Pulse Shape Analysis Techniques Research Articles

Experimental characterization of discrimination capabilities of EJ-276 and EJ-276G read by SiPM by pulse-shape analysis techniques

This work aims at the investigation of the Pulse Shape Discrimination (PSD) capabilities of two very compact detection systems composed of a 3x3x3 cm3 EJ 276 and a 3x3x3 cm3 EJ 276G (the latter green shifted version) plastic scintillators, both coupled to Silicon Photomultiplier (SiPM). The studied systems are supposed to be the basic elementary cell of a new multi-detector for simultaneous detection of neutrons and light charged particles called NArCoS (Neutron Array for Correlation Studies) having both high angular and energy resolution. In this perspective, the results of the experimental qualification of two prototypes with gamma and alpha radioactive sources and in beam tests are reported, in order to characterize the optimum choice for the new device.

Principal Component Analysis Applied to Digital Pulse Shape Analysis for Isotope Discrimination

Digital pulse shape analysis (DPSA) techniques are becoming increasingly important for the study of nuclear reactions since the development of fast digitizers. These techniques allow us to obtain the (A, Z) values of the reaction products impinging on the new generation solid-state detectors. In this paper, we present a computationally efficient method to discriminate isotopes with similar energy levels, with the aim of enabling the edge-computing paradigm in future field-programmable gate-array-based acquisition systems. The discrimination of isotope pairs with analogous energy levels has been a topic of interest in the literature, leading to various solutions based on statistical features or convolutional neural networks. Leveraging a valuable dataset obtained from experiments conducted by researchers in the FAZIA Collaboration at the CIME cyclotron in GANIL laboratories, we aim to establish a comparative analysis regarding selectivity and computational efficiency, as this dataset has been employed in several prior publications. Specifically, this work presents an approach to discriminate between pairs of isotopes with similar energies, namely, 12,13C, 36,40Ar, and 80,84Kr, using principal component analysis (PCA) for data preprocessing. Consequently, a linear and cubic machine learning (ML) support vector machine (SVM) classification model was trained and tested, achieving a high identification capability, especially in the cubic one. These results offer improved computational efficiency compared to the previously reported methodologies.

Development of digital electronics for the search of SHE nuclei using GARIS-II/III at RIKEN

This work describes the implementation of a digital data acquisition system for the search and study of short-lived isotopes or superheavy elements at the GARIS-III separator at the RIKEN Nishina Center. A reduction to 26±0.5 keV in the average value of the Full Width at Half Maximum of the alpha spectrum at 7.133 MeV was achieved, combined with a reduction of the dead time of the overall system to around a few tens of nanoseconds for the detection of very fast decay chains. Moreover, we measured a reduction of the background induced by particles crossing the implantation detector using pulse shape analysis techniques. The detail of the optimization and analysis will be presented.

Identification and simulation of surface alpha events on passivated surfaces of germanium detectors and the influence of metalisation

Events from alpha interactions on the surfaces of germanium detectors are a major contribution to the background in germanium-based searches for neutrinoless double-beta decay. Surface events are subject to charge trapping, affecting their pulse shape and reconstructed energy. A study of alpha events on the passivated end-plate of a segmented true-coaxial n-type high-purity germanium detector is presented. Charge trapping is analysed in detail and an existing pulse-shape analysis technique to identify alpha events is verified with mirror pulses observed in the non-collecting channels of the segmented test detector. The observed radial dependence of charge trapping confirms previous results. A dependence of the probability of charge trapping on the crystal axes is observed for the first time. A first model to describe charge trapping effects within the framework of the simulation software SolidStateDetectors.jl is introduced. The influence of metalisation on events from low-energy gamma interactions close to the passivated surface is also presented.

