Fast Neutron Response Research Articles

Response of LaBr2.85Cl0.15:Ce, LaBr[formula omitted]:Ce and NaI:Tl crystals to fast and thermal neutrons

This communication reports the measurement of direct response for fast and thermal neutrons of a newly marketed scintillation crystal LaBr2.85Cl0.15:Ce, along with the widely used LaBr3:Ce and NaI:Tl crystals. It continues our investigation on the possible utility of LaBr2.85Cl0.15:Ce crystal for the detection of high-energy γ-rays and neutrons. This is the first report of the neutron response of the LaBr2.85Cl0.15:Ce crystal. The neutron response was measured for the three detectors using a calibrated Am-Be source, which emits both neutrons and γ-rays. To eliminate the events due to the interaction of the 4.4 MeV γ-ray emitted from Am-Be, the γ-ray response of all three crystals was simulated using the Monte Carlo simulation toolkit GEANT4. Events due to 2.22 MeV γ-ray produced by the capture of thermal neutrons in the paraffin moderator were also eliminated with the aid of simulations. In addition, the response due to fast neutrons was also simulated using GEANT4 and compared with the measured spectrum. This is the first report of the simulation of fast neutron response of LaBr2.85Cl0.15:Ce using GEANT4. The fast neutron detection efficiency of the three detectors has been obtained from the measured spectra.

Fast neutron response of 6Li enriched CLYC and CLLB scintillators within 0.9–5.2 MeV

The performance of two 6Li enriched elpasolite scintillators Cs2LiYCl6 : Ce3+ (CLYC) and Cs2LiLaBr6 : Ce3+ (CLLB) were investigated using mono-energetic fast neutrons generated by a 3 MV tandem accelerator. The Pulse Shape Discrimination (PSD) ability, response and energy resolution to fast neutrons and scattered slow neutrons were studied. The 6Li enriched CLYC had excellent discrimination ability between neutron and gamma rays with the Figure of Merit (FOM) value better than 2.4. An Artificial neural network (ANN) model could discriminate fast neutrons and scattered slow neutrons effectively with an accuracy of 97%, which render the Li-6 enriched CLYC capable to detect thermal neutrons with high efficiency and the energy spectrum of fast neutrons simultaneously. The quenching factors are calculated to be 0.92 and 0.48 for the 35Cl(n, p)35S and 35Cl (n, α)32P reaction, while the detection efficiency is about 0.1%–0.75% for fast neutrons within the energy range of 0.9–5.2 MeV. Unlike CLYC, CLLB was only sensitive to slow neutrons, and the discrimination ability between neutrons and gamma rays is worse than CLYC, while the ANN model exhibit better performance which has an accuracy of 97%.

Fast neutron response characterization of an EJ-276 plastic scintillator for use as a neutron monitor

EJ-276 is a recent development in plastic scintillators that comes with improved pulse shape discrimination over its predecessor EJ-299-33. In this work, the neutron response functions, proton light output function, and detection efficiency of EJ-276 were measured using mono-energetic neutron reference fields at the national metrology laboratory, Physikalisch-Technische Bundesanstalt, in the neutron energy range 1.5–19 MeV. In particular, the proton light output in the range 14–19 MeV is presented for the first time. The optimization of Pulse Shape Discrimination has also been investigated using an 241Am-Be source and results are presented. Finally, we study how, using a shadow-cone method and time of flight method, we can discriminate scattered and other spurious neutrons from the mono-energetic beam. The characterized EJ-276 detector is to be used in the monitor of the neutron fields and gamma background at the new Neutron Irradiation Laboratory for Electronics facility at the Rutherford Appleton Laboratory.

