Spectrum Unfolding Technique Research Articles

Quantification of the light output anistropy in deuterated stilbene

Deuterated stilbene is an organic scintillator that is a desirable material for fast neutron spectroscopy using spectrum unfolding techniques without requiring time-of-flight information. Due to the crystal structure of the material, some anisotropy of the light output exists, which is dependent on the direction of heavy charged particle recoil relative to the crystal structure. The anisotropy of trans-stilbene (hereafter referred to as stilbene) has been well characterized in previous work, but for deuterated stilbene, the anisotropy has only been partially characterized along the a and b crystal axes, while the artificial c′ axis, which shows the largest anisotropy in stilbene, has not been characterized until this publication. In this work, two deuterated stilbene crystals were characterized with neutron energies up to 35 MeV at the Los Alamos Neutron Science Center. For one of the crystals, the response is characterized along the a, b, and c′ axes. This characterization shows a distinct anisotropy along the axes in deuterated stilbene, which is very similar to that found in regular stilbene, such that the a axis is the brightest, while the b and c′ axes are approximately 3% and 20%–35% lower relative to the a axis.

Machine Learning-based reconstruction of single and double radiation source distribution using plastic scintillation optical fiber

In this study, we propose a novel method for reconstructing the radiation distribution of a one-dimensional radioactive source using Machine Learning (ML) algorithms and plastic scintillation optical fiber. The wavelength spectrum unfolding technique is used to estimate the source position accurately. We compare the accuracy and time efficiency of three different algorithms, namely, Generalized reduced gradient (GRG), Maximum likelihood expectation maximization (MLEM), and ML, in the single-source and dual-source cases. Our results demonstrate that MLEM algorithm has a shorter reconstruction time with comparable accuracy of position and intensity compared to GRG algorithm for single-source case. For dual-source case, ML algorithm provides real-time estimation of position and intensity with acceptable errors, while GRG algorithm has a larger error in intensity estimation and longer computation time. Our proposed ML algorithms offer useful guidance for practical applications in radiation source location.

Neutron spectrum unfolding of the magnetic proton recoil spectrometer using the GRAVEL and MLEM algorithms

The magnetic proton recoil (MPR) spectrometer is a high-resolution neutron spectrometer, which provides critical information about the deuterium–tritium (DT) fusion yield, collective fuel velocity, and ion temperature. For precise measurements, the MPR spectrometer operates in a high-resolution mode; however, its detection efficiency is low. The trade-off between energy resolution and detection efficiency limits its widespread applications. Herein, we apply the spectrum unfolding technique to obtain a precise energy spectrum using the MPR spectrometer in a low-resolution mode and to achieve high detection efficiency. Numerical experiments and Geant4 simulations are conducted to compare the GRAVEL and the maximum likelihood expectation maximization (MLEM) algorithms in different cases. A new stopping criterion for iterations is adopted and works well. The source focuses on the primary DT neutrons with single Gaussian, asymmetric, and dual Gaussian distributions. All the cases demonstrate an accurate and stable unfolded energy spectrum, revealing the potential of the MPR spectrometer in a wide range of applications.

Low-energy cross-section measurement of the B10(α,n)N13 reaction and its impact on neutron production in first-generation stars

Nucleosynthesis in the first generation of massive stars offers a unique setting to explore the creation of the first heavier nuclei in an environment free of impurities from earlier stellar generations. In later generations of massive stars, hydrogen burning occurs predominantly through the CNO cycles, but without the carbon, nitrogen, and oxygen to catalyze the reaction sequence, first stars would have to rely on the inefficient $pp$ chains for their energy production. Observations of second and third generation stars show pronounced abundances of carbon and oxygen isotopes, which suggests a rapid conversion of the primordial abundances to heavier elements. While the triple-alpha-process primarily facilitates this conversion, there are alternative reaction sequences, such as $^{2}\mathrm{H}(\ensuremath{\alpha},\ensuremath{\gamma})^{6}\mathrm{Li}(\ensuremath{\alpha},\ensuremath{\gamma})^{10}\mathrm{B}(\ensuremath{\alpha},n)^{13}\mathrm{N}$, that may play a significant role. To study such alternate reaction pathways for production of carbon and heavier nuclei, a number of new measurements are needed. In this work, new measurements are reported for the $^{10}\mathrm{B}(\ensuremath{\alpha},n)^{13}\mathrm{N}$ reaction, extending the cross section down to 575 keV incident $\ensuremath{\alpha}$-particle energy. The measurements were made using a state-of-the-art deuterated liquid scintillator and a spectrum unfolding technique. An $R$-matrix analysis was performed in order to facilitate a comparison of the underlying nuclear structure with the reaction measurements. An unexpected upturn is observed in the low-energy $S$ factor that indicates the presence of a new low-energy resonance. A revised reaction rate is determined that takes into account the present data as well as other previous measurements from the literature that were previously neglected.

