Fast Neutron Detection Research Articles

Characterization of a 6Li-loaded liquid organic scintillator for fast neutron spectrometry and thermal neutron detection

The characterization of a liquid scintillator incorporating an aqueous solution of enriched lithium chloride to produce a scintillator with 0.40% 6Li is presented, including the performance of the scintillator in terms of its optical properties and neutron response. The scintillator was incorporated into a fast neutron spectrometer, and the light output spectra from 2.5MeV, 14.1MeV, and 252Cf neutrons were measured using capture-gated coincidence techniques. The spectrometer was operated without coincidence to perform thermal neutron measurements. Possible improvements in spectrometer performance are discussed.

Validation of Geant4 and MCNPX-PoliMi simulations of fast neutron detection with the EJ-309 liquid scintillator

In organic liquid scintillators, the energy distribution of neutrons is indirectly measured by characterizing the pulse height distribution of light output produced through nuclear reactions within the detectors. However, the complexity of the light generation on these detectors makes simulating their response difficult with standard Monte Carlo techniques. This paper presents the response of 7.62-cm-long×7.62-cm-diameter and 12.7-cm-long×12.7-cm-diameter EJ-309 liquid scintillators, simulated using the Geant4 and MCNPX-PoliMi Monte Carlo codes. The simulations are restricted to the neutron transport and interaction models. Monoenergetic neutrons as well as fission neutrons emitted by a 252Cf spontaneous fission source are used in these simulations. The energy deposition of neutrons in organic liquid scintillators is dependent upon their collisions with hydrogen and carbon nuclei; therefore simulating neutron collisions on an event-by-event basis is important to generate neutron pulse height distributions. There are no default physics models in Geant4 hence proper selection of physics models is required for accurate simulation. In this paper, we compare Geant4 (ver. 4.9.5) and MCNPX-PoliMi (ver. 2.0) neutron pulse height distributions. The simulations are validated with time-of-flight (TOF) measurements obtained with 12.7-cm-long×12.7-cm-diameter EJ-309 detectors. Results show that both Geant4- and MCNPX-PoliMi-generated distributions are in good agreement with the measured data.

Semi-insulating GaAs detectors optimized for fast neutron detection

Semi-insulating (SI) GaAs detectors with a HDPE (High Density PolyEthylene) conversion layer were optimized for detection of fast neutrons (from 0.5 MeV to 12 MeV). Based on previous simulations of neutron transport in HDPE and SI GaAs carried out by the Monte Carlo radiation transport computer code MCNPX (Monte Carlo N-Particle eXtended, version 2.5.0) we used a SI GaAs wafer with a larger thickness of 270 μm. The area of a single AuZn Schottky contact was enlarged from 6.25 mm2 to 7.36 mm2. Thanks to the pixel structure of the new metallization the breakdown voltage increased from 60 V to 280 V as deduced from the measured I−V characteristics. Various thicknesses of HDPE layers in the range from 100 μm to 2000 μm were used with SI GaAs detectors for neutron conversion. The measured relative detection efficiency for fast neutrons using SI GaAs detectors with various HDPE thicknesses varied from 0.07 to 0.12% at lower applied voltages, with a maximum for a 500 μm thick conversion layer.

The effect of material purity on the optical and scintillation properties of solution-grown trans-stilbene crystals

Large, 10cm-size, single crystals of trans-stilbene were grown for the first time from solution by a temperature reduction technique. Their scintillation performance for fast neutron detection depended on the purity of the initial starting material. A light-yield-inhibitor-free starting material was specially synthesized and crystals from this material were compared to those grown from commercially available powders of trans-stilbene. The use of pure material made possible the growth of the large size crystals (>400g) with excellent neutron–gamma discrimination performance, which are needed for use as a radiation detection material.

