Organic Scintillation Detectors Research Articles

Fast neutron spectra measurement in a copper using a 252Cf standard neutron source

The presented paper focuses on the measurement of fast neutron leakage spectra in the copper assembly using an intense standard 252Cf(sf) source. The neutron spectra were measured by a stilbene organic scintillation detector and compared with MCNP6.2 calculations using copper nuclear data from the ENDF/B-VIII.0, ENDF/B-VII.1, JEFF-3.3, JEFF-3.2, JENDL-4.0 and CENDL-3.1 libraries in the energy range of 1–10 MeV. Discrepancies appear in the intermediate neutron energy range between experiment and calculation. The best agreement with an experiment was achieved using ENDF/B-VIII.0 library in the region of 1–4.5 MeV. In the region of 4.5–10 MeV, the best agreement was achieved using JENDL-4.0 or CENDL-3.1 library. The largest discrepancies in the whole energy range were found for calculations involving JEFF-3.3 copper evaluations.

GEANT4-based calibration of an organic liquid scintillator

A light-yield calibration of an NE 213A organic liquid scintillator detector has been performed using both monoenergetic and polyenergetic gamma-ray sources. Scintillation light was detected in a photomultiplier tube, and the corresponding pulses were subjected to waveform digitization on an event-by-event basis. The resulting Compton edges have been analyzed using a GEANT4 simulation of the detector which models both the interactions of the ionizing radiation as well as the transport of scintillation photons. The simulation is calibrated and also compared to well-established prescriptions used to determine the Compton edges, resulting ultimately in light-yield calibration functions. In the process, the simulation-based method produced information on the gain and intrinsic pulse-height resolution of the detector. It also facilitated a previously inaccessible understanding of the systematic uncertainties associated with the calibration of the scintillation-light yield. The simulation-based method was also compared to well-established numerical prescriptions for locating the Compton edges. Ultimately, the simulation predicted as much as 17% lower light-yield calibrations than the prescriptions. These calibrations indicate that approximately 35% of the scintillation light associated with a given gamma-ray reaches the photocathode. It is remarkable how well two 50 year old prescriptions for calibrating scintillation-light yield in organic scintillators have stood the test of time.

A dual‐scintillator time‐of‐flight spectrometer for secondary fast neutrons in proton radiotherapy

In order to measure the energy spectra of secondary neutrons in proton and heavy-ion radiotherapy, a neutron spectrometer has been developed. In this paper, two organic scintillation detectors combined with photomultiplier tubes were used to form a dual-scintillator time-of-flight spectrometer. The performance of this spectrometer was tested with a DT neutron generator, and the detection efficiency of each detector as well as the spectrometer were simulated by Geant4 toolkit. It’s shown that the time resolution of the spectrometer is 0.797 ns with an energy resolution of 26.4% for neutrons with an energy of 14 MeV at a flight distances over 150 cm.

Pulse shape simulations for organic scintillation detectors using Geant4

The accurate simulation of the temporal pulse shapes from organic scintillation detectors capable of pulse shape discrimination (PSD) presents the opportunity to assess the pulse shape discrimination of these detectors prior to fabrication. The aim of this study is the simulation of the temporal pulse shapes from EJ-276, a PSD-capable plastic scintillator developed by Eljen Technologies. PSD plastic scintillators are increasingly replacing organic liquid scintillators for the detection of neutrons in the presence of mixed radiation fields for nuclear security applications. Plastics are inexpensive, robust and can be fabricated in a variety of shapes and sizes. They offer a solid-state alternative to liquid scintillators which can be difficult to transport due to the risk of leakage. However, the PSD performance of plastic scintillators has been observed to decrease due to various factors which combine to influence the overall shape of the pulse. The Monte Carlo toolkit Geant4 has been used to simulate the temporal pulse shapes from an EJ-276 plastic scintillator coupled to a photomultiplier tube (PMT). All three decay time components of EJ-276 have been modelled, utilising new methods available in the latest version of Geant4, for two different scintillator geometries. The simulated n/γ pulse shapes reproduce the features important for PSD. Future work will involve integrating the temporal response of the PMT with existing pulse shape simulations. Simulated data will then be compared with experimental measurements.

