Scintillation Pulse Research Articles

Pulse shape discrimination of LSO and LuYAP scintillators based on Discrete Fractional Fourier Transform

Parallax error decreases the accuracy of the Positron Emission Tomography (PET) scanner. One of suitable solutions to reduce this error is to gain the depth of interaction information in a PET which uses the phoswich detectors. The pulse shape of the scintillator material can identify the corresponding layer of interaction within the phoswich detector by using Pulse Shape Discrimination (PSD) methods. In this work, we propose the PSD based on a Discrete Fractional Fourier Transform (DFRFT) to extract the features of the scintillation pulses. Then, we use the Support Vector Machine (SVM) to classify these features. A data set consists of 100 000 scintillation pulses for LSO and LuYAP crystals are discriminated using the proposed method. Different fraction factors of the DFRFT are studied to select the optimum one. Also, the SVM is applied using linear, radial basis function, or quadratic kernel. The highest efficiency of the proposed PSD is 96.3% when the applied fraction factor is 0.8 and a quadratic SVM kernel is used.

Delayed Neutrons from the Photofission of 238U at Eγ max ≈ 10 MeV in the Intervals between Pulses of Irradiation

Experiments on measuring the energy spectra and time distributions of delayed neutrons are described. The neutrons are produced in 238U photofission under the action of bremsstrahlung photons in the LUE-8-5 pulsed electron accelerator at the Russian Academy of Sciences’ Institute for Nuclear Research. The kinetic energy of incident electrons is ≈10 MeV. Measurements are made in the intervals between beam pulses. Fast neutrons are registered by a scintillation spectrometer, with identification of background γ quanta according to the shape of the scintillation pulses.

Fast neutron and gamma ray pulse shape discrimination in EJ-276 and EJ-276G plastic scintillators

In this paper, we report on the scintillation properties and pulse shape discrimination (PSD) performance of new plastic scintillators, EJ-276 and EJ-276G . Cylindrical samples with dimensions of 1×1 inch and 2×2 inches were tested. Additionally, four samples 2×2 inches in size were used in stack configurations in order to measure the PSD characteristics of large samples (up to 2×8 inches). The paper covers the measurements of neutron/gamma discrimination, emission spectra, photoelectron yield, and analysis of the light pulse shapes originating from events related to gamma-rays and fast neutrons. The results were compared to data recorded previously using plastic scintillators EJ-299-33, EJ-299-33G, EJ-299-34, EJ-299-34G, and an EJ-301 liquid scintillator.

Scintillator light yield measurements with waveform digitizers

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.

Analysis of 83mKr prompt scintillation signals in the PIXeY detector

Prompt scintillation signals from 83mKr calibration sources are a useful metric to calibrate the spatial variation of light collection efficiency and electric field magnitude of a two phase liquid-gas xenon time projection chamber. Because 83mKr decays in two steps, there are two prompt scintillation pulses for each calibration event, denoted S1a and S1b. We study the ratio of S1b to S1a signal sizes in the Particle Identification in Xenon at Yale (PIXeY) experiment and its dependence on the time separation between the two signals (Δ t), notably its increase at low Δ t. In PIXeY data, the Δ t dependence of S1b/S1a is observed to exhibit two exponential components: one with a time constant of 0.05 ± 0.02 μ s, which can be attributed to processing effects and pulse overlap and one with a time constant of 10.2 ± 2.2 μs that increases in amplitude with electric drift field, the origin of which is not yet understood.

Study of gamma-ray background noise for radioactive waste drum characterization with plastic scintillators

In the framework of the radioactive waste drum characterization using neutron coincidence counting, the Nuclear Measurement Laboratory of CEA Cadarache is studying plastic scintillators as an alternative to ideal but costly 3He gas proportional counters. Plastic scintillators are at least 5 times cheaper for the same detection efficiency, and in addition, they detect fast neutrons about three orders of magnitude faster than 3He detectors. However, they are sensitive to gamma rays, which implies the necessity to identify precisely gamma background sources that may affect the useful signal. This paper presents a detailed analysis of the gamma-ray spectrum of a radioactive waste drum containing glove box filters contaminated by plutonium dioxide. Gamma emissions accompanying inelastic scattering (n,n’) and (α,n) reactions that can lead to neutron-gamma coincidences parasitizing useful coincidences from plutonium spontaneous fissions are identified. Some of these parasitic gamma rays having energies up to several MeV, we plan to reject high-energy scintillator pulses with an electronics rejection threshold above 1 MeV, which should preserve the major part of useful fission neutron pulses.