Simulation and validation of a planar HPGe detector signal database for use in pulse shape analysis

Due to their excellent spectroscopic capabilities and sensitivity to the position of γ-ray interactions, segmented High-Purity Germanium (HPGe) detectors are frequently used in γ-ray imaging applications. The quality of produced images is heavily dependent on the precision of the identification of interaction position within the detector that can be achieved through segmentation and the application of pulse shape analysis (PSA) techniques. In this work, a simulated database of signals was produced for a HPGe planar detector used in a multi-tiered Compton camera system. The impurity concentration and temperature-dependent mobility parameters of the simulation were optimised and validated by evaluating the pulse-shape response at select positions using experimentally-measured collimated γ-beam data. A grid-search algorithm was developed for analysis of single and double-site γ-ray interactions using signal comparison PSA and is shown to improve the position resolution within the detector.

Pulse shape discrimination for GRIT: Beam test of a new integrated charge and current preamplifier coupled with high granularity Silicon detectors

The GRIT (Granularity, Resolution, Identification, Transparency) Silicon array is intended to measure direct reactions. Its design is based on several layers (three layers in the forward direction, two backward) of custom-made trapezoidal and square detectors. The first stage is 500 μm thick and features 128 × 128 orthogonal strips. Pulse shape analysis for particle identification is implemented for this first layer. Given the compacity of this array and the large number of channels involved (>7500), an integrated preamplifier, iPACI, that gives charge and current information has been developed in the AMS 0.35 μm BiCMOS technology. The design specifications and results of the test bench are presented. Considering an energy range of 50 MeV and an energy resolution (FWHM) of 12 keV (FWHM) for the preamplifier, the energy resolution for one strip obtained from alpha source measurement in real conditions is 35 keV. The current output bandwidth is measured at 130 MHz for small signals and the power consumption reaches 40 mW per detector channel. A first beam test was performed coupling a nTD trapezoidal double-sided stripped Silicon detector of GRIT with the iPACI preamplifier and a 64-channel digitizer. Z = 1 particles are discriminated with pulse shape analysis technique down to 2 MeV for protons, 2.5 MeV for deuterons and 3 MeV for tritons. The effect of the strip length due to the trapezoidal shape of the detector is investigated on both the N- and the P-side, showing no significant impact.

Characterisation of the charge collection properties in a segmented planar HPGe detector

Segmented germanium semiconductor detectors provide excellent γ-ray spectroscopic information whilst also being sensitive to the position of interactions within their active volume. Understanding the detector response to γ-ray interactions throughout its volume is required for interpreting the signal output. This work is the first assessment of the response and charge collection properties of an electrically-cooled Mirion Technologies-manufactured planar HPGe detector with boron implanted p+ and amorphous germanium based n+ strip contacts. The signal response of the detector as a function of γ-ray interaction position has been characterised by performing collimated beam scans with 59.5 keV and 661.7 keV γ-rays. A database of average signals, generated through the detector volume and across the detector surfaces, has been produced. A map of parameterised rise time response through depth has been generated and used to apply pulse shape analysis techniques, improving the position resolution within the detector.

Design and Experimental Validation of an Integrated Multichannel Charge Amplifier for Solid-State Detectors With Innovative Spectroscopic Range Booster

The circuit structure and the experimental performance of a newly integrated multichannel charge-sensitive preamplifier for silicon or germanium detectors are shown. The circuit has been conceived and optimized for the silicon detector GAL-TRACE, employed for light and heavy ion spectroscopy in nuclear physics experiments. The chip includes four channels optimized for anodic signals and one channel specifically designed for cathodic signals. An I2C interface is used to adjust, as needed, a host of key parameters of the preamplifier, such as sensitivity and bandwidth. An integrated range booster yields an exceptional dynamic range of 103 dB for the cathodic channel. The circuit features a low power consumption of 11 mW per channel, a fast rise time of the order of 10 ns, and low noise. Thanks to an accurate design of the input stage, the equivalent noise charge, measured with a commercial shaping amplifier, is as low as 130 rms electrons at a shaping time of 6 $\mu \text{s}$ with 5 pF of detector capacitance. The bandwidth of the preampifier is adequate for pulse shape analysis techniques used for particle discrimination.