Monte Carlo simulations of the particle transport in semiconductor detectors of fast neutrons

Several Monte Carlo all-particle transport codes are under active development around the world. In this paper we focused on the capabilities of the MCNPX code (Monte Carlo N-Particle eXtended) to follow the particle transport in semiconductor detector of fast neutrons. Semiconductor detector based on semi-insulating GaAs was the object of our investigation. As converter material capable to produce charged particles from the (n, p) interaction, a high-density polyethylene (HDPE) was employed. As the source of fast neutrons, the 239Pu–Be neutron source was used in the model. The simulations were performed using the MCNPX code which makes possible to track not only neutrons but also recoiled protons at all interesting energies. Hence, the MCNPX code enables seamless particle transport and no other computer program is needed to process the particle transport. The determination of the optimal thickness of the conversion layer and the minimum thickness of the active region of semiconductor detector as well as the energy spectra simulation were the principal goals of the computer modeling. Theoretical detector responses showed that the best detection efficiency can be achieved for 500μm thick HDPE converter layer. The minimum detector active region thickness has been estimated to be about 400μm.

Characteristics of a stilbene scintillation crystal in a neutron spectrometer

Various organic scintillators are commonly used as the detecting material for neutrons, but these detectors are less sensitive to gamma rays. In particular, stilbene crystals and BC501A (NE213, EJ301) have good pulse-shape discrimination (PSD) between neutron and gamma-ray events, and have been selected as the media for fast-neutron detection among the organic, inorganic and plastic materials in a mixed radiation field. Although some of the scintillation characteristics of stilbene crystals have been studied, the detailed scintillation characteristics of the crystal are not completely understood. In this study, the light yield, decay time and pulse shape discrimination capability of a stilbene crystal were measured because this crystal is an optimized detector in a large flux of neutrons such as those might be found in cyclotron and charged particle accelerator facilities. The pulse-shape discrimination of neutrons and gamma rays with a stilbene crystal was measured using a 252Cf neutron source at room temperature. A neutron tagger module was used for the neutron and gamma separation using the charge comparison method in real time. The total pulse width for the charge integration and the delay from the peak-to tail start time were optimized for a better neutron and gamma separation. The relative light yield and decay time of the stilbene crystal scintillator were also measured.

First studies of electron transport along small gas gaps of novelfoil radiation converters for fast-neutron detectors

Novel high-efficiency fast-neutron detectors weresuggested for fan-beam tomography applications. They combine multi-layerpolymer converters in gas medium, coupled to thick gaseous electronmultipliers (THGEM). Neutron-induced scattering on the converter's hydrogennuclei results in gas ionization by the escaping recoil-protons between twosuccessive converters. The electrons drift under the action of a homogeneouselectric field, parallel to the converter-foil surfaces, towards aposition-sensitive THGEM multiplying element.In this work we discuss the results of a systematic study of the electrontransport inside a narrow gap between successive converter foils, whichaffects the performance of the detector, both in terms of detectionefficiency and localization properties. The efficiency of transportingionization electrons was measured along a 0.6 mm wide gas gap in 6 and 10 mmwide polymer converters. Computer simulations provided conceptualunderstanding of the observations. For drift lengths of 6 mm, electrons wereefficiently transported along the narrow gas gap with minimaldiffusion-induced losses; an average collection efficiency of 95% wasachieved for ionization electrons induced by few keV photoelectrons. The 10mm height converter yielded considerably lower efficiency due to electricaland mechanical flaws of the converter foils. The results indicate thatdetection efficiencies of ∼ 7% can be expected for 2.5 MeV neutronswith 300-foils converters, of 6 mm height, 0.4 mm thick foils and 0.6 mm gasgap.