Neural-Network-Based Energy Calculation for Multivoltage Threshold Sampling

Multivoltage threshold (MVT) is an all digital scintillation pulse sampling method with advantages over traditional analog–digital mixed approaches in positron emission tomography (PET). However, MVT has limitations in energy calculation over the whole energy spectrum. We constructed a convolutional neural network (CNN) to establish an accurate mapping between MVT samples and energy values. We acquired pulse signals from five single-channel scintillation detectors with an oscilloscope. The CNN was trained with one detector and tested on all five detectors. It was compared with integration, CNN, and the biexponential fitting methods, with respect to the energy resolution (ER), mean absolute error, and R-squared. The CNN method was also preliminarily tested on a system with 12 672 channels. The results showed that the CNN method greatly outperformed the biexponential fitting and was comparable to integration. On average, the ER was improved from 21.0%@511 keV by biexponential fitting to 12.3%@511 keV by CNN on the signal channel level, and from 24.7%@511 keV to 20.3%@511 keV on the system level. The CNN also obtained energy spectra almost identical to those of the integration method.

A functional form for liquid scintillator pulse shapes

Digitization of detector signals enables analysis of the original waveform to extract timing, particle identification, and energy deposition information. Here we present the use of analytical functions based on sigmoids to model and fit such pulse shapes from liquid organic scintillators, though the method should also be applicable to other detector systems. Neutron and gamma interactions in NE213 detectors were digitized from the phototube anode and fit using a sigmoid-based function. The acuity of the fit in extracting timing information and performing neutron-gamma pulse-shape discrimination are presented and discussed.

Optical and scintillation properties of alkaline earth doped Ga2O3 single crystals prepared by the floating zone method

In this study, we synthesized Ga2O3 single crystals nominally doped with 1.0% alkaline earth elements (Be, Mg, Ca, Sr and Ba) by the floating zone method. Then, we evaluated the optical (photoluminescence spectrum and decay) and scintillation (scintillation spectrum, decay, and pulse height spectrum) properties systematically. In photoluminescence and scintillation spectra, we found an intense emission from the UV to blue range although spectral shapes were different in each sample. Decay times were a few to few tens ns in both photoluminescence and scintillation. When we irradiated α-rays from 241Am, the scintillation light yields of Be, Mg, Ca, Sr and Ba-doped Ga2O3 were 570, 1030, 1100, 1560 and 1600 ph MeV−1(α), respectively.

Synthesis and scintillation properties of cerium-doped Gd2SiO5 nanopowders under alpha radiation and the importance of selecting the appropriate calcination temperature

In this study, single phase cerium-doped Gd2SiO5 (GSO:Ce) nanopowders were successfully synthesized by a Pechini sol-gel process, and the effect of calcination temperature on the crystallinity and purity of GSO phase was investigated by XRD and FTIR analyses. The suitable temperature for calcination of nanopowders and the formation of single phase GSO is estimated to be about 1250 °C. The morphology of samples, elemental composition, and nanoparticle size distribution of the single phase sample were examined by using FESEM, EDX, and DLS analyses. In addition to crystallite size and crystal lattice strain, the crystal lattice parameters of this sample were obtained using a series of equations, the results of which are very close to those obtained from the Rietveld refinement method. Studies show that unlike undoped GSO single crystal, the optical absorption spectra of synthesized GSO nanopowders are continuous and doping with 2 mol% cerium causes the highest photoluminescence intensity at 435 nm under excitation at 345 nm. The scintillation pulse height spectra of these samples were also investigated under alpha radiation. Finally, the importance of selecting the appropriate calcination temperature and its effect on the size of the nanoparticles, photoluminescence intensity, and scintillation pulse height spectrum has been studied.

Features of pulse shape discrimination capability of organic heterogeneous scintillators

Abstract This work studies the PSD (pulse shape discrimination) capability of organic heterogeneous (polycrystalline and composite) scintillation materials, i.e. their ability to separate ionizing radiations by the difference in the form of scintillation pulses generated by the radiations with different specific energy losses (dE/dx). It analyses the parameters describing the formation of a scintillation pulse, the knowledge of which is useful for the design and creation of electronic units optimally solving the PSD problem for a given scintillator. The comparative analysis of the results shows that the PSD capability of organic heterogeneous scintillators is close to the corresponding values for reference single crystals. Stilbene-based materials have the best PSD capability to a separate detection of the ionizing radiations with different values of dE/dx. Organic heterogeneous scintillators based on stilbene and p-terphenyl are promising candidates for development of effective detectors for the ionizing radiations with high dE/dx-values (e.g., fast neutrons and alpha particles) in a presence of gamma background.

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.