Nuclear fragment identification with [formula omitted]E-E telescopes exploiting silicon carbide detectors

A study of the response of three ΔE-E telescopes to fragments produced in nuclear interactions at 40AMeV is presented. All the employed telescopes feature silicon carbide (SiC) detectors for at least one detection stage. Two identification methods have been used and their performance discussed: the ΔE-E technique and the Pulse Shape Analysis technique (for identification of nuclear fragments stopped in a single SiC layer). Identification capabilities similar to those obtained with the best available silicon detectors have been found for the SiC detector prototypes studied in this work.

Towards the lowest-energy limit for light ions identification with silicon pixel-type detectors

.An in-beam test of two pixel-type silicon detectors of the TRACE detector project has been performed at Laboratori Nazionali di Legnaro (Italy). The aim was to investigate the lowest kinetic energy values at which isotopic identification of heavy-ion reactions products with mass Asim 10 is possible, by using a single-layer silicon detector. Two separate read-out chains, analog and digital, were used, and the Pulse Shape Analysis technique was employed to obtain the particle identification matrices for the digitally processed data. The results confirmed the high capability of the Pulse Shape Analysis method which can be used for light ion identification, with performances similar to the analog approach. Separation in both charge and mass was obtained for Li and Be isotopes, however, the presence of a significant background from alpha particles severely limited the data analysis in the lower energy region. Due to this effect, the identification of the light products (7, 6Li isotopes) could be possible down to sim 24.5 MeV only, while the 9, 7Be isotopes were separable down to sim 29 MeV. This gives the value of < 4 MeV/nucleon as the lowest kinetic energy for light products identification by using the pixel-type detectors of the TRACE project, in the present experimental conditions.

New methods to identify low energy 3He with Silicon-based detectors

Methods to discriminate low energy mass-3 particles using Silicon detectors are reviewed. The time-of-flight technique based on analog electronics and measuring time between a reference signal and the charge signal from the Silicon detector does not allow to discriminate 3He from 3H. One method consists in adding a thin layer of Silicon (typically of 20μm) to recover the standard discrimination by ΔE-E identification matrix. Strong inhomogeneities in thickness are observed for these thin detectors. Difficulties in handling such detectors have led to investigate numerical approaches using reverse-mounted neutron transmutation doped Silicon detectors. First, the pulse shape analysis (PSA) technique has been investigated on the current and charge signals. PSA enables the discrimination of tritons and 3He and also 3He and 4He with good figures of merit. Secondly, the time of flight between two digitized signals (a reference signal from the beam pulse and the current or the charge signal) is presented. Both numerical techniques give promising results for the identification of light particles with Silicon detectors.

Digital fast pulse shape and height analysis on cadmium-zinc-telluride arrays for high-flux energy-resolved X-ray imaging.

Cadmium-zinc-telluride (CZT) arrays with photon-counting and energy-resolving capabilities are widely proposed for next-generation X-ray imaging systems. This work presents the performance of a 2 mm-thick CZT pixel detector, with pixel pitches of 500 and 250 µm, dc coupled to a fast and low-noise ASIC (PIXIE ASIC), characterized only by the preamplifier stage. A custom 16-channel digital readout electronics was used, able to digitize and process continuously the signals from each output ASIC channel. The digital system performs on-line fast pulse shape and height analysis, with a low dead-time and reasonable energy resolution at both low and high fluxes. The spectroscopic response of the system to photon energies below (109Cd source) and above (241Am source) the K-shell absorption energy of the CZT material was investigated, with particular attention to the mitigation of charge sharing and pile-up. The detector allows high bias voltage operation (>5000 V cm-1) and good energy resolution at moderate cooling (3.5% and 5% FWHM at 59.5 keV for the 500 and 250 µm arrays, respectively) by using fast pulse shaping with a low dead-time (300 ns). Charge-sharing investigations were performed using a fine time coincidence analysis (TCA), with very short coincidence time windows up to 10 ns. For the 500 µm pitch array (250 µm pitch array), sharing percentages of 36% (52%) and 60% (82%) at 22.1 and 59.5 keV, respectively, were measured. The potential of the pulse shape analysis technique for charge-sharing detection for corner/border pixels and at high rate conditions (250 kcps pixel-1), where the TCA fails, is also shown. Measurements demonstrated that significant amounts of charge are lost for interactions occurring in the volume of the inter-pixel gap. This charge loss must be accounted for in the correction of shared events. These activities are within the framework of an international collaboration on the development of energy-resolved photon-counting systems for high-flux energy-resolved X-ray imaging (1-140 keV).