Измерения спектров гамма- и нейтронного излучения в ядерных реакциях с ионами -=SUP=-3-=/SUP=-He и -=SUP=-9-=/SUP=-Be

The results of the measurements of the neutron and gamma radiation produced in the 9 Be( 3 He,pγ) 11 B and 9 Be( 3 He,nγ) 11 C nuclear reaction at the 2.57 MeV energy of the 3 He ions are represented. The measurements were carried out with organic scintillation detector BC-501A and semiconductor HPGe spectrometers. Gamma-ray spectra were measured at 0°, 30°, 60°, 90° and 120° angles. The shapes of the gamma lines 6.9 MeV from the 9 Be( 3 He,nγ) 11 C reaction and 8.92 MeV from the 9 Be( 3 He,nγ) 11 C were obtained. The angular distribution of protons populating the 8.92 MeV energy level of the 11 B nucleus was reconstructed. The energy distribution of the neutrons produced in the 9 Be( 3 He,nγ) 11 C reaction was measured (obtained).

Genetic algorithm optimization of tin–copper graded shielding for improved plutonium safeguards measurements

Plutonium nuclear material safeguards is essential to ensure that states do not divert plutonium to clandestine nuclear weapons programs. Traditional safeguards methods rely on correlated neutron measurements. Detection systems based on organic scintillators, which are sensitive to fast neutrons and photons, are gaining support for use in safeguards applications due to increased capabilities and shorter measurement times compared to the current state-of-the-art. A drawback of organic scintillators in plutonium measurements is data throughput limitations; 241Am builds up in plutonium samples and emits 60-keV gamma rays with a high specific activity. In this work, a genetic algorithm is used to design graded shielding made of tin and copper to attenuate the 60-keV gamma rays. The shield is designed to limit the attenuation of the higher-energy, fission photons, because fission photons can be used as a signature. The genetic algorithm optimization takes days to run, while a complete search space exploration would take years. The genetic algorithm was run for different levels of 60-keV gamma-ray attenuation, and the thickness of the required shielding layers was determined at each level. Tin–copper shielding was placed in front of an array of organic scintillation detectors for a measurement of approximately 4.5 kg of alpha-phase, weapons-grade plutonium, an AmLi source, and a 252Cf source. We show that the shield successfully attenuated low-energy, non-fission gamma rays, which significantly improved the organic scintillator performance.

Using a small-core graphite calorimeter for dosimetry and scintillator quenching corrections in a therapeutic proton beam

Organic plastic scintillation detectors (PSDs) are known to produce less light per absorbed dose in highly dense radiations in comparison with e.g. 60Co gamma beams. This so-called ionization density quenching can be experimentally determined by comparison of the scintillator output with the absorbed dose established with a reference detector. The hypothesis of this work was that a newly developed small-core graphite calorimeter (core size: ø5mm × 7mm) can be used as reference for such measurements. The potential benefit of a calorimetric reference would be to have a robust and accurate reference with well-understood dosimetry properties even in high-intensity FLASH beams. As a first step, the hypothesis was tested by comparing previously established quenching parameter estimates for the BCF-60 scintillating material with data obtained with the new instrument at different depths along the central axis of a 170 MeV scanned proton beam. After the calorimetric measurements, scintillator measurements were acquired under equivalent conditions by positioning the PSD in a replica graphite core nominally identical to the core used for calorimetry. To experimentally document details of the irradiations, the spot width was mapped along the central beam axis using a new technique based on a PSD and a time-to-distance conversion procedure. Analysing the proton data in the framework of the Birks model, the graphite calorimeter gave a quenching parameter for BCF-60 in agreement with literature values. The consistency between the calorimetric results and the other sources of information supports the validity of the new method, and we therefore aim to apply it for characterization of other detectors in more intense beams where ionometry cannot serve as reference.

