Point-contact Germanium Detector Research Articles

Machine Learning for Accurate Energy Analysis of Double Beta Decay Events

Point-contact germanium detectors are used to detect and analyze particles and their decay processes. This research focuses on the search for a phenomenon known as neutrinoless double beta decay, where two neutrons within a nucleus transform into two protons, emitting electrons in the process. Unlike standard double beta decay, no neutrinos are emitted, suggesting that the neutrino may act as its own antiparticle, effectively canceling itself out. Detecting this process would provide crucial insights into the nature of neutrino mass. During such an event, the detector registers a spike in energy due to the emitted electrons, producing a step-like pulse in the output channels. The energy of the event is determined by the difference in charge between the ‘top’ and ‘bottom’ steps of the pulse. However, accurately determining the energy is complicated by the presence of a resistor in the detector, which causes the top step to decay exponentially back to a baseline in preparation for the next event. This decay makes it challenging to determine the true energy of the pulse. My work this summer focused on developing machine learning models to reconstruct the step-like nature of pulses from their decayed versions recorded by the detector. This involved constructing deep neural networks trained on inputs (decayed pulses) and corresponding targets (the original step pulses) to learn to accurately reconstruct the target from the input. Initially, the networks were trained on simulated detector data. Once proficient in reconstruction, electronic noise was added to the input pulses to simulate real-world conditions, under which the models were trained to remove both electronic noise and decay. The models were trained to handle pulses with varying decay constants to improve their generalization ability. These neural networks can now be applied to real detector data for accurate energy analysis of double beta decay events.

Machine learning-based discrimination of bulk and surface events of germanium detectors for light dark matter detection

Surface events that exhibit incomplete charge collection are an essential background source in the light dark matter detection experiments with p-type point-contact germanium detectors. We propose a machine learning-based algorithm to identify bulk and surface events according to their pulse shape features. We construct the training and test set with part of the γ-source calibration data and use the rising edge of the waveform as the model input. This method is verified with the test set and another part of the γ-source calibration data. Results show that this method performs well on both datasets, and presents robustness against the bulk events’ proportion and the dataset size. Compared with the previous approach, the uncertainty is reduced by 16% near the energy threshold on the physics data of CDEX-1B. In addition, the key pattern identified in the waveform is verified to be consistent with its physical nature by digging into this algorithm.

Comparative study of ratio and spectral shape discrimination technique in case of physics event selection for point contact germanium detector in sub-$$keV_{ee}$$ range

Comparative study of ratio and spectral shape discrimination technique in case of physics event selection for point contact germanium detector in sub-$$keV_{ee}$$ range

Search for a Nonrelativistic Boson in Two-Body Antimuon Decay.

We demonstrate the feasibility of probing the charged lepton-flavor-violating decay μ^{+}→e^{+}X^{0} for the presence of a slow-moving neutral boson X^{0} capable of undergoing gravitational binding to large structures and, as such, able to participate in some cosmological scenarios. A short exposure to surface antimuons from beam line M20 at TRIUMF generates a branching ratio limit of ≲10^{-5}. This is comparable to or better than previous searches for this channel, although in a thus-far-unexplored region of X^{0} phase space very close to the kinematic limit of the decay, where m_{X^{0}} approaches m_{μ^{+}}. The future improved sensitivity of the method using a customized p-type point-contact germanium detector is described.

Temperature dependence of the electron-drift anisotropy and implications for the electron-drift model

The electron drift in germanium detectors is modeled making many assumptions. Confronted with data, these assumptions have to be revisited. The temperature dependence of the drift of electrons was studied in detail for an n-type segmented point-contact germanium detector. The detector was mounted in a temperature controlled, electrically cooled cryostat. Surface events were induced with collimated 81 keV photons from a 133Ba source. A detailed analysis of the rise time of pulses collected in surface scans, performed at different temperatures, is presented. The longitudinal anisotropy of the electron drift decreases with rising temperature. A new approach, making use of designated rise-time windows determined by simulations using SolidStateDetectors.jl, was used to isolate the longitudinal drift of electrons along different axes to quantify this observation. The measured temperature dependence of the longitudinal drift velocities combined with the standard electron-drift model as widely used in relevant simulation packages results in unphysical predictions. A modification of the electron-drift model based on assuming phonons to be the dominating scattering centers for electrons is motivated and described. The results of a first implementation of the modified model in SolidStateDetectors.jl are shown. They describe the temperature dependence of the data reasonably well. A general review of the model and the standard input values for mobilities is suggested.