Background model for the Majorana Demonstrator

The Majorana Collaboration is constructing a system containing 44 kg of high-purity Ge (HPGe) detectors to demonstrate the feasibility and potential of a future tonne-scale experiment capable of probing the neutrino mass scale to ∼15 meV. To realize this, a major goal of the Majorana Demonstrator is to demonstrate a path forward to achieving a background rate at or below 1 count/(ROI-t-y) in the 4 keV region of interest (ROI) around the Q-value at 2039 keV. This goal is pursued through a combination of a significant reduction of radioactive impurities in construction materials and analytical methods for background rejection, for example using powerful pulse shape analysis techniques profiting from the p-type point contact (PPC) HPGe detectors technology. The effectiveness of these methods is assessed using simulations of the different background components whose purity levels are constrained from radioassay measurements. Preliminary background results obtained during the engineering runs of the Demonstrator are presented.

New semi-automatic method for reaction product charge and mass identification in heavy-ion collisions at Fermi energies

This article presents a new semi-automatic method for charge and mass identification of charged nuclear fragments using either ΔE−E correlations between measured energy losses in two successive detectors or correlations between charge signal amplitude and rise time in a single silicon detector, derived from digital pulse shape analysis techniques. In both cases different nuclear species (defined by their atomic number Z and mass number A) can be visually identified from such correlations if they are presented as a two-dimensional histogram (‘identification matrix’), in which case correlations for different species populate different ridge lines (‘identification lines’) in the matrix. The proposed algorithm is based on the identification matrix's properties and uses as little information as possible on the global form of the identification lines, making it applicable to a large variety of matrices. Particular attention has been paid to the implementation in a suitable graphical environment, so that only two mouse-clicks are required from the user to calculate all initialization parameters. Example applications to recent data from both INDRA and FAZIA telescopes are presented.

Application of neural networks to digital pulse shape analysis for an array of silicon strip detectors

The new generation of nuclear physics detectors that used to study nuclear reactions is considering the use of digital pulse shape analysis techniques (DPSA) to obtain the (A,Z) values of the reaction products impinging in solid state detectors. This technique can be an important tool for selecting the relevant reaction channels at the HYDE (HYbrid DEtector ball array) silicon array foreseen for the Low Energy Branch of the FAIR facility (Darmstadt, Germany). In this work we study the feasibility of using artificial neural networks (ANNs) for particle identification with silicon detectors. Multilayer Perceptron networks were trained and tested with recent experimental data, showing excellent identification capabilities with signals of several isotopes ranging from 12C up to 84Kr, yielding higher discrimination rates than any other previously reported.

Front-end electronics for the FAZIA experiment

FAZIA is a multidetector specifically designed to optimize A and Z reaction product identification in heavy-ion collision experiments. This multidetector is modular and based on three-layer telescopes made of two silicon detectors followed by a thick (10 cm) CsI(Tl) scintillator read-out by a photodiode. Its electronics is fully digital. The goal to push at maximum identification capability while preserving excellent energy resolution, can be achieved by using pulse-shape analysis techniques and by making an intensive use of high-speed flash ADCs. This paper presents the front-end part of the electronics.