The Study of Zinc Sulphide Scintillator for Fast Neutron Radiography

Fast neutron radiography is a promising application for accelerators. The potential effectiveness of this technique depends on the development of suitable imaging detectors for fast neutrons. Zinc sulphide based scintillators have the largest light output per event in the family of imaging scintillators used so far in fast neutron radiography. This paper investigated different aspects of this scintillator in order to determine the factors which might affect the light output. A mathematical model was established to estimate effectiveness of this scintillator. Zinc sulphide screens were prepared with ZnS particles of different concentration in polypropylene matrix. A 14MeV fast neutron source was used in the experiments. The light output was detected using a CCD camera or a film coupled to the scintillator screen. The results showed that the optimum scintillators is 3-mm in thickness with the weight ratio of 1:1 to 2:1 for ZnS and polypropylene.

Development of 14-MeV Neutron Measurement with Nuclear Emulsion for DT Burning Plasma Diagnostics

A method for the measurement of energetic DT neutron tails resulting from knock-on alpha particles is needed to study plasma physics in a fusion reactor. A nuclear emulsion offers satisfactory performance for the detection of fast neutrons and measures their energies using three-dimensional tracking information. However, the time required for analysis forms a bottleneck in the implementation of this measurement technique. Recently, the analysis speed of nuclear emulsion has dramatically increased because of our development. In this report, we propose the use of nuclear emulsion for DT burning plasma diagnostics using the latest analysis technology and the validation of the methodology. In addition, we discuss the prospects of improving nuclear emulsion technologies for fusion plasma diagnostics.

Directional detection of fast neutrons by the Timepix pixel detector coupled to plastic scintillator with silicon photomultiplier array

Fast neutrons are conventionally detected by scintillators of large volume, low spatial resolution and poor, if any, directional sensitivity. In this paper we present a detection technique based on the tracking of protons recoiled by fast neutrons. In this approach we use the silicon pixel detector Timepix attached in contact planar geometry to a fast plastic scintillator. The protons recoiled by neutrons in the scintillator are detected by the pixel detector while scintillation light is sensed by a 4 × 4 array of silicon photomultipliers (SiPM). Each photomultiplier is equipped with an independent amplifier and discriminator providing a fast trigger signal to the pixel detector. Variable threshold level allows adjustment of the trigger sensitivity. Single events in the pixel detector can be tagged and triggered by the scintillating detector. Position and energy sensitivity of the scintillator together with the position and the energy sensitivity of the pixel detector allow obtaining information about the position and the spectrum of the neutron source. The Timepix detector is operated with the FITPix readout interface and the Pixelman software package providing control, DAQ and online visualization. The assembled prototype has been tested with fast neutrons from a laboratory radioactive source (AmBe) and a Van de Graaff accelerator (D-T reaction). The detector architecture, comprising the Timepix device, the scintillator and the segmented SiPM, allows stacking several such units for increased detection efficiency and enhanced directional sensitivity.

NGEM fast neutron detectors for beam diagnostics

Fast neutron detectors with a sub-millimetric space resolution are required in order to qualify neutron beams in applications related to magnetically-controlled nuclear fusion plasmas and to spallation sources. A nGEM detector has been developed for the CNESM diagnostic system of the SPIDER NBI prototype for ITER and as beam monitor for fast neutrons lines at spallation sources. The nGEM is a triple GEM gaseous detector equipped with polypropylene and polyethylene layers used to convert fast neutrons into recoil protons through the elastic scattering process. This paper describes the results obtained by testing a nGEM detector at the ISIS spallation source on the VESUVIO beam line. Beam profiles (σx=14.35mm, σy=15.75mm), nGEM counting efficiency (around 10-4 for 3MeV<En<15MeV), detector stability (≈4.5%) and the effect of filtering the beam with different type of materials were successfully measured. The x beam profile was compared to the one measured by a single crystal diamond detector. Finally, the efficiency of the detector was simulated exploiting the GEANT4 tool.