Characterization of plastic scintillator bars using fast neutrons from D-D and D-T reactions

We report results of fast neutron response in plastic scintillator (PS) bars from deuterium-deuterium (D-D) and deuterium-tritium (D-T) reactions using Purnima Neutron Generator Facility, BARC, Mumbai. These measurements are useful in context of Indian Scintillator Matrix for Reactor Anti-Neutrino (ISMRAN) detection, an array of 10×10 PS bars, used to measure reactor anti-neutrinos through inverse beta decay (IBD) signal. ISMRAN detector, an above-ground experiment close to the reactor core (∼13m), deals with an active fast neutron background inside the reactor hall. A good understanding of fast neutron response in PS bars is an essential pre-requisite for suppression and discrimination of fast neutron background from IBD events. A monoenergetic neutron beam from the fusion reaction of D-D at 2.45 MeV and D-T at 14.1 MeV are used to characterize the energy response in these bars. The neutron energy response function has been simulated using the GEANT4 package and are compared with the measured data. A reasonable agreement of deposited energies by fast neutrons in PS bars between data and simulation are obtained for these reactions. The ratio of energy deposition in adjacent bars is used to discriminate between prompt IBD, fast neutron and neutron capture cascade gamma events.

A Monte-Carlo study on the fluorescent nuclear track detector (FNTD) response to fast neutrons: Which information can be obtained by single layer and 3D track reconstruction analyses?

Fluorescence Nuclear Track Detectors (FNTDs) are part of a new technology developed for particle detection and applicable to personal neutron dosimetry. The objective of this study is to simulate the FNTD fast neutron response to: (i) assess and understand the performance of the existing neutron dosimeter design (Landauer Inc.) and its associated single layer track-spots analysis; and (ii) evaluate the potential information that can be obtained by the analysis of the 3D reconstructed recoil proton trajectories. To achieve that, a FLUKA Monte Carlo (MC) model of the current FNTD design was developed and the FNTD response was investigated for mono-energetic neutrons and the 252Cf and 241AmBe neutron sources. The investigation of the recoil proton densities behind the different converters showed that the single layer analysis and dose calculation algorithm, based on the comparison and subtraction of the track densities behind the different converters, works properly only up to neutron energies ~13 MeV. Above this neutron energies, recoil protons generated in the detector housing (PE) have a range larger than the thickness of the PTFE and 6Li-enriched glass, reaching the FTND and, therefore, adding to the signal in these detection regions and influencing the secondary electron discrimination and the energy determination algorithm. MC simulations show that the FNTD 3D reconstructed recoil proton tracks can provide estimates of the irradiation angles and average neutron energy. The results show that the angle or displacement (dX/dZ or dY/dZ) distributions of the recoil proton tracks can be used to obtain information on irradiation angle; the angle with the detector's normal (polar angle), the most important because of its influences on the FTND sensitivity, can be determined in laboratory and for irradiation angles < 60° with an 4° uncertainty already for doses > 4.5 mSv in the case of a214AmBe neutron irradiation. The neutron field mean energy can also be determined for normal irradiation by analysing the depth distribution of the recoil proton tracks already for a minimum of 150 tracks, or 2.5 mSv for 241AmBe, assuming a scanned area is ~2.0 mm2. Therefore, the present study contributes to understanding the performance of the current FNTD design and analysis for neutron dosimetry and investigates a new detector evaluation approach to gain additional information on the irradiation conditions.

Fast-neutron response of the novel scintillator caesium hafnium chloride

This paper reports on the performance of the inorganic scintillator caesium hafnium chloride (CHC) under exposure to the mixed radiation field of an AmBe neutron source and coupled to a silicon photomultiplier (SiPM). The neutron response is determined using the pulse shape discrimination charge comparison technique which can clearly identify both the (n,α) and (c) reactions in the material. Figures of merit for the pulse shape discrimination are presented and the quenching of the different channels is assessed through comparison to Monte Carlo simulations.