Neutron spectrum unfolding for the development of a novel neutron detector for fusion

In future fusion power plants, such as DEMO, the D-T neutron emission rate is predicted to exceed 1 × 1021 n s−1. Accurately monitoring neutron energies and intensities will be the primary method for estimating fusion power, and calculating key nuclear parameters, including the tritium breeding ratio and nuclear heating. The noVel nEutRon Detector for fusIon (VERDI) project, implemented under the EUROFusion Enabling Research 2017 program, aims to develop a detector capable of withstanding the harsh environment of a future fusion power plant. The VERDI detector is based on the foil activation technique, which relies on neutron spectrum unfolding methods to process the convolution of gamma-ray measurement and detector response function to infer the neutron energy spectrum. This paper details the experimental method and results collected using six prototype VERDI detectors during an initial experiment performed in July 2017 at the ENEA Frascati Neutron Generator (FNG) under D-T neutrons (14 MeV). The measured activities of product isotopes are compared with equivalent data calculated using the FISPACT-II code to provide an average C/Eact agreement of 1.05±0.13. Experimental results from the FNG have been applied to neutron spectrum unfolding techniques using established unfolding codes, MAXED and GRAVEL.

Review of recent gamma spectrum unfolding algorithms and their application

Abstract Gamma spectrum analysis is regarded as a fast, reliable and non-destructive technology on determining the type and intensity of radionuclides. It is widely used in nuclear physics research, geological exploration, environmental assessment. Under the certain condition (detector, electronic components and measuring environment), the data obtained by different unfolding algorithms have different results. The accuracy of qualitative and quantitative analysis of radionuclides is determined by gamma spectrum unfolding technique. This paper mainly analyzes the development and application of the gamma spectrum unfolding algorithms and technologies in denoising, background subtraction and overlapping peak separation, reviews the performances, results and highlights of the typical algorithms, discusses representative examples and proposes potential research directions.

Neutron Spectroscopy for pulsed beams with frame overlap using a double time-of-flight technique

A new double time-of-flight (dTOF) neutron spectroscopy technique has been developed for pulsed broad spectrum sources with a duty cycle that results in frame overlap, where fast neutrons from a given pulse overtake slower neutrons from previous pulses. Using a tunable beam at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory, neutrons were produced via thick-target breakup of 16 MeV deuterons on a beryllium target in the cyclotron vault. The breakup spectral shape was deduced from a dTOF measurement using an array of EJ-309 organic liquid scintillators. Simulation of the neutron detection efficiency of the scintillator array was performed using both GEANT4 and MCNP6. The efficiency-corrected spectral shape was normalized using a foil activation technique to obtain the energy-dependent flux of the neutron beam at zero degrees with respect to the incoming deuteron beam. The dTOF neutron spectrum was compared to spectra obtained using HEPROW and GRAVEL pulse height spectrum unfolding techniques. While the unfolding and dTOF results exhibit some discrepancies in shape, the integrated flux values agree within two standard deviations. This method obviates neutron time-of-flight spectroscopy challenges posed by pulsed beams with frame overlap and opens new opportunities for pulsed white neutron source facilities.

Neutron spectrum unfolding using genetic algorithm in a Monte Carlo simulation

A spectrum unfolding technique GAMCD (Genetic Algorithm and Monte Carlo based spectrum Deconvolution) has been developed using the genetic algorithm methodology within the framework of Monte Carlo simulations. Each Monte Carlo history starts with initial solution vectors (population) as randomly generated points in the hyper dimensional solution space that are related to the measured data by the response matrix of the detection system. The transition of the solution points in the solution space from one generation to another are governed by the genetic algorithm methodology using the techniques of cross-over (mating) and mutation in a probabilistic manner adding new solution points to the population. The population size is kept constant by discarding solutions having lesser fitness values (larger differences between measured and calculated results). Solutions having the highest fitness value at the end of each Monte Carlo history are averaged over all histories to obtain the final spectral solution. The present method shows promising results in neutron spectrum unfolding for both under-determined and over-determined problems with simulated test data as well as measured data when compared with some existing unfolding codes. An attractive advantage of the present method is the independence of the final spectra from the initial guess spectra.

原子炉ドシメトリーの高精度化・高信頼化に向けた中性子スペクトルアンフォールディング法の現状と課題

Neutron spectrum unfolding is a widely used technique for characterizing neutron fields for various types of reactor dosimetry, where the neutron spectrum is derived from integral measured data such as multiple-foil activation rates and moderated neutron detector counts. Many spectrum unfolding codes have been developed so far and their performances have been compared. However, a standardized metrology for neutron spectrum unfolding has not been satisfactorily established yet from the viewpoints of adequate selection and usage of unfolding codes, response function database and input data set preparation. This article reviews the present status of the neutron spectrum unfolding technique that is mainly related to reactor dosimetry with activation foils and discusses the validity of the solution spectra obtained from different unfolding codes under a typical fast reactor neutron field. The results show that the solution spectrum strongly depends on a priori (i.e., guess) spectrum required for the input data as well as the theoretical assumption in each unfolding code. The issues that must be resolved to improve the accuracy of reactor dosimetry are summarized for the a priori input spectrum, the nuclear database, and the standardization of the unfolding procedure.