New cerium-based metal–organic scintillators for radiation detection

We have previously shown that a new class of scintillating materials can be developed based on the synthesis and crystal growth of rare-earth metal–organic compounds. The first scintillator of this type consisted of single crystals of CeCl3(CH3OH)4 that were grown from a methanol solution. These crystals were shown to be applicable to both gamma-ray and fast neutron detection. Subsequently, metal–organic scintillators consisting of the compound LaBr3(CH3OH)4 activated with varying levels of Ce3+ and of CeBr3(CH3OH)4 were grown in single crystal form. We have now extended the development of this new class of scintillators to more complex organic components by reacting rare-earth halides such as CeCl3 or CeBr3 with different isomers of propanol and butanol—including 1-propanol, isobutanol, n-butanol, and tert-butanol. The reaction of CeCl3 or CeBr3 with these organics results in the formation of new and relatively complex molecular crystals whose structures were determined using single-crystal X-ray diffraction. These new metal–organic scintillating materials were grown in single crystal form from solution, and their scintillation characteristics have been investigated using X-ray-excited luminescence plus energy spectra obtained with gamma-ray and alpha-particle sources. If the reactions between the inorganic and organic components are not carried out under very dry and highly controlled conditions, molecular structures will be formed that incorporate waters of hydration. The present observation of scintillation in these hydrated rare-earth metal–organic compounds is apparently an original finding, since we are not aware of any previous reports of scintillation being observed in a material that incorporates waters of hydration.

Simulation and prototyping of 2 m long resistive plate chambers for detection of fast neutrons and multi-neutron event identification

Resistive plate chamber (RPC) prototypes of 2m length were simulated and built. The experimental tests using a 31MeV electron beam, discussed in details, showed an efficiency higher than 90% and an excellent time resolution of around σ=100ps. Furthermore, comprehensive simulations were performed by Geant4 toolkit in order to study the possible use of these RPCs for fast neutron (200MeV–1GeV) detection and multi-neutron event identification. The validation of simulation parameters was carried out via a comparison to experimental data. A possible setup for invariant mass spectroscopy of multi-neutron emission is presented and the characteristics are discussed. The results show that the setup has a high detection efficiency. Its capability of determining the momentum of the outgoing neutrons and reconstructing the relative energy between the fragments from nuclear reactions is demonstrated for different scenarios.

Fast neutron detection under intense gamma-ray fields with novel nuclear emulsion technique

Abstract In order to measure neutron flux and energy in the presence of high-intensity background gamma rays, a new nuclear emulsion based on a non-sensitized OPERA film was synthesized, which had small AgBr grain sizes of 60, 90 and 160 nm. The sensitivity of this new emulsion, which is described in terms of the stopping power and grain density, was estimated experimentally by neutron irradiation at several energies. The response of the emulsion to gamma-ray-induced electrons was also simulated and the results were compared with experimental data obtained using a 60 Co gamma source. The results indicated that each AgBr grain has a threshold energy that must be deposited before the AgBr can develop into metallic Ag. Based on these results, the efficiency of the new emulsion in detecting gamma rays and neutrons was evaluated.

Reconstruction Algorithms for Directional Neutron Detection Using a Time Projection Chamber

Directional detection of fast neutrons emitted by special nuclear materials can be performed with a time projection chamber. This device permits particle identification and full three-dimensional reconstruction of charged-particle tracks produced by interaction of fast neutrons in the chamber active volume. Single-recoil-proton reconstruction allows rapid pointing, while the reconstruction of two recoil protons produced by a single incident neutron event can enable a measurement with very high angular resolution. Kinematic reconstruction algorithms for both of these cases are presented and their performance assessed using data generated by a simple Monte Carlo simulation and experimental data where those exist. The simulation data are also used to estimate the relative efficiency of both neutron imaging modalities as a function of the volume and pressure of the time projection chamber.

Statistical characterization of the reproducibility of neutron emission of small plasma focus devices

The purpose of this work is to discuss the techniques related to the detection of fast pulsed neutrons produced in plasma focus (PF) devices, the statistical analysis of the corresponding data, and the methodologies for evaluation of the device performance in low emission neutron sources. A general mathematical framework is presented for the assessment of the reproducibility of the neutron emission of small PF devices given the shot-to-shot distribution and detector efficiency. The effect on the reproducibility in case of using two independent detectors is also discussed. The analysis is applied to the neutron emission of the plasma focus device PF-50J operating in repetitive mode (0.1–0.5 Hz and 65 J bank energy).