Neutron Response of the EJ-254 Boron-Loaded Plastic Scintillator

Organic scintillators doped with capture agents provide a detectable signal for neutrons over a broad energy range. This work characterizes the fast and slow neutron response of EJ-254, an organic plastic scintillator with 5% natural boron loading by weight. For fast neutrons, the primary mechanism for light generation in organic scintillators is n-p elastic scattering. To study the fast neutron response, the proton light yield of EJ-254 was measured at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. Using a broad-spectrum neutron source and a double time-of-flight technique, the EJ-254 proton light yield was obtained over the energy range of approximately 270 keV to 4.5 MeV and determined to be in agreement with other plastic scintillators comprised of the same polymer base. To isolate the slow neutron response, an AmBe source with polyethylene moderator was made incident on the EJ-254 scintillator surrounded by an array of EJ-309 observation detectors. Events in the EJ-254 target coincident with the signature 477.6 keV $\gamma$ ray (resulting from deexcitation of the residual $^{7}$Li nucleus following boron neutron capture) were identified. Pulse shape discrimination was used to evaluate the temporal differences in the response of EJ-254 scintillation signals arising from $\gamma$-ray and fast/slow neutron interactions. Clear separation between $\gamma$-ray and fast neutrons signals was not achieved and the neutron capture feature was observed to overlap both the $\gamma$-ray and fast neutron bands. Taking into account the electron light nonproportionality, the neutron-capture light yield in EJ-254 was determined to be 89.4$\pm$1.1 keVee.

Fast-neutron response of LaBr3(Ce) and LaCl3(Ce) scintillators

The response of LaBr3(Ce) and LaCl3(Ce) scintillators to fast neutrons is investigated. Neutron-induced charged-particle reactions are observed in both materials when exposed to the fast neutrons produced by an AmBe source, with pulse-shape discrimination used to separate channels. LaBr3(Ce) is found to have the best separation between reaction channels, while LaCl3(Ce) has a significantly higher efficiency.

Characteristic of a Cs[formula omitted]LiLaBr[formula omitted]:Ce scintillator detector and the responses for fast neutrons

The elpasolite crystal CLLB is proposed to detect both γ-ray and thermal neutron events with the ability to distinguish them by pulse shape discrimination (PSD) method. Pulse shapes of γ-ray, alpha particle, thermal and fast neutron events were measured and determined that PSD can be performed to separate γ-ray from alpha and thermal neutron events with a Figure of Merit (FoM) 1.23. The γ-ray non-proportional response is less than 2% in the energy region of 59.6 keV–4438 keV. Fast neutron responses for CLLB scintillator were investigated and analyzed using deuterium tritium (DT) reaction neutrons. The activity of 511 keV γ-ray emitted by 78Br and 80Br can reaches up to ∼0.709 Bq⋅n−1, some high-energy events at 15 MeVee – 18 MeVee were found that generated according to the reaction 6Li(n, T)4He.

A Study of the Fast Neutron Response of a Single-Crystal Diamond Detector at High Temperatures

The fast neutron response of a single-crystal diamond (SCD) detector was studied up to 300 °C. The SCD detector was fabricated using a $5\,\,\text {mm}\times 5\,\,\text {mm}\times 0.4\,\,\text {mm}$ substrate with premetallization treatments for obtaining oxygen terminated surfaces. The electrical contacts to the detector were provided by Cr/Au metallization. The performance of the fabricated detector was investigated using leakage current versus bias voltage (I–V) measurements and using pulse height spectra measurements with 5.5 MeV alphas and 14 MeV fast neutrons. The leakage current was observed to be about 400–600 pA at a field of 1 V/ $\mu \text{m}$ . From the full-width half-maximum of the alpha peak at 5.5 MeV, an energy resolution of about 2.1 ± 0.1% was estimated. The fast neutron response measurements were carried out using a 14 MeV deuterium–tritium neutron source. No measurable degradation of the detector performance was observed up to the maximum studied temperature of 300 °C. The neutron spectrum obtained at 300 °C showed a well-defined peak at 8.5 MeV due to the 12C(n, $\alpha)^{9}$ Be reaction which was the same as that observed at room temperature. Considering the total integrated counts, the detector sensitivity was observed to be $10^{-2}$ count per neutron. The results presented in this paper demonstrate for the first time the high temperature operation of an SCD detector up to 300 °C. These results also suggest that such SCD detectors could be used at higher temperatures for neutron diagnostics in the future fusion plasma facilities such as International Thermonuclear Experimental Reactor.

