{76}$Ge. In this paper we report on the process development for the hydrogen reduction of germanium dioxide enriched in $^{76}$Ge as part of the effort to manufacture detectors for the LEGEND experiment. The process was optimized via a kinetic un-reacted shrinking model and tested with a batch of natural GeO$_2$. We completed the reduction of a batch of 23 kg isotopically enriched Ge with an average yield of 99.85%. Subsequently, the Ge was purified to…
A pulse-shape discrimination method based on artificial neural networks was applied to pulses simulated for different background, signal and signal-like interactions inside a germanium detector. The simulated pulses were used to investigate variations of efficiencies as a function of used training set. It is verified that neural networks are well-suited to identify background pulses in true-coaxial high-purity germanium detectors. The systematic uncertainty on the signal recognition efficiency derived using signal-like evaluation samples from calibration measurements is estimated to be 5\%. Th…
We describe the upgrade and performance of the high-purity germanium counting facility Gator, which is dedicated to low-background $γ$-ray spectrometry. Gator is operated at the Gran Sasso Underground Laboratory in Italy, at an average depth of 3600 meter water equivalent, and employed for material screening and selection in ultra-low background, rare-event search experiments in astroparticle physics. The detector is equipped with a passive shield made of layers of copper, lead and polyethylene, and the sample cavity is purged with gaseous nitrogen maintained at positive pressure for radon sup…
Searches for new physics push experiments to look for increasingly rare interactions. As a result, detectors require increasing sensitivity and specificity, and materials must be screened for naturally occurring, background-producing radioactivity. Furthermore the detectors used for screening must approach the sensitivities of the physics-search detectors themselves, thus motivating iterative development of detectors capable of both physics searches and background screening. We report on the design, installation, and performance of a novel, low-background, fourteen-element high-purity germaniu…
The goal of the \textsc{Majorana} \textsc{Demonstrator} project is to search for 0$νββ$ decay in $^{76}\mathrm{Ge}$. Of all candidate isotopes for 0$νββ$, $^{76}\mathrm{Ge}$ has some of the most favorable characteristics. Germanium detectors are a well established technology, and in searches for 0$νββ$, the high purity germanium crystal acts simultaneously as source and detector. Furthermore, p-type germanium detectors provide excellent energy resolution and a specially designed point contact geometry allows for sensitive pulse shape discrimination. This paper will summarize the experiences th…
An infrastructure to characterize germanium detectors has been designed and constructed at the HADES Underground Research Laboratory, located in Mol (Belgium). Thanks to the 223m overburden of clay and sand, the muon flux is lowered by four orders of magnitude. This natural shield minimizes the exposure of radio-pure germanium material to cosmic radiation resulting in a significant suppression of cosmogenic activation in the germanium detectors. The project has been strongly motivated by a special production of germanium detectors for the GERDA experiment. GERDA, currently collecting data at t…
A novel Compton Scanner setup has been built, commissioned and operated at the Max-Planck-Institute for Physics in Munich to collect pulses from bulk events in high-purity germanium detectors for pulse shape studies. In this fully automated setup, the detector under test is irradiated from the top with 661.660 keV gammas, some of which Compton scatter inside the detector. The interaction points in the detector can be reconstructed when the scattered gammas are detected with a pixelated camera placed at the side of the detector. The wide range of accepted Compton angles results in shorter measu…
High-purity germanium (HPGe) detectors are widely used for the measurement of the low concentrated radioactivities. For the Super-Kamiokande-Gadolinium project, the concentration of radium in Gd$_2$(SO$_4$)$_3{\cdot}$8H$_2$O is determined by using HPGe detectors. The amount of the sample is generally limited by the size of the sample space of a HPGe detector. This leads the limitation of the detector efficiency. A new method of chemical extraction using the molecular recognition resin was developed to minimize this problem. Using the developed method, radium could be extracted from Gd$_2$(SO$_…
The \textsc{Gerda} experiment searches for the neutrinoless double beta ($0νβ$beta$) decay of $^{76}$Ge using high-purity germanium detectors made of material enriched in $^{76}$Ge. For Phase II of the experiment a sensitivity for the half life $T_{1/2}^{0ν}\,\,\sim2\cdot10^{26}$ yr is envisioned. Modified Broad Energy Germanium detectors (BEGe) with thick n$^+$ electrodes provide the capability to efficiently identify and reject background events, while keeping a large acceptance for the $0νβ$beta$-decay signal through novel pulse-shape discrimination (PSD) techniques. The viability of produc…
We present a convolutional autoencoder to denoise pulses from a p-type point contact high-purity germanium detector similar to those used in several rare event searches. While we focus on training procedures that rely on detailed detector physics simulations, we also present implementations requiring only noisy detector pulses to train the model. We validate our autoencoder on both simulated data and calibration data from an $^{241}$Am source, the latter of which is used to show that the denoised pulses are statistically compatible with data pulses. We demonstrate that our denoising method is …