DUAL-PARTICLE DOSEMETER BASED ON ORGANIC SCINTILLATOR

Traditionally available handheld dosemeters are generally sensitive to only one type of radiation: neutrons or photons. Some dosemeters also rely on very specific attenuation correlations between response and dose, are not scalable in size and multiple dosemeters are required to characterise mixed-particle fields. The research presented here serves as a proof-of-concept for a method to simultaneously measure dose rates from neutrons and photons using a particle discriminating organic scintillation detector without the need for spectral deconvolution. The method was compared with traditional instruments and to simulation. Isotopic photon dose rates measured with this method were within 4% of simulated truth, whereas fission spectrum neutron dose rates were measured within 21%. Measurements of dose rates from both particles agree with simulated truth better than traditional instruments. This new method allows for measurement of dose equivalent from both neutrons and photons with a single instrument and no reliance on spectral deconvolution.

Angular distribution of neutron production by proton and carbon-ion therapeutic beams

Carbon-ion beams are increasingly used in the clinical practice for external radiotherapy treatments of deep-seated tumors. At therapeutic energies, carbon ions yield significant secondary products, including neutrons, which may be of concern for the radiation protection of the patient and personnel. We simulated the neutron yield produced by proton and carbon-ion pencil beams impinging on a clinical phantom at three different angles: 15°, 45° and 90°, with respect to the beam axis. We validated the simulated results using the measured response of organic scintillation detectors. We compared the results obtained with FLUKA 2011.2 and MCNPX 2.7.0 based on three different physics models: Bertini, Isabel, and CEM. Over the different ions, energies, and angles, the FLUKA simulation results agree better with the measured data, compared to the MCNPX results. Simulations of carbon ions at low angles exhibit both the highest deviation from measured data and inter-model discrepancy, which is probably due to the different treatment of the pre-equilibrium stage. The reported neutron yield results could help in the comparison of carbon-ion and proton treatments in terms of secondary neutron production for radiation protection applications.

Development of an efficient technique for constructing energy spectrum of NaI(Tl) detector using spectrum of NE102 detector based on supervised model-free methods

The motivation of this study is development of a technique to construct energy spectrum of higher price/high resolution detectors (e.g. NaI (Tl)) using spectrum of lower price/low resolution detectors (e.g. NE102). Since there is no explicit mathematical model between these type of detectors (i.e. organic and inorganic scintillator detectors), it is necessary to utilize model-free methods. Construction of mapping function to generate spectrum of inorganic scintillator using spectrum of organic scintillator can be done by supervised model-free methods. Different supervised learning methods including localized neural networks, statistical methods, feed-forward neural networks, and conditional methods are utilized for spectrum construction. Experimental spectrums of the different radioisotopes (i.e. Co-60, Cs-137, Na-22, Am-241) including 15 spectrums of NaI (Tl) detector and 15 spectrums of NE102 detector are respectively used as training data and test data in the supervised methods. Results demonstrate that localized network (i.e. radial basis network) is the more appropriate method for the spectrum construction. The results of statistical method (i.e. support vector machine) is acceptable while conditional method (i.e. decision tree) does not give acceptable results and multi-layer perceptron does not learn the spectrums. The developed technique can be applied with an interesting ratio of training set to test set (i.e. r/(2r-1-r)). In other words, constructing spectrums of all possible combinations of r radioisotopes (i.e. 2r-1-r) is possible only with training of single radioisotopes spectrums (i.e. r). The developed method for generation of spectrums is more appropriate for identification of the radioisotopes and is not so useful for spectrum tracking. Spectrum tracking can be done by training of supervised learning method using generated pulses of detector.

Measurement of the detection efficiency response curve of stilbene detectors based on a Back-n white neutron source at the CSNS

The time-of-flight (TOF) of an associated white neutron beam line exploiting back-streaming neutrons (Back-n) at China Spallation Neutron Source (CSNS) was obtained using a scintillator detector consisting of stilbene crystal, a photomultiplier tube (PMT), a screening can, and a high sampling rate oscilloscope. An original energy-count spectrum was acquired based on the time-of-flight method. Four kind of stilbene crystals were placed at different angles and distances from the polyethylene scattering target, with different sizes of Φ15 mm × 5 mm, Φ25 mm × 10 mm, Φ50 mm × 10 mm and Φ50 mm × 30 mm. The detection efficiency of multiple energy points was calculated based on the reference energy spectrum, and 10 detection efficiency response curves between 0.5 MeV and 20 MeV were obtained in accordance with a Monte Carlo simulation. The results demonstrated the feasibility of using a scintillator detector for neutron detection efficiency at a white neutron source. This method shows great potential in neutron detection efficiency calibration for all organic scintillation detectors.