A novel approach to neutron dosimetry.

Having been overlooked for many years, research is now starting to take into account the directional distribution of neutron workplace fields. Existing neutron dosimetry instrumentation does not account for this directional distribution, resulting in conservative estimates of dose in neutron workplace fields (by around a factor of 2, although this is heavily dependent on the type of field). This conservatism could influence epidemiological studies on the health effects of radiation exposure. This paper reports on the development of an instrument which can estimate the effective dose of a neutron field, accounting for both the direction and the energy distribution. A 6Li-loaded scintillator was used to perform neutron assays at a number of locations in a 20 × 20 × 17.5 cm3 water phantom. The variation in thermal and fast neutron response to different energies and field directions was exploited. The modeled response of the instrument to various neutron fields was used to train an artificial neural network (ANN) to learn the effective dose and ambient dose equivalent of these fields. All experimental data published in this work were measured at the National Physical Laboratory (UK). Experimental results were obtained for a number of radionuclide source based neutron fields to test the performance of the system. The results of experimental neutron assays at 25 locations in a water phantom were fed into the trained ANN. A correlation between neutron counting rates in the phantom and neutron fluence rates was experimentally found to provide dose rate estimates. A radionuclide source behind shadow cone was used to create a more complex field in terms of energy and direction. For all fields, the resulting estimates of effective dose rate were within 45% or better of their calculated values, regardless of energy distribution or direction for measurement times greater than 25 min. This work presents a novel, real-time, approach to workplace neutron dosimetry. It is believed that in the research presented in this paper, for the first time, a single instrument has been able to estimate effective dose.

Fabrication and characterization of polycrystalline diamond detectors for fast neutron monitoring

Diamond detectors have been fabricated using commercially available detector grade polycrystalline CVD substrates for fast neutron measurement in the Indian TBM Experiment at the upcoming ITER facility. Subsequent to fabrication, the detectors were characterized for leakage current and for response to alpha particles from 238+239Pu source. The detectors were observed to have low leakage currents at a field of 1Vµm−1. The stability of the alpha response and improvement in the count rate were achieved by β-irradiation using 90Sr β source. The fast neutron response of the detectors was studied using a D–T fast neutron source in India. The detectors showed linear response in the measured neutron flux of 2.86×105–8.76×106ncm−2s−1. The neutron response of detectors of 100µm and 300µm thicknesses was compared experimentally to study the neutron response dependence on detector thickness. Experimental results show that the detector of 100µm thickness has better performance compared to the detector of thickness of 300µm. The results presented in this paper confirm the suitability of commercially available films from Diamond Materials, GmbH, Germany for detector fabrication for fast neutron monitoring. The details of detector fabrication and results of characterization are presented in this paper.

Fast neutron response of 6Li-depleted CLYC detectors up to 20 MeV

The response of 6Li-depleted Cs2LiYCl6 (CLYC) to high-energy neutrons has been investigated using a pair of 1in.×1in. crystals. These are the first two detectors of their kind, which will comprise a 16-element array for studies in fast neutron spectroscopy. Their thermal neutron response has been compared with standard CLYC crystals with a 6Li enrichment of 95%, demonstrating excellent suppression of the overwhelming thermal neutron background. The response to mono-energetic neutrons over a range of 0.5 to 20MeV was tested. From this, the response function, energy resolution, and pulse-shape discrimination up to 20MeV were characterized. Detailed Monte Carlo investigations with MCNPX have been used to show that the dominant reaction mechanisms contributing to the observed response are 35Cl(n,p) and 35Cl(n,α). Preliminary investigations have also demonstrated the possibility for separating events from these two reactions.