Ultrafast inorganic scintillator-based front imager for Gigahertz Hard X-ray imaging

State-of-the art X-ray imaging cameras using silicon sensors for X-ray detection have demonstrated a frame-rate of 10 MHz and excellent performance for X-ray energies below 20 keV. We proposed a pixelated ultrafast inorganic scintillator-based front imager for GHz hard X-ray imaging. The proposed imager is featured with a total absorption for hard X-ray photons, and provides sub-ns scintillation pulse width crucial for X-ray bunches of a few ns spacing foreseen at the proposed MaRIE facility. We measured temporal response of a dozen ultrafast and fast inorganic scintillators at the 10-ID-B site of the Advanced Photon Source (APS) of ANL. Crystal’s response to hybrid X-ray beam of 30 keV, consisting of singlet bunches of 50 ps width and septuplet bunches of 27 ps width with 2.83 ns bunch spacing, was measured. Ultrafast inorganic scintillators, such as BaF2:Y and ZnO:Ga, show clearly resolved X-ray bunches for septuplets, as well as no degradation of amplitude for continuous eight septuplets, providing a proof of principle for the ultrafast inorganic scintillator-based total absorption front imager for the proposed MaRIE project.

Time-based signal sampling using sawtooth-shaped threshold

Energy measurement of scintillation pulses by using time-over-threshold (TOT) has advantages of low readout cost, low power consumption, and less complexity compared with conventional analog-to-digital converter (ADC) approaches. Therefore, TOT is attractive in positron emission tomography (PET) systems based on silicon photomultiplier (SiPM) arrays requiring many readout channels. However, poor energy resolution and linearity of TOT leads to degradation of the overall PET detector and system performance, which is unsuitable for high-performance PET systems. To overcome these limitations, we propose a novel time-based signal-sampling method, sawtooth threshold sampling (STS), for PET scintillation detectors. This method uses a sawtooth-shaped threshold signal generated adaptively to the input scintillation pulse. Based on the time difference between the rising and falling edges of the digital pulse train generated by comparing the input scintillation pulse to the sawtooth signal, we can estimate the input scintillation pulse amplitude at the several time points. We compared several curve-fitting and numerical integration methods for energy estimation from the STS samples. Coincidence data between two identical scintillation detectors composed of one-to-one coupled SiPM and LGSO crystal (3 × 3 × 20 mm3) was measured using the proposed STS circuit. For timing- and energy-resolution measurement, STS (10.5% ± 0.21% and 200 ± 5.8 ps) was superior than simple TOT (16.4% ± 0.52% and 224 ± 8.2 ps) and was similar to high-speed ADC (9.84% ± 0.11% and 193 ± 4.3 ps). In conclusion, the proposed method can be a cost-effective solution for data collection in future SiPM-based PET systems.

Pulse shape discrimination based on Fractional Discrete Cosine Transform

One of the most serious errors in PET systems is the parallax error, which occurs due to spatial resolution limitation in PET. This error can be solved by using multilayer scintillation detector (phoswish detector) where each layer has a different decay constant. Each phoswish detector is optically coupled to a single PMT. Hence, by applying pulse shape discrimination (PSD) methods to identify the scintillated layer, the parallax error can be reduced. This paper proposes a PSD method relied on a Fractional Discrete Cosine Transform (FrDCT) and a Support Vector Machine (SVM) classifier. The proposed method can be used to differentiate between two arbitrary types of scintillation pulses where each type has a different decay constant. For demonstration of its performance, the proposed FrDCT-based method is applied to a data set of scintillation pulses of LSO and LuYAP crystals. Furthermore, FrDCT with different factors as well as different kernels of SVM; such as Linear, Quadratic, and Radial Basis Function (RBF) kernels, are used to study their effects on the discrimination efficiency. The highest achieved efficiency is equal to 95.9% for the 0.9 factor of FrDCT using a quadratic kernel SVM. On the other hand, the linear kernel consumes the lowest execution time at the expense of slightly reducing the discrimination efficiency.

A digital pulse shortening method for the mitigation of pulse pile-up effect in scintillation radiation detectors

The pulse pile-up effect can significantly degrade the spectroscopic performance of scintillation radiation detectors at high counting rates. This paper reports on a digital pulse processing method for shortening the duration of scintillation pulses, thereby alleviating the pulse pile-up effect. The method operates based on replacing the decay-time constant of the scintillation pulses with a shorter decay-time constant. The details of the digital algorithm are presented and the performance of the method at a high counting rate of 795 kHz is experimentally examined with a NaI(Tl) detector. The effects of the pulse shortening on the spectroscopic performance of the system are also discussed.

