Digital Pulse Shape Discrimination Research Articles

Wavelet-based digital pulse-shape method for discrimination between neutron and gamma-rays with organic scintillation detectors

Organic scintillator detectors are central instruments in many applications involving fast neutrons. Many organic scintillator detectors exhibit pulse-shape discrimination (PSD) properties and are widely used to discriminate between neutron events and background gamma rays. PSD methods have been traditionally implemented using analog electronic circuits; however, in recent years, digital techniques have proven to outperform analog methods in areas such as count rate capability. Many digital PSD algorithms with various levels of achievement have been proposed, and those based on the wavelet transform of digitized scintillation pulses have shown great promise for operation at high event rates, where the pulse pile-up effect limits the performance of PSD methods. However, the proposed wavelet-based PSD methods involve intricate calculations that limit their practical use. In this work, we describe a modified version of the wavelet-based PSD methods that offers significant simplification of the PSD process while still producing excellent PSD performance. The method employs the Haar wavelet transform, which is the simplest available wavelet function and is easily implemented on digital hardware, such as field-programmable gate arrays (FPGA). We describe the details of the method, and different aspects of its performance are experimentally demonstrated using an experimental setup comprising a NE213 liquid scintillation detector. A figure-of-merit (FOM) of 1.47 ± 0.07 is achieved with an energy threshold of 500 keVee (electron equivalent energy). An excellent FOM value (1.32) is achieved with a short pulse processing window of only 26 ns, indicating the resilience of the method against the pulse pile-up effect.

Logistic regression-time–frequency algorithm for α/β discrimination in silicon photomultiplier–based Phoswich detectors

Phoswich (phosphor sandwich) systems composed of ZnS(Ag) and plastic dual scintillators have the ability to distinguish α and β particles via digital pulse-shape discrimination. In this work, a novel logistic regression algorithm based on time–frequency features (LR-TF) is presented and evaluated for its ability to digitally discriminate α and β particles in a mixed field. The LR-TF algorithm employs the charge comparison method (CCM) and Fourier gradient analysis (FGA) to extract time–frequency features from pulse shapes, thereby constructing a logistic regression model. Training of the model is shown to enable the discrimination α and β particles. The CCM, FGA, and LR-TF were applied to thousands of pulses obtained with a data acquisition system based on a Phoswich coupled with a silicon photomultiplier. The experimental results show that the LR-TF algorithm provided the best overall performance with misidentification ratios of 1.9 % and 0.63 % for α particles and β particles, respectively, achieving a figure of merit of 4.30, and an area under the receiver operating characteristic of 0.996.

Evidence of fast neutron detection capability of the CLLB scintillation detector

For the first time we report the ability of the CLLB scintillator to detect and discriminate fast neutrons (2–10 MeV energy range) from gamma-rays and thermal neutrons, using a digital pulse shape discrimination technique. Firstly, we conducted measurements in single mode with an Am–Be source, unshielded and shielded, and we tagged a new cluster of events as the fast neutron-induced zone. The Figure of Merit obtained for the fast neutron/gamma discrimination was 1.1. Then, in order to better characterize these new cluster of events induced by fast neutrons, we conducted measurements in coincidence mode. We used a mono-energetic pulsed neutron source, at the Van de Graaff accelerator facility at the Legnaro National Laboratories (INFN-LNL, Legnaro-Italy), and also, a continuous neutron spectrum (using a 252Cf source). Combining all experimental results with a Monte Carlo model of the CLLB detector, built using the GEANT4 tool kit, it was possible to obtain the light output function of 7Li ions in the CLLB crystal (dL/dE = κ; κ = 0.36 ± 0.02). Moreover, the intrinsic fast neutron efficiency was modeled by Monte Carlo, obtaining similar values 1.5%–2% for neutrons between 2 MeV and 10 MeV. Finally, an application of this new feature, in the field of nuclear security and nuclear safeguards, is given.

Rejection of the internal α background in LaBr3:(Ce) detectors by using a wavelet-based pulse-shape discrimination method

