Study on spatial resolution of micromegas as a neutron detector under condition of high neutron flux and γ ray background

In this paper Micromegas has been designed to detect neutrons. The simulation of the spatial resolution of Micromegas as neutron detector is carried out by GEANT4 toolkit. The neutron track reconstruction method based on the time coincidence technology is employed in the present work. The influence of the flux of incident 14 MeV neutron and high gamma background on the spatial resolution is carefully studied. Our results show that the spatial resolution of the detector is sensitive to the neutron flux, but insensitive to the intensity of γ background if the neutron track reconstruction method proposed by our group is used. The γ insensitivity makes it possible for us to use the Micromegas detector under condition which has high γ-rays background.     

A capture-gated fast neutron detection method

To address the problem of the shortage of neutron detectors used in radiation portal monitors(RPMs),caused by the ~3He supply crisis, research on a cadmium-based capture-gated fast neutron detector is presented in this paper. The detector is composed of many 1 cm × 1 cm × 20 cm plastic scintillator cuboids covered by 0.1 mm thick film of cadmium. The detector uses cadmium to absorb thermal neutrons and produce capture γ-rays to indicate the detection of neutrons, and uses plastic scintillator to moderate neutrons and register γ-rays. This design removes the volume competing relationship in traditional ~3He counter-based fast neutron detectors, which hinders enhancement of the neutron detection efficiency. Detection efficiency of 21.66% ± 1.22% has been achieved with a 40.4 cm × 40.4cm × 20 cm overall detector volume. This detector can measure both neutrons and γ-rays simultaneously. A small detector(20.2 cm × 20.2 cm × 20 cm) demonstrated a 3.3 % false alarm rate for a ~(252)Cf source with a neutron yield of 1841 n/s from 50 cm away within 15 s measurement time. It also demonstrated a very low(0.06%) false alarm rate for a 3.21 × 10~5 Bq ~(137)Cs source. This detector offers a potential single-detector replacement for both neutron and the γ-ray detectors in RPM systems.     

Dosimetry Methods of Fast Neutron Using the Semiconductor Diodes

Semiconductor detectors based on a silicon pin diode are frequently used in the detection of different nuclear radiations. For the detection and dosimetry of fast neutrons, these silicon detectors are coupled with a fast neutron converter. Incident neutrons interact with the converter and produce charged particles that can deposit their energy in the detectors and produce a signal. In this study, three methods are introduced for fast neutron dosimetry by using the silicon detectors, which are： recoil proton spectroscopy, similarity of detector response function with conversion function, and a discriminator layer. Monte Carlo simulation is used to calculate the response of dosimetry systems based on these methods. In the different doses of an 241Am-Be neutron source, dosimetry responses are evaluated. The error values of measured data for dosimetry by these methods are in the range of 15-25%. We find fairly good agreement in the 241Am-Be neutron sources.     

Simulation of large-scale fast neutron liquid scintillation detector

Neutron background measurement is always very important for dark matter detection due to almost the same effect for the recoiled nucleus scattered off by the incident neutron and dark matter particle. For deep under-ground experiments, the flux of neutron background is so low that large-scale detection is usually necessary. In this paper, by using Geant4, the relationship between detection efficiency and volume is investigated, meanwhile, two geometrical schemes for this detection including a single large-sized detector and arrayed multi-detector are compared under the condition of the same volume. The geometrical parameters of detectors are filtrated and detection efficiencies obtained under the similar background condition of China Jingping Underground Laboratory (CJPL). The results show that for a large-scale Gd-doped liquid scintillation detector, the detection efficiency increases with the size of detector at the beginning and then trends toward a constant. Under the condition of the same length and cross section, the arrayed multi-detector has almost similar detection performance as the single large-sized detector, while too much detector number could cause degeneration of detection performance. Considering engineering factors, such as testing, assembling and production, the 4 × 4 arrayed detector scheme is flexible and more suitable. Furthermore, the conditions for using fast and slow signal coincidence detection and the detectable lower limit of neutron energy are evaluated by simulating the light process.     

