Search

Precise measurement under high counting rates has been a major challenge for γ -ray spectroscopy, where signals generally pile up with each other and their characteristic values are deeply coupled. An off-line pile-up pulse recovery algorithm based on template matching is developed for LaBr 3 scintillation detectors and has been applied to data from China Spallation Neutron Source (CSNS) Back-n white neutron beamline. As an improvement to the traditional Template Matching methods, the predetermined template shape is associated with pulse amplitudes, which is found crucial to achieve the best fitting results. Moreover, a universal strategy of setting the initial values of fitting allows to resolve signals that are separated in time by less than the characteristic rise time. Processing of simulated pulses demonstrates that the lower limit of interval to distinguish successive events is 10 ns, which is a significant improvement to the previous works. The algorithm performs well under a counting rate of 6 × 1 0 7 s−1, and is still effective up to 1 0 8 s−1. Analysis of experimental data indicates that this algorithm recovers over 98% count yield under a counting rates of no more than 4 × 1 0 6 s−1 in this work, providing a more reliable approach to deduce neutron-induced reaction cross sections. [ABSTRACT FROM AUTHOR]

Abstract Pulse digitization at high rates has paved the way for the use of complex, non-standard algorithms, beyond the conventional constant fraction discriminator (CFD) option to obtain the time-stamps of gamma photons arriving to nuclear detectors. Detector pulses can be shaped with digital filters in digital signal processing (DSP) hardware or software, and timestamps can be derived by interpolating or extrapolating the acquired samples. Digitized pulses can be stored in a computer for off-line analysis allowing for many different algorithm variations to be tried on the same data. One difficulty is that the number of filters and parameters to be tuned in order to optimize the timing performance may become too large to be done by hand. For this, in this work we use a genetic algorithm to tune in-silico digital filters, digital time pickup algorithms and their associated parameters. The data set consisted of 500,000 pulses obtained from the last dynode of two ultrafast photomultiplier tubes (model R9779 by Hamamatsu) coupled to monolithic fast inorganic LaBr 3 (Ce) scintillators, in a truncated cone geometry 1.5 × 1 × 1.5". With finely tuned conventional (CFD+TAC) timing electronics, a CRT of 155 ps (FWHM) for a 60Co source has been measured by our group, within the state of the art for detectors of this size. Our alternate digital, in-silico implementation of CFD+TAC, fine-tuned with the same parameters obtained during manual adjustment of the conventional DAQ and applied to pulses digitized at 5 GS/s and 14 bits of vertical resolution, reproduces the performance of the conventional DAQ. On the other hand, a genetic algorithm (GA) is employed to optimize digital recursive filters with up to 8 parameters, and different time-pickup strategies (upper level thresholding, CFD, time extrapolation). The same 500,000 pulses are employed for which the GA finds several families of filters outperforming the manually tuned algorithm by more than 10%. [ABSTRACT FROM AUTHOR]

Abstract An analysis of various n/ γ separation algorithms for a digital neutron spectrometer based on a stilbene crystal is performed. The same set of digital signals is used to test the following n/ γ separation algorithms: the algorithm based on the correlation analysis; the algorithms based on the direct comparison of the pulse fast component and the pulse tail; SDCC (Simplified Digital Charge Collection) algorithm; PGA (Pulse Gradient Analysis) and FGA (Frequency Gradient Analysis) algorithms. It is shown that the algorithm based on the correlation analysis makes it possible to carry out the most effective n/ γ separation for the entire pulse areas range. The shape of the distributions of the separation parameter for electrons and protons is analyzed. The correctness of these distributions approximation by the Gaussian curve is justified. An algorithm for the selection of the recoil protons resulting from the neutron interaction with the scintillator is proposed. It is based on the dynamic change of the separation parameter threshold depending on the pulse area. The distributions of the separation parameter for electrons and recoil protons strongly overlap for small pulse areas. In this case, the threshold for the separation parameter is chosen on the basis of the fixed probability of the event false identification. In our work, for example, the contribution of false events was fixed at 1%. With the increase of the pulse area, the distributions for electrons and protons begin to separate well. In this case it is convenient to set the threshold on the separation parameter at the point of their intersection. The use of this algorithm makes it possible to control the fraction of the falsely identified events for the entire range of the pulse areas. This is especially important for low-energy recoil protons, for which the probability of false event identification reaches its maximum. This algorithm was applied to the measurements performed by the time-of-flight method with a 252 Cf source. The suppression degree of the prompt gamma-ray peak was 1100 at the neutron detection threshold of 275 keV. The contribution of the falsely identified events for the different parts of the time-of-flight spectrum using the proposed event selection algorithm was studied. The largest contribution to the false pulses was found for the neutron distribution edges — for the neutrons with the energies above 8 MeV and below 0.5 MeV, and it did not exceed 0.23% in the entire range of the pulse areas. [ABSTRACT FROM AUTHOR]