LaBr3:(Ce) exhibits the best energy resolution among scintillation γ-ray detectors. The excellent energy resolution of this detector has made it suitable for many radiation detection applications such as environmental monitoring, medical imaging, nuclear security, nuclear physics experiments, etc. However, LaBr3:(Ce) crystal endures from internal radioactivity which produces a considerable background in the measurements. The problem of internal radiation is particularly acute with large size detectors, measuring low-intensity γ-rays. A considerable part of the internal radiation comes from the α-decay of 227Ac and its daughters which are present as an impurity in the detector’s crystal. Unfortunately, due to the very similar chemical properties of Actinium and Lanthanum, it is very difficult to remove the 227Ac impurity by using purification processes, and therefore, the α-background is present in all the commercial crystals. A practical approach for the reduction of this problem is to identify the α-events through an electronic analysis of the shape of output pulses of the detector and reject them. In the past, several digital pulse-shape discrimination (PSD) methods have been proposed to suppress the internal α background in LaBr3:(Ce) detectors. These methods exploit the small difference in the shape of scintillation pulses from α- and γ-ray interactions. The PSD methods have demonstrated some degree of success, which are limited to only a partial rejection of the α-background while further reduction of the α background is highly desirable for low-background γ-ray measurements. In this work, a novel digital PSD method that significantly improves the rejection of the internal α background was developed. The PSD method is based on the wavelet transform of the scintillation pulses, leading to an almost complete rejection of the α background. This method is very useful for low-level radiation measurements with LaBr3:(Ce) detectors involving high-energy γ rays.

Characterization of the neutron/[formula omitted]-ray discrimination performance in an EJ-301 liquid scintillator for application to prompt fission neutron spectrum measurements at CSNS

The FIssion Neutron spectrum Detector Array (FINDA) has been designed for measurement of prompt fission neutron spectrum on the Back-n beam line at the China Spallation Neutron Source (CSNS). In order to test the neutron-γ discrimination performance of the EJ301 scintillator that will be used as a detector unit in the FINDA, a comparative study using three conventional digital pulse shape discrimination (PSD) methods has been carried out. An Am–Be source has been used to generate the mixed neutron-γ radiation and the waveform data has been obtained with a fast digitizer. Pulse interpolation, baseline restoration and pulses pile-up rejection have been applied for digital pulse processing. Charge comparison (CC) method and simplified charge comparison(SCC) method show a better capability of neutron-γ discrimination than rise time comparison(RTC) method over the entire energy range. Rise time comparison method needs a very fast digitizer or pulse interpolation to improve the PSD ability. The analysis of neutron-γ discrimination performance of the EJ301 scintillator provides a strong technical support for fast neutron measurement using the FINDA at CSNS.

Scintillation light yield of Tb:Sr2Gd8(SiO4)6O2

We have constructed the scintillation light yield evaluation system for the scintillator with millisecond decay time, which is based on the digital signal processing and the analog integration circuit with the decay time of 0.1 ms. The digital pulse shape discrimination technique is introduced to check and/or remove the influence of the pile-up events. As the first trial of the absolute scintillation light yield evaluation for scintillators with millisecond decay time, we attempt to acquire the scintillation light yield spectrum of the Tb:Sr2Gd8(SiO4)6O2 (Tb:SrGS). The pulse area spectrum obtained from the Tb:SrGS scintillator shows a clear peak corresponding to the full-energy absorption events of 662 keV gamma-rays from a Cs-137 source. The peak channels in the pulse area spectra are almost the same both for the Tb:SrGS and bismuthgermanate (BGO) scintillator. Considering the difference in the wavelength matching between scintillators and the photomultiplier tube, the Tb:SrGS scintillator has a 2.8 times higher scintillation light yield compared with the BGO. We finally conclude that the scintillator light yield of the Tb:SrGS is evaluated to be 23 000 photons MeV−1.

Experimental comparison of the measurement results of two different silicon photomultipliers and organic scintillator to detect fast neutrons

We report on the study of experimental comparison between the results of two types of silicon photomultiplier (SiPM), to discriminate the signals from fast neutrons and gamma-rays by the digital charge pulse shape discrimination (PSD) method. This uses a total to partial charge ratio analysis. The signals are digitized by a high-speed digital oscilloscope LeCroy WR 620zi with a sample rate of 10 GS/s. This oscilloscope was used to acquire data, which were later processed offline in a specially developed software package written in LABVIEW. The tested detector part consisted from a stilbene crystal (3x3x6 mm3 and 6x6x6 mm3) optically coupled to a SiPM (KETEK 3x3 mm2 and SensL 6x6 mm2 respectively) as a photodetector. Measurements with a Cf-252 source were performed and a figure of merit (FOM) for discrimination between neutron and gamma-ray was calculated for each assembly. The resulting value of FOM for the KETEK 3x3 mm2 SiPM and SensL 6x6 mm2 SiPM was 1.6±0.03 and 1.76±0.05, respectively, the SensL assembly having a slightly better discrimination factor than KETEK one. The obtained results prove a good fast neutron detection performance of the SiPMs, which makes it possible to use these types of neutron detectors in the several fields of radiation protection applications and safety of nuclear facilities.