Study of Borromean halo nuclei by the neutron wall with simulation

The model of three-body Borromean halo nuclei breakup was described by using standard phase space distributions and the Monte Carlo simulation method was established to resolve the detection problem of two neutrons produced from breakup reaction on the neutron wall detector. For 6He case, overall resolution ~rEk for the Oaussiaal part of the detector response and the detection efficiency including solid angle acceptance with regard to the excitation energy Ek are obtained by the simulation of two neutrons from 6He breakup into the neutron wall. The effects of the algorithm on the angular and energy correlations of the fragments are briefly discussed.     

Comparison of thermal neutron detection efficiency of 6Li scintillation glass and 3He gas proportional tube

We report on a comparison study of the ³He gas proportional tube and the ⁶Li incorporated scintillation glasses on thermal neutron detection efficiency. Both ³He and ⁶Li are used commonly for thermal neutron detection because of their high neutron capture absorption coefficient. By using a neutron source ²⁵²Cf and a paraffin moderator in an alignment system, we can get a small beam of thermal neutrons. A flash ADC is used to measure the thermal neutron spectrum of each detector, and the detected number of events is determined from the spectrum, then we can calculate the detection efficiency of different detectors. Meanwhile, the experiment has been modeled with GEANT4 to validate the results against the Monte Carlo simulation.     

Monte Carlo simulation of GEM-based for 14 MeV neutron detector

A new concept of neutron detector based on Gas Electron Multiplier(GEM) technology is presented in this paper,in which a novel multi-layer high density polyethylene(HDPE) as neutron-to-proton converter is proposed and studied with Geant4 toolkit for fast 14 MeV neutron.Our preliminary results show that the detection efficiency of the detector with 400 converter units is higher than 2.3% and reconstruction accuracy of the incident neutron position is higher than 2.6%.     

Toward a fractal spectrum approach for neutron and gamma pulse shape discrimination

Accurately selecting neutron signals and discriminating 7 signals from a mixed radiation field is a key research issue in neutron detection.This paper proposes a fractal spectrum discrimination approach by means of different spectral characteristics of neutrons and 7 rays.Figure of merit and average discriminant error ratio are used together to evaluate the discrimination effccts.Different neutron and γ signals with various noise and pulse pile-up are simulated according to real data in the literature.The proposed approach is compared with the digital charge integration and pulse gradient methods.It is found that the fractal approach exhibits the best discrimination performance,followed by the digital charge integration method and the pulse gradient method,respectively.The fractal spectrum approach is not sensitive to high frequency noise;and pulse pile-up.This means that the proposed approach has superior performance for effective and efficient anti-noise and high discrimination in neutron detection.     

Characteristics of the sampling paddle modules of neutron wall from the cosmic ray test

In the construction of a large area neutron detector （neutron wall） that is used to detect neutrons at GeV energies, the performances of all the sampling paddle modules prepared for the neutron wall are investigated with a specially designed test bench. Tested by cosmic rays, an average intrinsic time resolution of 222.5 ps is achieved at the center of the modules. The light attenuation length and the effective speed of the light in the module are also investigated.     