MCNPX Monte Carlo simulations of particle transport in SiC semiconductor detectors of fast neutrons

The aim of this paper was to investigate particletransport properties of a fast neutron detector based on silicon carbide.MCNPX (Monte Carlo N-Particle eXtended) code was used in our study becauseit allows seamless particle transport, thus not only interacting neutronscan be inspected but also secondary particles can be banked for subsequenttransport. Modelling of the fast-neutron response of a SiC detector wascarried out for fast neutrons produced by 239Pu-Be source with the meanenergy of about 4.3 MeV. Using the MCNPX code, the following quantities havebeen calculated: secondary particle flux densities, reaction rates ofelastic/inelastic scattering and other nuclear reactions, distribution ofresidual ions, deposited energy and energy distribution of pulses. Thevalues of reaction rates calculated for different types of reactions andresulting energy deposition values showed that the incident neutronstransfer part of the carried energy predominantly via elastic scattering onsilicon and carbon atoms. Other fast-neutron induced reactions includeinelastic scattering and nuclear reactions followed by production of α-particles and protons. Silicon and carbon recoil atoms, α-particles and protons are charged particles which contribute to thedetector response. It was demonstrated that although the bare SiC materialcan register fast neutrons directly, its detection efficiency can beenlarged if it is covered by an appropriate conversion layer. Comparison ofthe simulation results with experimental data was successfully accomplished.

The Effect of Atmospheric Water Vapor on Neutron Count in the Cosmic-Ray Soil Moisture Observing System

Abstract The cosmic-ray method for measuring soil moisture, used in the Cosmic-Ray Soil Moisture Observing System (COSMOS), relies on the exceptional ability of hydrogen to moderate fast neutrons. Sources of hydrogen near the ground, other than soil moisture, affect the neutron measurement and therefore must be quantified. This study investigates the effect of atmospheric water vapor on the cosmic-ray probe signal and evaluates the fast neutron response in realistic atmospheric conditions using the neutron transport code Monte Carlo N-Particle eXtended (MCNPX). The vertical height of influence of the sensor in the atmosphere varies between 412 and 265 m in dry and wet atmospheres, respectively. Model results show that atmospheric water vapor near the surface affects the neutron intensity signal by up to 12%, corresponding to soil moisture differences on the order of 0.10 m3 m−3. A simple correction is defined to identify the true signal associated with integrated soil moisture that rescales the measured neutron intensity to that which would have been observed in the atmospheric conditions prevailing on the day of sensor calibration. Use of this approach is investigated with in situ observations at two sites characterized by strong seasonality in water vapor where standard meteorological measurements are readily available.

Electrical Characteristics and Fast Neutron Response of Semi-Insulating Bulk Silicon Carbide

The electrical characteristics and fast neutron response of a High Temperature Chemical Vapour Deposition (HTCVD) grown semi-insulating bulk SiC wafer has been measured. Current-Voltage measurements demonstrated a low leakage current in the region of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-10</sup> to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12v</sup> A with a bulk resistivity of at least 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> Ω.cm. Alpha particle spectroscopy measurements demonstrated an electron charge collection efficiency of up to 90% with reasonable reproducibility of the acquired spectra. Evidence of (incident particle) rate dependent polarisation was seen following a constant applied bias combined with alpha irradiation over a period of time (order of tens of minutes). The ability of the wafer to detect fast neutrons was demonstrated and a comparison drawn with the MCNPX simulated response of a bulk SiC device. Comparing the MCNPX simulated response of a bulk SiC device to that of a silicon device suggests a superior ability to detect fast neutrons with an intrinsic efficiency 1.7 times that of silicon.