Design of a monolithic CMOS sensor for high efficiency neutron counting

The development of CMOS Pixel Sensors (CPS) for charged particles detection had led to promising applications for dosimetry. Based on our previous studies on the detection of fast and thermal neutrons with a CMOS sensor originally designed for particle tracking in high energy physics, a dedicated integrated CMOS sensor for efficient neutron counting is presented. This sensor has been designed and fabricated in a 0.35μm CMOS process. It is a low noise, low power consumption sensor with a sensitive area of 6.55mm2 and a digital output. The test results indicate that it is suitable for neutron detection, thanks to its equivalent input noise charge of less than 400e−, and a 20kHz detection frequency. In this paper the prototype is presented for both its circuit design and test results.

Optimizing Cs2LiYCl6 for fast neutron spectroscopy

Cs2LiYCl6 (CLYC) has generated recent interest as a thermal neutron detector due to its excellent n/γ-ray pulse-shape discrimination and energy resolution. Here, the capabilities of CLYC as a fast neutron detector and spectrometer are reported. A 1in.×1in. CLYC detector was used to measure the response of mono-energetic neutrons over a range of 0.8–2.0MeV produced via the 7Li(p,n) reaction at the University of Massachusetts Lowell 5.5MV Van de Graaff accelerator. A broad continuum from the 6Li(n, α) reaction was observed, as well as additional peaks below the thermal capture peak. Based on possible reactions in CLYC, the additional peaks are determined to be due to the 35Cl(n,p)35S reaction, with a Q-value of +615keV, and corroborated in simulations using MCNPX. The average resolution of 9% for these peaks makes CLYC a promising candidate for a fast neutron spectrometer.

Polycrystalline chemical-vapor-deposited diamond for fast and ultra-fast neutron detection

An intrinsic sensor (diameter 16 mm, thickness 300 μm ) based on chemical vapor deposition derived polycrystalline diamond was developed. Ultra low dark current under 100 pA and high n/γ discrimination of 15.8 were obtained on this device, which enable it to be used for neutron detection in multi-radiation field. Moreover, the neutron sensitivity of this detector was characterized theoretically and experimentally. And for fast and ultra-fast neutron, the detection sensitivity is in the magnitude of 10−17 to 10−16 C cm2/n with good stability and negligible pump effect. All these results indicate that the as-prepared sensor to be ideal for fast and ultra-fast neutron monitoring, and it may pave the way to build neutron detector with low cost and large sensitive area with diamond.

Fast-neutron detectors for nuclear physics experiments

Fast-neutron detectors are used in a wide range of nuclear physics experimentsincluding studies of elastic and inelastic neutron scattering, charge-exchange reactions, photonuclear reactions, neutron-inducedfission, and, especially recently, reactions of radioactive nuclei. Although many of thedetectors being developed now are based on technologies that are several decades old, newphysics is now accessible due to the advent of advanced accelerators, and these facilities presentchallenging opportunities for detecting fast neutrons. The choice of detectors, theirappropriateness for particular measurements and how they are integrated into experiments willbe discussed. Detector arrays are of particular importance these days to study angulardistributions or simply to increase the solid angle coverage to increase the data rate. Modelingthe response of the detectors has become much more important in order to understand bettertheir response and to calculate effects of neutron scattering in the experimental area, includingdetector-to-detector scattering. Data acquisition through waveform digitizers is now commonand leads to more information from each event as well as significant reductions in dead time andin the complexity of the electronics. At the same time, analyzing waveforms in real timepresents challenges in terms of handling large amounts of information. Examples of significantimprovements in the utilization of neutron detectors in physics experiments, in thecharacterization of the detector response, and in signal processing will be presented.