Measurement of the Compton scattering in germanium with a p-type point-contact germanium detector for dark matter detection

Low-energy background through Compton scattering from the ambient γ rays can be contaminated in direct dark matter search experiments. In this paper, we report comparable measurements on low-energy spectra via Compton scattering from several γ-ray sources with a p-type point-contact germanium detector. The spectra between 500 eV and 18 keV have been measured and analyzed. Moreover, the features of the electron binding effect, particularly at the edges of the K- and L-shells in the germanium atom, were observed with different gamma sources and were consistent with the models in the Geant4 simulation. An empirical background model is proposed that provides insights into understanding the low-energy background in germanium for direct dark matter experiments.

Modeling the charge collection efficiency in the Li-diffused inactive layer of P-type high purity germanium detector

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 discussed based on the model. This model can be easily extended to other P-type germanium detectors, for instance, the P-type broad-energy Ge detector, and the P-type inverted-coaxial point-contact detector.

Performance of a convolutional autoencoder designed to remove electronic noise from p-type point contact germanium detector signals

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 able to preserve the underlying shapes of the pulses well, offering improvement over traditional denoising methods. We also show that the shaping time used to calculate energy with a trapezoidal filter can be significantly reduced while maintaining a comparable energy resolution. Under certain circumstances, our denoising method can improve the overall energy resolution. The methods we developed to remove electronic noise are straightforward to extend to other detector technologies. Furthermore, the latent representation from the encoder is also of use in quantifying shape-based characteristics of the signals. Our work has great potential to be used in particle physics experiments and beyond.

Exotic Dark Matter Search with the CDEX-10 Experiment at China’s Jinping Underground Laboratory

A search for exotic dark matter (DM) in the sub-GeV mass range has been conducted using 205kg day data taken from a p-type point contact germanium detector of the CDEX-10 experiment at China's Jinping underground laboratory. New low-mass dark matter searching channels, neutral current fermionic DM absorption (χ+A→ν+A) and DM-nucleus 3→2 scattering (χ+χ+A→ϕ+A), have been analyzed with an energy threshold of 160eVee. No significant signal was found; thus new limits on the DM-nucleon interaction cross section are set for both models at the sub-GeV DM mass region. A cross section limit for the fermionic DM absorption is set to be 2.5×10^{-46} cm^{2} (90%C.L.) at DM mass of 10 MeV/c^{2}. For the DM-nucleus 3→2 scattering scenario, limits are extended to DM mass of 5 and 14 MeV/c^{2} for the massless dark photon and bound DM final state, respectively.

A novel wide-angle Compton Scanner setup to study bulk events in germanium detectors

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 measurement times in comparison to similar setups where only perpendicularly scattered gammas are selected by slit collimators. In this paper, the construction of the Compton Scanner, its alignment and the procedure to reconstruct interaction points in the germanium detector are described in detail. The creation of a first pulse shape library for an n-type segmented point-contact germanium detector is described. The spatial reconstruction along the beam axis is validated by a comparison to measured surface pulses. A first comparison of Compton Scanner pulses to simulated pulses is presented to demonstrate the power of the Compton Scanner to test simulation inputs and models.

Search for Spontaneous Radiation from Wave Function Collapse in the Majorana Demonstrator.