Rossi-alpha measurements of fast plutonium metal assemblies using organic scintillators

The reactivity and the k-effective multiplication factor (keff) of a fissionable assembly are quantities of widespread interest. These values can be inferred from Rossi-alpha measurements of the prompt neutron decay constant (or the inverse: α−1). It has been shown that 3He-gas proportional counter-based detection systems are insensitive to α−1 of fast assemblies (much faster than tens of microseconds). Therefore, it is of interest to investigate fast detection systems such as those based on organic scintillation detectors. In this work, an array of 12 cylindrical, 5.08 cm × 5.08 cm diameter trans-stilbene organic scintillators was used to measure five subcritical assemblies. One assembly was a sphere of approximately 4.5 kg of alpha-phase, weapons-grade plutonium (keff=0.773, α−1=11.84ns) known as the BeRP ball encased in a thin stainless-steel clad. The other assemblies used the same encased sphere and 7.62 cm of iron, nickel, copper, or tungsten reflectors (keff=0.884,0.916,0.924,0.939, respectively and α−1=36.60,41.56,49.60,70.32ns, respectively). This work (1) validates Rossi-alpha measurements with organic scintillators by demonstrating good agreement between measurements and simulations; and (2), demonstrates that organic scintillator-based systems are sensitive to α−1 on the order of 10–100 ns.

Fast neutron and γ-ray backscatter radiography for the characterization of corrosion-born defects in oil pipelines

In this research, a combined fast-neutron/γ-raybackscatter imaging technique is described. The aim of this work is to understand corrosion defects in pipelines by measuring differences in the scattered radiation flux, generated when different steel thicknesses are irradiated by a neutron and γ-ray focused beam. A californium-252 radiation source is used to produce fast neutron and γ rays, exploiting its spontaneousfission. This mixed radiation field is collimated and directed towards the steel samples. Backscattered neutrons and γ rays aremeasured as a function of the steel thickness using 4 liquid organic scintillation detectors linked to a real-time, pulse-shape discrimination system, which separates and retains the neutron and γ-ray event data. In this paper, we describe how, using asingle radiation source and detection system, it is possible to perform and combine two complementary imaging modalities. This research is validated by an MCNP6 computer simulation study. The backscatter imaging system developed for this research and the experimental results of the measurements carried out using the National Physical Laboratory neutron low-scatter facility are also presented in this paper.

Constructing energy spectrum of inorganic scintillator based on plastic scintillator by different kernel functions of SVM learning algorithm and TSC data mapping

In this paper, a novel idea is developed to construct energy spectrum of inorganic scintillator detector (e.g. NaI(Tl)) using energy spectrum of organic scintillator detector (e.g. NE102) by means of a model-free method. For this purpose, support vector machine (SVM) accompanied with different kernel functions (i.e. linear, polynomial, and Gaussian) is applied. The spectra of NE102 and NaI(Tl) detectors of the single radioisotopes (i.e. Co60, Cs137, Na22, and Am241) are utilized for training of SVM. In other words, data of NE102 detector are input spectrums of training patterns and data of NaI(Tl) detector are target spectrums of training patterns. To construct an appropriate mapping function between spectrums of detectors, a kind of cross-correlation technique namely mapping of totality of channels to single channel (TSC) is utilized. In the test process, spectrums of different combinations of the target radioisotopes are constructed and the results are compared with the measured spectrums. Polynomial kernel function gives good results. Linear and Gaussian kernel functions do not give so appropriate results. The major advantages of the developed method are: 1- The NaI(Tl) spectrums of different combinations of the target radioisotopes are constructed only by training of single radioisotopes spectrums. 2- Energy spectrums of high price/ high resolution detectors (i.e. inorganic scintillator detectors) can be constructed using low price/ low resolution detectors (i.e. plastic scintillator detectors). 3- This method can be used to construct energy spectrum of any type of inorganic scintillator (e.g. BGO detector) using either plastic or liquid scintillators. Liquid scintillator detectors (e.g. NE213) are more appropriate for detection/identification of radioactive sources which emit both gamma and neutron radiations. However, these detectors give low resolution gamma spectrums. The developed method can be appropriate for construction of higher resolution gamma spectrums for this type of sources. This application of the developed method is discussed in the manuscript.