Monte Carlo simulation of the experimental pulse height spectra produced in diamond detectors by quasi-mono-energetic neutrons

This work was carried out in view of the possible use of diamond detectors as high resolution neutron spectrometers for the ITER project. An MCNP5(X) based computational tool has been developed to simulate the fast neutron response of diamond detectors. The source neutrons are generated by a source routine, developed earlier, that includes deuteron beam energy loss, angular straggling, and two-body relativistic kinematics. The diamond detector routine calculates a pulse height spectrum that is built up by elastic and inelastic scattering, (n,a), (n,p), and (n,d) reaction channels. A combination of nuclear data from ENDF/B-VII.0, TENDL-2010, and ENSDF is used. The simulated spectra are compared with measured spectra. It is shown that the simulation tool allows an interpretation of most of the characteristic features in the spectrum. This is an important step towards the use of diamond detectors for spectral analysis and fluence measurements. © 2001 Elsevier Science. All rights reserved.

Fast neutron detection efficiency of ATLAS-MPX detectors for the evaluation of average neutron energy in mixed radiation fields

Within the framework of the ATLAS-MPX project, the ATLAS-MPX detectors (based on Medipix2 silicon devices) are covered with converting layers of 6LiF and polyethylene (PE) to make them sensitive to thermal and fast neutrons, respectively. Two ATLAS-MPX reference detectors were exposed to two calibrated neutron sources, 252Cf (2.2 MeV mean neutron energy) and 241AmBe (4.08 MeV mean neutron energy), in order to determine their fast neutron detection efficiency. Measurements were performed at low energy threshold (∼8 keV) and high energy threshold (∼230 keV). Fast neutron detection efficiency is primarily achieved via the use of a 1.3 mm thick polyethylene (PE) converter. Recoil protons from the elastic collision between neutron and hydrogen are detected from their tracks in the 300 μm thick silicon pixel detector. Calibrated neutron sources were placed at different distances from the detectors, both separately and simultaneously in order to obtain single and superposed neutron energy spectra. As expected, the neutron detection efficiency in the PE layer increases when the neutron mean energy increases due to the decrease of proton self-absorption in the PE converter itself. The variation of the cluster size as a function of the proton and alpha energy (at low energy threshold) was also studied for better understanding of the neutron response using the PE converter. The determination of the ratio of the fast neutron responses in each detector region at high energy threshold made it possible to establish a relation between the ratios and the mean neutron energy. At low energy threshold, a relation between the neutron energy spectrum and the cluster size distribution of heavy charged particles has been established.

Calculation of CR-39 [formula omitted] efficiency for fast neutrons using the MCNP and SRIM codes

Elastic scattering of light nuclei, especially hydrogen, is widely used for the detection of fast neutrons. The fast neutron is detected in state nuclear track detectors (SSNTDs), such as CR-39, through interactions with H, C and O nuclei. The fast-neutron response can be increased by using a converter layer with high hydrogen content, if the front face of the dosimeter is being processed. In this work, only elastic scattering of hydrogen has been considered, and the optimum thickness of the converter layer has been determined. Some different thicknesses of aluminium layers have been used as discriminators for which the efficiency of a detector to a 241Am–Be neutron source has been calculated. A program FTRAC has been developed for the calculation of the detection efficiency. This software determines the specifications of recoil protons (using the PTRAC card of the MCNP code) and also the kinematics of interactions. Transport of recoiled protons has been carried out using SRIM code. Good agreement have been shown between measurement and simulation result.

The fast neutron response of 4H silicon carbide semiconductor radiation detectors

Fast neutron response measurements are reported for radiation detectors based on large-volume SiC p-i-n diodes. Multiple reaction peaks are observed for 14-MeV neutron reactions with the silicon and carbon nuclides in the SiC detector. A high degree of linearity is observed for the /sup 28/Si(n,/spl alpha//sub i/) reaction set of six energy levels in the product /sup 25/Mg nucleus, and pulse height defect differences between the observed /sup 12/C(n,/spl alpha//sub 0/)/sup 28/ Si(n,/spl alpha//sub i/) energy responses are discussed. Energy spectrometry applications in fission and fusion neutron fields are also discussed.