Application of deconvolution to recover frequency-domain multiplexed detector pulses

Multiplexing of radiation detectors reduces the number of readout channels, which in turn reduces the number of digitizer input channels for data acquisition. We recently demonstrated frequency domain multiplexing (FDM) of pulse mode radiation detectors using a resonator that converts the detector signal into a damped sinusoid by convolution. The detectors were given unique "tags" by the oscillation frequency of each resonator. The charge collected and the time-of-arrival of the detector pulse were estimated from the corresponding resonator output in the frequency domain. In this paper, we demonstrate a new method to recover the detector pulse from the damped sinusoidal output by deconvolution. Deconvolution converts the frequency-encoded detector signal back to the original detector pulse. We have developed a new prototype FDM system to multiplex organic scintillators based on convolution and deconvolution. Using the new prototype, the charge collected under the anode pulse can be estimated from the recovered pulse with an uncertainty of about 4.4 keVee (keV electron equivalent). The time-of-arrival can be estimated from the recovered pulse with an uncertainty of about 102 ps. We also used a CeBr3 inorganic scintillator to measure the Cs-137 gamma spectrum using the recovered pulses and found a standard deviation of 13.8 keV at 662 keV compared to a standard deviation of 13.5 keV when the original pulses were used. Coincidence measurements with Na-22 using the deconvolved pulses resulted in a timing uncertainty of 617 ps compared to an uncertainty of 603 ps using the original pulses. Pulse shape discrimination was also performed using Cf-252 source and EJ-309 organic scintillator pulses recovered by deconvolution. A figure of merit value of 1.08 was observed when the recovered pulses were used compared to 1.2 for the original pulses.

Separate detection of ionizing radiation with different specific energy losses by organic heterostructured scintillators

Molecular organic scintillation materials are the most effective objects for creating systems that detect the kinds of radiation, that the most harmful to humans (alpha particles, fast neutrons, etc.). In addition, organic crystals and liquids are capable to separate these types of radiation from photons of background gamma radiation. In these scintillators, ionizing radiation generates two types of luminescent response– prompt and delayed radioluminescence Ionizing radiation with a high specific energy loss dE/dx, i.e. energy loss E per unit path length x, generate a scintillation pulse in these media with a high proportion of the slow component. Recently, new types of scintillators have been developed, namely, heterogeneous organic scintillators containing single-crystal scintillation grains that can be combined by hot pressing sintering (polycrystals or Van der Waals ceramics) or can be incorporated into a transparent gel composition (composite scintillators). the ability of heterogeneous organic scintillators to separate signals from radiation with different dE/dx and the physical basis of this process in heterogeneous scintillation materials remain one of the urgent, unexplored problems. This work presents the results of the study of the form of scintillation pulse shapes for the samples of organic single crystals, polycrystals and compositional scintillators based on stilbene in comparison with the same results obtained for p-terphenyl and anthracene for various types of ionizing radiation excitations. The peculiarities of the influence of the triplet-triplet annihilation process on the formation of a slow component of the radioluminescence pulse in these systems have being studied. We found that the ability of new types of organic heterogeneous materials (polycrystals and composite scintillators) to the separate registration of ionizing radiation in the shape of the scintillation pulse is close to the corresponding values that characterize this ability of structurally perfect single crystals.

Preparation and scintillation properties of Eu2+:CaF2 scintillation ceramics

Different concentration of Eu2+ -doped CaF2 ceramics, designed to the low (0.01 at% and 0.04 at%) and the high (0.2 at% and 0.5 at%), have been prepared by hot-press sintering method. Under 355 nm laser excitation, all the ceramics showed emission bands at 425 nm due to 5d-4f transitions of Eu2+ ions and weak emission peaks at 574, 617 and 630 nm due to the 4-4f transitions of Eu3+ ions. Under X-rays irradiation, the emission bands at 315 nm and 422 nm, ascribed STE and 5d-4f transitions of Eu2+ ions, were observed and the weak emission peaks, ascribed 4f‐4f transitions of Eu3+ ions, were also observed in all the samples. Furthermore, the scintillation decay lifetime and pulse height spectra of the samples were investigated.

Pulse shape analysis of individual gamma events—Correlation to energy resolution and the possibility of its improvement

The measurement in 2014 of the nonproportionality of each decay component in CsI:Tl found opposite slopes of the fast and tail nonproportionality curves above about 10 keV. Somewhat earlier experiments on nonproportionality and resolution versus shaping time in NaI:Tl and CsI:Tl showed that proportionality and intrinsic resolution could be improved by including the slower “tail” component of the scintillation pulse. The observed opposing nonproportionality trends of fast and tail components constitute a basis for improvement of scintillator nonproportionality if they are added in a suitable linear combination. We examine whether combining the rise and decay components pulse by pulse with an algorithm of optimized proportions may also improve energy resolution. The premise is that a scintillation pulse carries more information about the particle stopping event than is conveyed in a simple measurement of the pulse height. In this work, we measured pulse shapes of individual gamma events in CsI:Tl and other scintillators using an eMorpho multichannel analyzer as a digital oscilloscope. Decomposition of every scintillation pulse into a rise and three exponential decay components allowed us to represent the pulse height spectrum as a linear combination of them. We found that energy resolution of CsI:Tl can be altered and improved through changing the weight of decay components in the linear combination.