In this article we present a method of characterizing scintillating materials by digitization of each individual scintillation pulse followed by digital signal processing. With this technique it is possible to measure the pulse shape and the energy of an absorbed gamma photon on an event-by-event basis. In contrast to time-correlated single photon counting technique, the digital approach provides a faster measurement, an active noise suppression, and enables characterization of scintillation pulses simultaneously in two domains: time and energy. We applied this method to study the pulse shape change of a CsI(Tl) scintillator with energy of gamma excitation. We confirmed previously published results and revealed new details of the phenomenon. [ABSTRACT FROM AUTHOR]

We report on the dedicated implementation of the blocking technique for a LaBr 3 (Ce) detector as well as associated electronics and data acquisition system for border protection applications. The detector is meant to perform delayed γ -ray spectroscopy of fission fragments produced via photofission induced by a high intensity pulsed photon beam. The gating technique avoids saturation of the detection chain during irradiation. The resulting setup allows us to successfully perform delayed γ -ray spectroscopy starting only 30 ns after the gating operation. The measured energy resolution ranges from 5% to 6.5% at 662 keV depending on the γ -ray detection time after the gating operation. [ABSTRACT FROM AUTHOR]

Deuterated ( 2 H -based) organic scintillators, unlike conventional 1 H -based scintillators can generate pulse-shape gated neutron energy spectra 1 to 30 MeV without need for time-of-flight (ToF). They are especially appropriate for measurements at high-intensity, low-energy accelerators where a pulsed beam is not available. Even when the latter is available, the loss in effective beam intensity combined with the need for suitable long-path ToF measurements can be very inefficient. In contrast, as we demonstrate, arrays of d- benzene scintillators (NE230, BC537, EJ315) and recently xylene-d10 (EJ301D) together with related digital signal processing (DSP) can provide highly efficient neutron energy measurements w/o n-ToF. We also report the recent synthesis and initial testing of crystalline d-stilbene as a neutron detector. [ABSTRACT FROM AUTHOR]

X-ray computed tomography (CT) is non destructive visualization inside the human body. Recently, X-ray photon-counting CT (PC-CT) has drawn attention for reducing the high doses used for patients and acquiring spectral information to identify materials. To enable photon-counting imaging with a wide region (∼ 60 mm long) of a small animal, we developed a photon-counting system using a 64-channel multi-pixel photon counter (MPPC) array with a fast temporal response (a few nanoseconds) and a very large signal gain (∼ 1 0 6 ), combined with a 64-channel yttrium–gadolinium–aluminum–gallium garnet scintillator array. In particular, to realize ultra-fast analog and digital signal processing (> 10 MHz/channel), we developed a 64-channel large-scale integrated circuit (LSI) named MPPC-CT64. We had previously developed a 16-channel PC-CT system with a 16-channel LSI (MPPC-CT16). Although the MPPC-CT16 realized photon-counting imaging for a ∼ 16-mm-long phantom, there were some energy uncertainties in the LSI, which degraded the obtained CT image quality. The MPPC-CT64 implements a function for correcting the threshold energies and also increases the number of energy thresholds from four to six, which provides more precise measurements of CT values dependent on X-ray energy. In this paper, we briefly present the electric architecture and performance evaluation of the LSI combined with MPPCs. [ABSTRACT FROM AUTHOR]

The in-beam positron emission tomography (ibPET) is dedicated to radiotherapy imaging of the heavy-ion medical machine (HIMM), which requires precise online measurement, as well as real-time signal processing capability of the electronics. In this paper, we present the implementation of real-time digital signal processing in the data acquisition unit (DAQU) for an ibPET system based on the photomultiplier tube (PMT) readout of lutetium-yttrium oxyorthosilicate (LYSO) scintillator crystals. We have designed 10-channel customized signal processing circuit on a high-performance FPGA, which supports 3 working modes (single event processing mode, self-calibration mode and raw ADC data mode), position calculation, crystal locating, event sorting, and etc. Moreover, the implementation of real-time corrections is capable of dealing with photon peak correction to 511 keV and crystal ID-based time correction. The implemented real-time ibPET system can achieve 46% data compression and beyond 1,000,000 events/sec/channel signal processing capabilities. A series of initial tests is conducted, and the results indicate that this design meets the application requirement on online processing. [ABSTRACT FROM AUTHOR]