Performance of the AGATA γ-ray spectrometer in the PreSPEC set-up at GSI

In contemporary nuclear physics, the European Advanced GAmma Tracking Array (AGATA) represents a crucial detection system for cutting-edge nuclear structure studies. AGATA consists of highly segmented high-purity germanium crystals and uses the pulse-shape analysis technique to determine both the position and the energy of the γ-ray interaction points in the crystals. It is the tracking algorithms that deploy this information and enable insight into the sequence of interactions, providing information on the full or partial absorption of the γ ray. A series of dedicated performance measurements for an AGATA set-up comprising 21 crystals is described. This set-up was used within the recent PreSPEC–AGATA experimental campaign at the GSI Helmholtzzentrum für Schwerionenforschung. Using the radioactive sources 56Co, 60Co and 152Eu, absolute and normalized efficiencies and the peak-to-total of the array were measured. These quantities are discussed using different data analysis procedures. The quality of the pulse-shape analysis and the tracking algorithm are evaluated. The agreement between the experimental data and the Geant4 simulations is also investigated.

Characterization of light particles (Z ≤ 2) discrimination performances by pulse shape analysis techniques with high-granularity silicon detector

Pulse shape analysis for light particles ( $Z \leq 2$ ) is studied in a 500 μm thick Double-Sided Stripped Silicon Detector (DSSSD) of nTD type with a pitch lower than 500 μm. Good separation between the $Z = 1$ isotopes is achieved irrespective of the side used for signal pick up with the detector biased at depletion voltage. The low energy threshold for discrimination between $Z = 1$ isotopes is found to be around 2.5 MeV at depletion voltage and the quality of the separation can be slightly improved by using filtering methods. On the other hand, the discrimination performances are enhanced when lowering the bias of the detector at the expense of energy resolution. At nominal bias (i.e. overdepletion) where the energy resolution is the best, no separation between the three hydrogen isotopes is achieved when using the amplitude of the current signal. Discrimination can still be obtained by acquiring the time over a threshold set at 10% of the amplitude after applying a square bipolar filter to the current signal. Besides, in view of the design of the front-end electronics, the effect of the sampling rate needed for pulse shape analysis has been investigated and shows that below 200 MSa/s, the discrimination quality is strongly reduced.

Limits on Spin-independent Couplings of Light Dark Matter WIMPs with a p-type Point-contact Germanium Detector

New limits on spin-independent WIMP-nucleon coupling using 39.5 kg-days of data taken with a p-type point-contact germanium detector with fiducial mass of 840g at the Kuo-Sheng Reactor Neutrino Laboratory (KSNL) is presented. Charactering and understanding the anomalous surface behaviour is of particular significance to this study. The slow rise-time of surface events is identified via software pulse shape analysis techniques. In addition, the signal-retaining and background-rejecting efficiencies are implied to clarify the actual bulk and surface events in the mixed regime at sub-keV range. Both efficiencies are evaluated with calibration sources and a novel n-type point-contact germanium detector. Efficiencies-corrected background spectra from the low-background facility at KSNL are derived. Part of the parameter space in cross-section versus WIMP-mass is probed and excluded.

Background Model for the Majorana Demonstrator

The Majorana Collaboration is constructing a system containing 40kg of HPGe detectors to demonstrate the feasibility and potential of a future tonne-scale experiment capable of probing the neutrino mass scale in the inverted-hierarchy region. To realize this, a major goal of the Majorana Demonstrator is to demonstrate a path forward to achieving a background rate at or below 1 cnt/(ROI-t-y) in the 4 keV region of interest around the Q-value at 2039 keV. This goal is pursued through a combination of a significant reduction of radioactive impurities in construction materials with analytical methods for background rejection, for example using powerful pulse shape analysis techniques profiting from the p-type point contact HPGe detectors technology. The effectiveness of these methods is assessed using simulations of the different background components whose purity levels are constrained from radioassay measurements.