The Majorana Demonstrator neutrinoless double-beta decay experiment comprises a 44kg (30kg enriched in ^{76}Ge) array of p-type, point-contact germanium detectors. With its unprecedented energy resolution and ultralow backgrounds, Majorana also searches for rare event signatures from beyond standard model physics in the low energy region below 100keV. In this Letter, we test the continuous spontaneous localization (CSL) model, one of the mathematically well-motivated wave function collapse models aimed at solving the long-standing unresolved quantum mechanical measurement problem. While the CSL predicts the existence of a detectable radiation signature in the x-ray domain, we find no evidence of such radiation in the 19-100keV range in a 37.5kg-y enriched germanium exposure collected between December 31, 2015, and November 27, 2019, with the Demonstrator. We explored both the non-mass-proportional (n-m-p) and the mass-proportional (m-p) versions of the CSL with two different assumptions: that only the quasifree electrons can emit the x-ray radiation and that the nucleus can coherently emit an amplified radiation. In all cases, we set the most stringent upper limit to date for the white CSL model on the collapse rate, λ, providing a factor of 40-100 improvement in sensitivity over comparable searches. Our limit is the most stringent for large parts of the allowed parameter space. If the result is interpreted in terms of the Diòsi-Penrose gravitational wave function collapse model, the lower bound with a 95%confidence level is almost an order of magnitude improvement over the previous best limit.

Identification of anomalous fast bulk events in a p-type point-contact germanium detector

The ultralow detection threshold, ultralow intrinsic background, and excellent energy resolution of p-type point-contact germanium detectors are important for rare-event searches, in particular for the detection of direct dark matter interactions, coherent elastic neutrino-nucleus scattering, and neutrinoless double beta decay. Anomalous bulk events with an extremely fast rise time are observed in the CDEX-1B detector. We report a method of extracting fast bulk events from bulk events using a pulse shape simulation and reconstructed source experiment signature. Calibration data and the distribution of X-rays generated by intrinsic radioactivity verified that the fast bulk experienced a single hit near the passivation layer. The performance of this germanium detector indicates that it is capable of single-hit bulk spatial resolution and thus provides a background removal technique.

Pulse shape analysis in Gerda Phase II

The GERmanium Detector Array (Gerda) collaboration searched for neutrinoless double-beta decay in ^{76}Ge using isotopically enriched high purity germanium detectors at the Laboratori Nazionali del Gran Sasso of INFN. After Phase I (2011–2013), the experiment benefited from several upgrades, including an additional active veto based on LAr instrumentation and a significant increase of mass by point-contact germanium detectors that improved the half-life sensitivity of Phase II (2015–2019) by an order of magnitude. At the core of the background mitigation strategy, the analysis of the time profile of individual pulses provides a powerful topological discrimination of signal-like and background-like events. Data from regular ^{228}Th calibrations and physics data were both considered in the evaluation of the pulse shape discrimination performance. In this work, we describe the various methods applied to the data collected in Gerda Phase II corresponding to an exposure of 103.7 kg year. These methods suppress the background by a factor of about 5 in the region of interest around Q_{beta beta }= 2039 keV, while preserving (81pm 3)% of the signal. In addition, an exhaustive list of parameters is provided which were used in the final data analysis.

Development of planar P-type point contact germanium detectors for low-mass dark matter searches

The detection of low-energy deposition in the range of sub-eV through ionization using germanium (Ge) with a bandgap of sim 0.7 eV requires internal amplification of the charge signal. This can be achieved through high electric field that accelerates charge carriers, which can then generate more charge carriers. The minimum electric field required to generate internal charge amplification is derived for different temperatures. We report the development of a planar point contact Ge detector in terms of its fabrication and the measurements of its leakage current and capacitance as a function of applied bias voltage. With the determination of the measured depletion voltage, the field distribution is calculated using GeFiCa, which predicts that the required electric field for internal charge amplification can be achieved in proximity to the point contact. The energy response to an Am-241 source is characterized and discussed. We conclude that such a detector with internal charge amplification can be used to search for low-mass dark matter.

First experimental measurements with the Segmented Inverted-coaxial GerMAnium (SIGMA) detector

The SIGMA project aims to develop a p-type segmented inverted coaxial point contact germanium detector for gamma-ray tracking and imaging. The performance of the first prototype of this type of detector is reported. The measurements performed verified the size and shape of the crystal and determined the location of the complex active volumes of the detector segments. The latter determines the charge collection times that are related to the position of interaction. Good agreement is found between the results and the specified crystal design. The energy resolution of the point contact has been measured to be 0.84 keV and 2.21 keV at 122 keV and 1332 keV, respectively.