Detection and discrimination of α- and β-radiation with plastic scintillators based on pulse shape discrimination

There is a need for more convenient and effective methods for detecting and discriminating between α- and β-radiation. In this work, two plastic scintillator detectors were used to detect and discriminate between α- and β-radiation based on digital pulse shape discrimination (PSD) . The charge comparison method (CCM) performed better than the time comparison method (TCM), and Detector A (with ZnS doping) showed better results than those of Detector B (without ZnS doping). With Detector A, the total rate of misidentification was approximately 0.25% with the appropriate discrimination parameters; with Detector B, the total rate of misidentification was approximately 1.5%. Both detectors showed improved discrimination compared with most commercial detectors. This method is a promising means for improving the effectiveness of the detection and discrimination of α- and β-radiation based on PSD.

Performance of linear classification algorithms on α/γ discrimination for LaBr3:Ce scintillation detectors with various pulse digitizer properties

With the development of high-speed readout electronics, the digital pulse shape discrimination (PSD) methods have attracted the attention of more researchers, especially in the field of high energy physics and neutron detection. How to choose a PSD algorithm and corresponding data acquisition system (DAQ) naturally becomes a critical problem to settle down for the detection system designers. In this paper, the relationship between the classification performance of different PSD algorithms and digitizers' sampling properties (including sampling rate and the effective number of bits) has been researched based on LaBr3:Ce scintillation detectors. A self-developed integrated digitizer configured with five different ADCs and a WavePro 404HD oscilloscope were deployed to digitize the waveforms from LaBr3:Ce scintillators. Moreover, three PSD methods, charge comparison method (CCM), least square for classification method (LS) and Fisher's linear discriminant analysis (LDA), based on linear model were applied to discriminate the alpha signals from the intrinsic isotope 227Ac. With the LS method and a 125 MSPS 14-Bit ADC, the FoM value was 1.424±0.042, which is similar to the result from LDA but 31% better than the result of CCM. The discrimination results showed that the performances of LS and LDA are less affected by the sampling rate with respect to the CCM method, which reflects in a better PSD capability. The results of this paper can help the developers of detector systems to make a trade-off among sampling properties, desirable discrimination results and the cost of systems.

Digital pulse shape discrimination technique for liquid scintillation detector EJ-301

A digital pulse shape discrimination (PSD) system has been built using a high speed digital oscilloscope NI-5772 and tested with an EJ-301 liquid scintillation detector. The sampling frequency of this system is 1.6 Gs/s. In this work, the charge comparison method, the rise time method and the zero-crossing time method are used to perform neutron-gamma discrimination. Experimental results show that the studied digital PSD algorithms can discriminate neutron and gamma rays in mixed radiation fields. The digital rise time method shows the optimal neutron-gamma discrimination with integration parameter RC of 10000 ns. Moreover, the zero-crossing time method is investigated through CR-R1C1 and CR-R1C1-R2C2 shaping networks. It is found that for the same pair of values for the two kinds of network, the CR-R1C1-R2C2 shaping network presents better neutron-gamma discrimination with R2C2 of 5 ns. In addition, through the investigation of the distribution of neutron and gamma peaks for different energy regions, the relationship between the ionization density of liquid scintillation detector and the contribution of slow component in the light outputs is demonstrated.

Modernization of the pulse shape discrimination method for neutron and gamma quanta in scintillation detector

In this paper, we investigated the efficiency of several known and new methods of digital pulse shape discrimination for neutrons and gamma quanta. Experimental data were obtained on a setup consists of a Pu-Be neutron source, organic p-terphenyl scintillation detector and 14 bits, 500 MHz sampling rate flash-ADC with capability to store and upload to the host computer long waveforms for further analysis. A comparison is made in between the results of using traditional and new methods for calculating the signal separation efficiency of Figure of Merit (FOM). The best known from the literature value of the efficiency of neutron and gamma quanta discrimination for the Pu-Be source is FOM = 1.5. We obtained the separation efficiency FOM = 1.77 in the scintillation detector with the p-terphenyl crystal, by a new method. Note also that for the known liquid scintillator BC-501A FOM≈1. A new method of scintillation detector pulse shape discrimination from neutrons and gamma quanta is used to detect the neutron yield from compact neutron generator that is created on the basis of carbon nanotubes.