Pulse shape discrimination applied to certain fast scintillators is usually performed offline. In sufficiently high-event rate environments data transfer and storage become problematic, which suggests a different analysis approach. In response, we have implemented a general purpose pulse shape analysis algorithm in the XIA Pixie-500 and Pixie-500 Express digital spectrometers. In this implementation waveforms are processed in real time, reducing the pulse characteristics to a few pulse shape analysis parameters and eliminating time-consuming waveform transfer and storage. We discuss implementation of these features, their advantages, necessary trade-offs and performance. Measurements from bench top and experimental setups using fast scintillators and XIA processors are presented. [ABSTRACT FROM AUTHOR]

This paper presents structure, design and implementation of a novel technique for determining the optimal shaping, in time-domain, for spectrometers by means of a Genetic Algorithm (GA) specifically designed for this purpose. The proposed algorithm is able to adjust automatically the coefficients for shaping an input signal. Results of this experiment have been compared to a previous simulated annealing algorithm. Finally, its performance and capabilities were tested using simulation data and a real particle detector, as a scintillator. [ABSTRACT FROM AUTHOR]

Abstract: In this paper we investigate the effects of full digitization of scintillator signals. The requirements on the analog to digital converter (ADC), in terms of sampling rate (f s) and bit resolution, are investigated. Two applications for scintillator detectors are studied, pulse timing and particle species identification. We find that signal reconstruction using sinc interpolation can be used e.g. for high-precision timing of a sampled electric pulse. Timing performances better than 6ps (FWHM) were obtained if f s equals or exceeds twice the maximum frequency of the scintillator pulse. Failing to meet this criterion deteriorates both the performance of pulse timing and particle identification. We find that the bit resolution of the ADC is very important also for timing of pulses. [Copyright &y& Elsevier]

Abstract: The energy resolution of a LaBr(Ce) detector has been studied as a function of the count rate up to 340kHz by using a 12 bit 250MS/s V1720 digitizer. The time resolution achieved by processing off line the digitized signals has been also determined. It appears that the energy resolution obtained with the digitizer is better than that achievable using standard NIM electronics. The time resolution yielded by the digitizer with a software CFTD is about δt=0.8ns (FWHM), slightly worse with respect to δt=0.65ns (FWHM) obtained from standard NIM. However, this time resolution lies well within the requirements for applications in Non-Destructive Analysis of large objects with tagged neutron beams. [Copyright &y& Elsevier]

Abstract: A new signal processing method for PET application has been developed, with discrete circuit components to measure energy and timing of a gamma interaction based solely on digital timing processing without using an amplitude-to-digital convertor (ADC) or a constant fraction discriminator (CFD). A single-channel discrete component time-based readout (TBR) circuit was implemented in a PC board. Initial circuit functionality and performance evaluations have been conducted. Accuracy and linearity of signal amplitude measurement were excellent, as measured with test pulses. The measured timing accuracy from test pulses reached to less than 300ps, a value limited mainly by the timing jitter of the prototype electronics circuit. Both suitable energy and coincidence timing resolutions (∼18% and ∼1.0ns) have been achieved with 3×3×20mm3 LYSO scintillator and photomultiplier tube-based detectors. With its relatively simple circuit and low cost, TBR is expected to be a suitable front-end signal readout electronics for compact PET or other radiation detectors requiring the reading of a large number of detector channels and demanding high performance for energy and timing measurement. [Copyright &y& Elsevier]

We present a digital signal processing system based on a photon counting approach which we developed for a thermal neutron detector consisting of ZnS(Ag): 6 LiF scintillating layers read out with WLS fibers and SiPMs. Three digital filters have been evaluated: a moving sum, a moving sum after differentiation and a digital CR-RC 4 filter. The performances of the detector with these filters are presented. A full analog signal processing using a CR-RC 4 filter has been emulated digitally. The detector performance obtained with this analog approach is compared with the one obtained with the best performing digital approach. [ABSTRACT FROM AUTHOR]