Probing the Neutrino-Nucleus Elastic Scattering with Point Contact Germanium detectors and its Quantum-Mechanical Coherency Effects

Neutrino Nucleus Elastic scattering (υAel ) is a well understood Standard Model process. A generic scale of Eυ <50 MeV is usually taken to characterize the requirement of coherency in vAel process. To quantify this transition, we formulated a universal parametrization of the degree of coherency. The TEXONO experiment is pursuing measurement of the υAel interactions with the reactor neutrinos at Kuo-Sheng Reactor Neutrino Laboratory (KSNL), in a kinematic regime where the Quantum Mechanical coherency is complete. Point-contact germanium detectors at sub-keV sensitivities are adopted to detect the low-energy nuclear recoils in υAel . We report our latest status and plan of this program. A complementing line of research is on the formulation of a universal scheme to quantify the QM coherency effects in υAel , and study their dependence on experimental configurations. Constraints from existing measurements from COHERENT experiment are presented.

Surface Characterization of P-Type Point Contact Germanium Detectors

P-type point contact (PPC) germanium detectors are used in rare event and low-background searches, including neutrinoless double beta (0νββ) decay, low-energy nuclear recoils, and coherent elastic neutrino-nucleus scattering. The detectors feature an excellent energy resolution, low detection thresholds down to the sub-keV range, and enhanced background rejection capabilities. However, due to their large passivated surface, separating the signal readout contact from the bias voltage electrode, PPC detectors are susceptible to surface effects such as charge build-up. A profound understanding of their response to surface events is essential. In this work, the response of a PPC detector to alpha and beta particles hitting the passivated surface was investigated in a multi-purpose scanning test stand. It is shown that the passivated surface can accumulate charges resulting in a radial-dependent degradation of the observed event energy. In addition, it is demonstrated that the pulse shapes of surface alpha events show characteristic features which can be used to discriminate against these events.

First results from a search for coherent elastic neutrino-nucleus scattering at a reactor site

The deployment of a low-noise 3 kg p-type point contact germanium detector at the Dresden-II power reactor, 8 meters from its 2.96 GW$_{th}$ core, is described. This location provides an unprecedented (anti)neutrino flux of 8.1$\times 10^{13} ~\bar{\nu_{e}}/$cm$^{2}$s. When combined with the 0.2 keV$_{ee}$ detector threshold achieved, a first measurement of CE$\nu$NS from a reactor source appears to be within reach. We report on the characterization and abatement of backgrounds during initial runs, deriving improved limits on extensions of the Standard Model involving a light vector mediator, from preliminary data.

Pulse shape simulation of p-type point contact germanium detector used in the rare physics events search

In this work, we have developed the simulation tools to study the pulse shape of the p-type point contact germanium (pPCGe) detectors. Charge signal pulse has been simulated within the pPCGe detectors, based on the “Shockley–Ramo modeling” and has been compared to the experimentally measured charge signal pulse. We have also utilized the “finite difference method” technique for simulating the electric field within the pPCGe detector volume to study the drifting process of the charge carrier. The simulated charge signal pulse, based on our simulation tools, has been found compatible with the experimental results.

Simulation of semiconductor detectors in 3D with SolidStateDetectors.jl

The open-source software package SolidStateDetectors.jl to calculate the fields and simulate the drifts of charge carriers in solid state detectors, especially in large volume high-purity germanium detectors, together with the corresponding pulses, is introduced. The package can perform all calculations in full 3D while it can also make use of detector symmetries. The effect of the surroundings of a detector can also be studied. The package is programmed in the user friendly and performance oriented language julia, such that 3D field calculations and drift simulations can be executed efficiently and in parallel. The package was developed for high-purity germanium detectors, but it can be adjusted by the user to other types of semiconductors. The verification of the package is shown for an n-type segmented point-contact germanium detector. Additional features of SolidStateDetectors.jl, which are under development are listed.