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.

Comparative study between the developed simulators based on the direct and indirect methods for calculation of the energy spectrum of the neutron source

The purpose of the present paper is to compare the various computer codes used in fast-neutron spectroscopy for spectrum unfolding and calculation of the response of the organic scintillation detectors. For neutron spectrum unfolding, the Fredholm integral equation is solved using iterative and soft computing algorithms. In the present paper, the Modified Least SQuaRe (MLSQR) method (iterative algorithm) and some soft computing algorithms are presented. The latter comprise Artificial Neural Network (ANN), Support Vector Machine (SVM) and Adaptive Group of Ink Drop Spread (AGIDS) methods. Furthermore, a MCNPX based algorithm for calculating the neutron response of organic scintillation detectors is presented which is used for Time-Of-Flight based fast-neutron spectroscopy (nTOF) . To this end, the ability for simulating the emission and tracking of two simultaneously emitted particles as well as the light production and light transport in the scintillator were added to the MCNPX computer code. Also, a post-processing software was developed to analyze the massive amounts of data in the output of the PTRAC card. The presented techniques are benchmarked with a published spectrum from an 241Am-9Be neutron source. The results obtained from the calculation using the proposed methods have an acceptable agreement with the standard ISO-8592 spectrum of the 241Am-9Be neutron source. However, the accuracy of the calculation using the SVM and AGIDS is better than the accuracy of the other presented methods.

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.

Nuclear recoil scintillation linearity of a high pressure 4He gas detector

We investigate scintillation linearity of a commercial high pressure 4He gas detector using monoenergetic 2.8 MeV neutrons from a deuterium-deuterium fusion neutron generator. The scintillation response of the detector was measured for a range of recoil energies between 83 keV and 626 keV by tagging neutrons scattering into fixed angles with a far-side organic scintillator detector. Detailed Monte Carlo simulations were compared to experimental data to determine the linearity of the detector response by comparing the scaling of the energy deposits in the simulations to the detector output. In this analysis, a linear scintillation response corresponds to a consistent value for the scaling factor between simulated energy deposits and experimental data for several different scattering angles. We demonstrate that the detector can be used to detect fast neutron interactions down to 83 keV recoil energies and can be used to characterize low-energy neutron sources, one of its potential applications.

The ORNL Deuterated Spectroscopic Array — ODeSA

Abstract An array consisting of 12 deuterated organic liquid scintillator detectors for fast-neutron spectroscopy was designed and built at Oak Ridge National Laboratory (ORNL). This versatile array is designed for measurements with low reaction yields, such as those performed with rare isotope beams, as well as at high current DC facilities used for underground nuclear astrophysics research. Because some measurements also offer limited, or no, additional timing information, the ORNL Deuterated Spectroscopic Array (ODeSA) was optimized to utilize spectrum unfolding to extract neutron energy spectra . This array was characterized for n/ γ pulse shape discrimination, light response, resolution, and intrinsic efficiency by using the neutron time-of-flight tunnel at the Edwards Accelerator Laboratory at Ohio University. Results and future plans are discussed.

Neutron-gamma discrimination based on support vector machine combined to nonnegative matrix factorization and continuous wavelet transform

Recent developments of digital signal processing have played an effective role to achieve a fast and accurate neutron-gamma discrimination. Thus, we present in this research work, a novel method which combines supervised and unsupervised machine learning to perform the neutron-gamma discrimination task at the output of a stilbene organic scintillation detector. We propose a three steps procedure that highly qualifies the discrimination. First, the detector’s output signals are processed as mixtures of several unknown sources, through nonnegative matrix factorization algorithms. Second, the continuous wavelet transform is performed to characterize the recovered original sources. The resulting time-scale representation is considered as an image which is segmented in order to extract main features of neutron signals versus the gamma ones. The features are then used as input of a nonlinear support vector machine classifier to finally achieve the neutron-gamma discrimination in mixed radiation field. Furthermore, the proposed method provides the classification precision for each radiation.