Hybrid method for digital pulse shape discrimination in organic scintillation detectors

Digital signal processing (DSP) simplifies the implementation of pulse shape discrimination (PSD) methods in a cost-effective manner, no matter how complicated they are complicated. This paper deals with the development of a new concept in DSP for PSD, meaning to combine several basis PSD features and obtain a hybrid factor, deeming to inherit benefits of its basis methods. In this regard, we will consider the neutron-gamma discrimination problem, by appropriately combining different PSD methods, namely the Charge Integration Method (CIM), the Zero-Crossing Method (ZCM) and the so-called Simplified Charge Collection Method (SCCM) among several possibilities. It is revealed that the hybrid method should be fed by independent basis PSD factors, keeping in mind that different PSD methods might not be truly uncorrelated. Here, the proposed hybrid PSD factors (via combining CIM-ZCM-SCCM) were implemented. A quantitative analysis of the results, shows that this hybridization concept is generally able to improve the discrimination quality, increasing the FoM over the studied dynamic range. The proposed hybrid formula is simply based on a multiplication of individual PSD factors, but one may define more complex hybrid features in this framework, aiming to improve existing PSD methods by combining them.

A technique for the reduction of pulse pile-up effect in pulse-shape discrimination of organic scintillation detectors

Abstract A technique for the reduction of pulse pile-up effect in digital pulse-shape discrimination (PSD) of neutrons and gamma-rays with organic scintillation detectors is presented. The technique is based on an electronic reduction of the effective decay-time constant of scintillation pulses while retaining the PSD information of the pulses. The experimental results obtained with a NE213 liquid scintillation detector in a mixed radiation field of neutrons and gamma-rays are presented, demonstrating a figure of merit (FOM) of 1.20 ± 0.05 with an energy threshold of 350 keVee (electron equivalent energy) when the effective length of the pulses is reduced to 50 ns.

A General-Purpose Digital Pulse Shape Discrimination Algorithm

Digital pulse shape discrimination (PSD) systems are widely used to extract various kinds of information from the shape of output pulses of radiation detectors. Such systems replace the traditional analog PSD circuits with numerical algorithms running on a digital processor, which makes them more flexible for implementing various PSD methods. However, the available digital PSD algorithms have been generally tailored to the characteristics of the concerned detectors, which limit their general use. Here, for the purpose of building a general-purpose digital PSD module, we present a PSD algorithm that, with minimum alterations, can be used with a wide range of radiation detectors. Our approach is based on using the cosine similarity measure to quantify the variations in the shape of detectors’ pulses with respect to the shape of a unit step pulse. The method is described in detail, and the results of the test experiments with different types of radiation detectors, including a boron tetrafluoride (BF3) proportional counter, a liquid scintillation detector, and high purity germanium (HPGe) detector are shown. The performance of the algorithm is also compared to that of the common rise time discrimination method.

Fast pulse sampling module for real-time neutron–gamma discrimination

An adaptable and compact fast pulse sampling module was developed for the neutron–gamma discrimination. The developed module is well suited for low-cost and low-power consumption applications. It is based on the Domino Ring Sampler 4 (DRS4) chip, which offers fast sampling speeds up to 5.12 giga samples per second (GSPS) to digitize pulses from front-end detectors. The high-resolution GSPS data is useful for obtaining precise real-time neutron–gamma discrimination results directly in this module. In this study, we have implemented real-time data analysis in a field programmable gate array. Real-time data analysis involves two aspects: digital waveform integral and digital pulse shape discrimination (PSD). It can significantly reduce the system dead time and data rate processed offline. Plastic scintillators (EJ-299-33), which have proven capable of PSD, were adopted as neutron detectors in the experiments. A photomultiplier tube (PMT) (model #XP2020) was coupled to one end of a detector to collect the output light from it. The pulse output from the anode of the PMT was directly passed onto the fast sampling module. The fast pulse sampling module was operated at 1 GSPS and 2 GSPS in these experiments, and the AmBe-241 source was used to examine the neutron–gamma discrimination quality. The PSD results with different sampling rates and energy thresholds were evaluated. The figure of merit (FOM) was used to describe the neutron–gamma discrimination quality. The best FOM value of 0.91 was obtained at 2 GSPS and 1 GSPS sampling rates with an energy threshold of 1.5 MeVee (electron equivalent).