Calculation of the light pulse distributions induced by fast neutrons in organic scintillation detectors

We develop a fully analytic and self-contained description of the amplitude distribution of light pulses in an organic scintillation detector due to a monoenergetic source of fast neutrons. To this end, two classes of problems have to be handled. One is a formula for the light pulse amplitude distribution for the complete life history of neutrons slowing down in a mixture of hydrogen and carbon as a statistical average over all collision sequences that can occur, accounting also for neutron leakage. A complete solution is given in terms of a non-recursive convolution integral expansion with respect to the various possible collision histories. These latter are dependent on the collision probabilities of neutrons of a given energy. The second is the calculation of this collision probability from analytical expressions for the geometry of the detector, in the present case a right cylinder. This quantity was taken from Monte Carlo simulations in previous work. Recursive formulae are derived for the probabilities of arbitrary collision sequences, and quantitative results are given for up to five consecutive collisions of all combinations. These probabilities can be used to determine how to truncate the non-recursive expansion of the full light amplitude distribution in quantitative work. The calculational method serves to lend insight and understanding into the structure of the pulse height spectra, as well as it provides a computationally cheap method of generating a large number of such spectra for various detector compositions, sizes and neutron energies, for the development and test of new spectrum unfolding techniques.

Validation of a method for neutron dosimetry and spectrometry using neutron activation of metal discs

A technique for neutron dosimetry and spectrometry based on neutron activation of different metal discs has been studied. After exposure to a neutron field, the radionuclides produced in the discs are detected using low-level gamma-ray spectrometry and the neutron spectrum is obtained using a spectrum unfolding technique. In order to validate the method, irradiation was performed in a well-characterised (252)Cf neutron reference field. Furthermore, the detector was used to determine the neutron fluence rate and spectrum at a storage place for MOX nuclear fuel. The results of the two measurements are reported and discussed.

Effects of the neutron spectrum on the neutron activation analysis constants for 94Zr and 96Zr

A novel approach for determining spectral parameters for neutron flux is used. It is based on recently updated consistent nuclear databases, direct calculations from differential cross sections, and a spectrum unfolding technique. Solutions are tested on the TRIGA Mark-II reactor, Ljubljana, making use of a Monte Carlo reconstructed flux shape and Cd-ratio measurements of several monitors. Results indicate that the epithermal spectrum shape a is energy-dependent. Estimated Q 0 values for 9 4 Zr and 9 6 Zr are 4.98 and 233.0, respectively, compared to 5.31 and 251.6, as reported recently by other authors.

Measurement of 90Sr in aqueous samples using liquid scintillation counting with full spectrum DPM method

90Sr activity was measured with the full spectrum DPM (FS-DPM) counting method after plotting the quench correction curves of 90Sr and 90Y, and the FS-DPM method was applied to determine the 90Sr in liquid waste samples. The optimum regions of 90Sr and 90Y were set for measuring 90Sr with the dual label DPM (DL-DPM) method. The direct measurement of purified 90Sr with spectrum unfolding techniques could overcome the disadvantages of the DL-DPM method. The activities of 90Sr in the 90Sr/ 90Y standardized solution measured with the FS-DPM protocols were close to the true activity. The concentrations of 90Sr in the liquid waste samples measured with the FS-DPM counting method were compared with those obtained with conventional methods.

Development of soft X-ray transmission grating spectrograph and its spectrum unfolding techniques for ultrashort pulse laser produced plasma diagnosis

A normal incident, transmission grating spectrograph with enhanced collection efficiency and high spectral resolution has been developed for ultrashort pulsed laser produced plasma diagnosis with a high spectral resolution of Δλ≤0.05nm. The partial coherent diffraction theory is applied to determine the imaging characteristics of the spectrograph. Intensity distributions on the observation plane are given in closed form. A novel method of spectrum unfolding techniques is put forward. Computer simulation shows that this method can give an uncertainty of ≤0.5% when applied in the radiation temperature measurement in indirect driven ICF experiments.

The development of the dual thin scintillator (DTS) in thy 1 + 2 coincidence configuration as a neutron spectrometer

Abstract The Dual Thin Scintillator-(DTS) when operated in the 1 + 2 coincidence configuration, exihibits a peaked response centered at the incident neutron energy. This is a more favouable response to be used with spectrum unfolding techniques and should provide a significant improvement over spectral determination with conventional proton recoil response functions.

Neutron elastic and inelastic differential scattering cross-sections by proton recoil spectrum unfolding

A neutron scattering system is described with which elastic and inelastic differential cross-section can be measured over the angular range 20° to 150°. Three NE213 liquid scintillation detectors are used to provide proton recoil spectra due to the scattered neutrons. These spectra are then unfolded to give the energy spectra of the scattered neutrons which can, in turn, give the differential scattering cross-sections. Results for 2.5 MeV neutron scattering by Fe, Sn and Bi are compared with previous cross-section measurements by conventional time-of-flight techniques. The potential usefulness of the proton recoil spectrum unfolding technique for differential cross-section measurement is assessed. Examples of measurements on nuclei with closely spaced excited states are also discussed.