A signal processing system based on field programmable gate arrays (FPGAs) was developed for a 64 channel×64 channel neutron imaging device using a wavelength shifting fiber read-out based on a photon-counting method. Light signals detected by 64 channel multi-anode photomultipliers are amplified and converted to digital photon signals containing incident-position and time information of neutron by using high-speed amplifiers and discriminators. By using coincidence circuits made in FPGAs, the incident position of neutron is determined based on these digital signals. A neutron imaging detector composed of the signal processing system and a neutron imaging device with a ZnS:Ag/6LiF scintillator was evaluated by using neutron beam. It was confirmed that the signal processing based on FPGAs was effective to determine a precise incident position of neutron with high speed and many channel. [Copyright &y& Elsevier]

Coincident events in two scintillator crystals coupled to photomultipliers (PMT) are detected by processing just the digital data of the recorded pulses. For this purpose the signals from both PMTs are continuously sampled by free-running ADCs at a sampling rate of 40 MHz. For each sampled pulse the starting time is determined by processing the pulse data. Even a fairly simple interpolating algorithm results in a FWHM of about 2 ns. [Copyright &y& Elsevier]

Gamma ray spectrometry measurements at high detector counting rates (on the order of 107 s−1 and higher) are relevant for high-temperature plasma diagnostics for existing tokamaks and during the development of gamma ray diagnostic systems for the ITER tokamak under construction. At high loads, to obtain spectra without distortion and with a small amount of dead time, it is necessary to use advanced scintillation detector signal processing methods, which can resolve superimposed pulses. Two algorithms that can be used for digital signal processing of scintillation gamma ray detectors with many piled-up pulses are considered in this article, the fitting and deconvolution methods. These algorithms are compared with both one another and two less sophisticated pulse-height analysis algorithms (maximum height and total sum under the peak) in applications that process model and measured signals. Baseline detection algorithms are also considered, which are necessary when processing signals from detectors. The algorithms are applied to LaBr 3 (Ce) detector signal processing. For the modeled signals, the best results in terms of the number of resolved events and energy resolution at counting rates up to 2 × 107 s−1 of the LaBr 3 (Ce) detector are demonstrated by the fitting method. In the real gamma ray measurements at a loading of 5.1 106 s−1, the deconvolution method demonstrated the best energy resolution. [ABSTRACT FROM AUTHOR]

The proton light yield of organic scintillators has been measured extensively in recent years using fast waveform digitizers and large discrepancies exist in the values reported by different authors. In this letter, we address principles of digital signal processing that must be considered when conducting scintillator light yield measurements. Digitized waveform pulse height values are only proportional to the amount of scintillation light if the temporal shape of the scintillation pulse is independent of the amount of energy deposited. This is not the case for scintillation pulses resulting from fast neutron interactions in organic scintillators. Authors measuring proton light yield should therefore report pulse integral values and ensure that the integration length is long enough to capture most of the scintillation light. [ABSTRACT FROM AUTHOR]

A digital data acquisition system together with MATLAB software for data analysis were utilised to measure the pulse specification of the scintillation detector assembly. Besides, the sensitivity of a photomultiplier tube (PMT) window to gamma-rays was studied to discriminate the signals produced as a result of gamma-ray interactions inside the glass window from those originating from the scintillator cell. The result showed that an algorithm based on the time features was capable to evaluate the contributions of these two different signals. [ABSTRACT FROM AUTHOR]

Coincidence timing resolution (CTR) of 10 ps FWHM would drastically increase the contrast of images in pre-clinical and clinical positron emission tomography (PET) due to the capability to localize the annihilation site along the line of response with unprecedented accuracy. Currently, the scintillators and the photodetectors with their electronic readout are the two limiting factors to reach such a timing resolution. On the photodetector side, the single photon timing resolution (SPTR) must be improved below 4 ps RMS to reach a CTR of 10 ps FWHM. To this end, we propose to use a 3D digital silicon photomultiplier (3DdSiPM) where each single photon avalanche diode (SPAD) has its own quenching circuit (QC) and time-to-digital converter (TDC). To reach an SPTR of 4 ps RMS in an array of SPAD requires to correct each readout pixel individually to minimize the impact of TDC least significant bit (LSB) non-uniformities and SPAD-to-SPAD skew. For this purpose, we developed an array of 256 pixels of SPAD readout circuits optimized for high timing precision with embedded digital signal processing. The latter is a code-to-time conversion where each pixel is individually corrected for TDC LSB variation and skew. Measurement results on single pixels (QC and TDC) show timing jitter down to 8 ps RMS. The uncorrected array timing jitter is about 87 ps RMS and is reduced to 18 ps RMS after skew correction and TDC LSB correction. [ABSTRACT FROM AUTHOR]