Distance metrics for digital pulse-shape discrimination of scintillator detectors

Digital pulse-shape discrimination (PSD) of scintillator detectors has gained popularity as an efficient technique for the discrimination between different types of particles. In this paper, we have investigated the use of simple distance metrics for digital PSD of neutron and gamma-rays in liquid scintillator detectors. The distance metrics are commonly used in the field of pattern recognition and offer the advantage of a simple and effective performance. Our method is based on quantifying the difference between a sampled detector pulse and an ideal step pulse. For this purpose, some of the common distance metrics including Euclidean, City-block, Canberra, root mean square (RMS) and Hellinger were experimentally examined through the discrimination of neutrons and gamma-rays in a liquid scintillator detector and their performances were compared to that of the conventional charge-comparison method. It was found that the City-Block distance metric exhibits a very simple and effective PSD approach by producing a figure-of-merit (FOM) of 0.72 against a FOM = 0.54 for the charge-comparison method in the very low light output range of below 160 keVee (electron equivalent energy) and a mean FOM value of 1.23 over the whole examined light output range of 60–1000 keVee.

Digital discrimination of neutrons and [formula omitted]-rays in liquid scintillation detectors by using low sampling frequency ADCs

It is well known that the digital pulse-shape discrimination (PSD) of fast neutrons and γ-rays in liquid scintillation detectors can be adversely affected by the low sampling frequency of the analog-to-digital converter (ADC). Previous studies have recommended that using an ADC with a sampling frequency of above 250 MHz is necessary to achieve a PSD performance comparable to that of the analog PSD systems. In the present study, we show that, in principle, a sampling frequency of above 32 MHz is sufficient to fully preserve the pulse-shape information of liquid scintillation detectors, though at such sampling frequencies a significant degradation of the PSD performance may arise from the used PSD algorithm. To avoid this problem, a new PSD algorithm in the frequency domain is presented and its excellent performance at low sampling frequencies is experimentally demonstrated. At the sampling frequency of 32 MHz, a Figure-of-Merit (FOM) of 1.31 ± 0.04 in the light output range of 200–1400 keVee (electron equivalent energy) is achieved with an ADC of 10-bit resolution.

A large area detector for thermal neutron flux measurements at the KamLAND site

A large area (0.5 m × 0.5 m) thermal neutron detector was developed based on 0.32 mm-thick 6LiF:ZnS(Ag) scintillator sheets. The detector was constructed for measurements of seasonal variations in low intensity thermal neutron flux at deep underground sites. The detector response to neutrons was studied using a low intensity 241Am/9Be(α,n) neutron source. The digital pulse shape discrimination technique was applied to separate neutron capture events from the background caused by traces of α-emitters from Uranium and Thorium decay chains in scintillator components. The thermal neutron detection efficiency was 0.368 ± 0.018 (0.155 ± 0.009) before (after) event selection based on the digital pulse shape discrimination method. The measured α-particle background event rate in the 6LiF:ZnS(Ag) scintillator was (4.88 ± 0.06(stat))×10−6α cm−2 s−1. As a test of detector performance we measured the thermal neutron flux at the KamLAND area of the Kamioka neutrino observatory and compared it with result of the neutron flux measurement carried out at the Super-Kamiokande site. The thermal neutron flux at KamLAND was (6.43 ± 0.50)×10−6 n cm−2 s−1 that included ∼10% contribution from neutrons with energies above 1 eV. During the detector construction we identified a source of high amplitude noise pulses generated by 5-inch R1250 photomultipliers manufactured by Hamamatsu Photonics K. K.

Particle identification using digital pulse shape discrimination in a nTD silicon detector with a 1 GHz sampling digitizer

In beam test experiments have been carried out for particle identification using digital pulse shape analysis in a 500 μm thick Neutron Transmutation Doped (nTD) silicon detector with an indigenously developed FPGA based 12 bit resolution, 1 GHz sampling digitizer. The nTD Si detector was used in a low-field injection setup to detect light heavy-ions produced in reactions of ∼ 5 MeV/A 7Li and 12C beams on different targets. Pulse height, rise time and current maximum have been obtained from the digitized charge output of a high bandwidth charge and current sensitive pre-amplifier. Good isotopic separation have been achieved using only the digitized charge output in case of light heavy-ions. The setup can be used for charged particle spectroscopy in nuclear reactions involving light heavy-ions around the Coulomb barrier energies.

Digital pulse shape discrimination between fast neutrons and gamma rays with para-terphenyl scintillator

In the presented work, we investigated several digital methods of a discrimination signals from fast neutrons and gamma quanta. The experimental setup consists of a Pu-Be neutron source, a scintillation detector with an organic para-terphenyl monocrystal, and a digitizer (CAEN DT5730, 500 MS/s). Mixed waveform sequences were stored and then separated by pulse shape. Four methods were used for signals separation. Comparison of the traditional and the new methods of Figure of Merit (FOM) calculation is given. FOM = 1.5 was obtained in our setup for the minimum threshold value. A scintillation detector with a para-terphenyl crystal was used to measure neutron yield in the neutron generator with carbon nanotubes.