应用于热中子的nTHGEM探测器性能研究

随着脉冲强流中子源的发展,对高性能中子探测器提出了更大的挑战,3He气体资源严重短缺和高计数率中子探测器的迫切需求,已开始制约着中子源应用技术的发展。中国科学院高能物理研究所针对中子的特殊性,专门研发了一种陶瓷基材的nTHGEM（neutron Thick Gaseous Electron Multiplier）探测器用于中子探测。基于nTHGEM的中子探测器具有高计数率、高位置与时间分辨能力、增益大、制作工艺简单,且便于大面积制作的特点,是目前国际上发展替代3He探测技术的重要方向之一。为了详细研究nTHGEM探测器的本身性能,本工作使用55Fe放射源研究了nTHGEM探测器的增益、计数率稳定性、能量分辨率等关键参数与nTHGEM膜间电压、收集场强、漂移场强之间的关系,优化了nTHGEM探测器在不同工作气体中的工作参数,为后续进一步优化nTHGEM探测器设计和工艺奠定了基础。实验结果表明,单层nTHGEM探测器在Ar（90%）+CO₂（10%）混合气体中增益能达到103,探测器计数率稳定性良好。另外,还在中国原子能科学研究院的CARR反应堆（China Advanced Research Reactor）上进行了中子束流实验,通过狭缝测量到探测器位置分辨率为（3.01±0.03）mm（FWHM）,已经接近高气压3He MWPC中子探测器水平。With the development of pulsed intense neutron source, the high-performance neutron detector poses more challenges. The severe shortage of 3He gas resources and the urgent need of neutron detector with high counting rate have begun to restrict the neutron source application technology development. In response to the particularity of neutrons, the Institute of High Energy Physics of CAS developed a nTHGEM(neutron Thick Gaseous Electron Multiplier)of ceramic substrate for neutron detection. The neutron detector based on nTHGEM is one of the most important directions for the development of alternative 3He detection technology in the world at present because of its high counting rate, high position and time resolution, large gain, simple fabrication process and large area production. In order to study the properties of nTHGEM detector in detail, this paper studied the relationship between nTHGEM detector's gain, counting rate stability, energy resolution and other key parameters and nTHGEM film voltage, collection field strength and drift field strength using 55Fe radioactive source, Optimized the working parameters of nTHGEM detector in different working gases, which laid the foundation for further optimization of nTHGEM detector design and process. The experimental results show that the single-layer nTHGEM detector has a gain of 103 in a Ar(90%)+CO₂(10%) mixed gas with good counting rate stability. In addition, a neutron beam experiment was performed on the China Advanced Research Reactor at the China Institute of Atomic Energy, and the position resolution of the detector was (3.01±0.03) mm (FWHM) measured by slits. Its performance is close to the high pressure 3He MWPC neutron detector level.     

The Monte-Carlo simulation on a scintillator neutron detector

A simulation of the properties of the shifting scintillator neutron detector using 6LiF/ZnS(Ag) scintillation screens is performed.The simulation results show that the light attenuation length of standard BC704 scintillator is about 0.65 mm.Its thermal neutron detection efficiency,gamma sensitivity and intrinsic spatial resolution can achieve around 50.0%,10 5and 0.18 mm(along X-axis) respectively.For the detector,air coupling position resolution is better than the silicone oil coupling.Some of the simulation results are compared with experimental results.They are in agreement.This work will be helpful for constructing neutron detector for high intensity powder diffractometer at Chinese spallation neutron source.     

Design and performance analysis of a compact magnetic proton recoil spectrometer for DT neutrons

A magnetic proton recoil (MPR) spectrometer is a novel instrument with superior performance, including high energy resolution, high count rate and good signal-to-noise ratio (SNR) for measurements of neutron spectra from inertial confinement fusion (ICF) experiments and high power Tokomaks. In this work, the design of a compact MPR spectrometer (cMPR) was evaluated for deuteron-tritium (DT) neutron spectroscopy. The characteristics of the spectrometer were analyzed using 2-D beam transport simulations, 3-D particle transport calculations and Monte-Carlo simulations. Based on the theoretical results, an instrument design that satisfies special experimental requirements is proposed. The energy resolution and efficiency of the spectrometer are also evaluated. The results indicate that the proposed cMPR spectrometer would achieve a detection efficiency and energy resolution of approximately 10^-8 and 4%, respectively, for DT neutrons.     

Performance study of the neutron-TPC

Fast neutron spectrometers will play an important role in the future of the nuclear industry and nuclear physics experiments, in tasks such as fast neutron reactor monitoring, thermo-nuclear fusion plasma diagnostics,nuclear reaction cross-section measurement, and special nuclear material detection. Recently, a new fast neutron spectrometer based on a GEM(Gas Electron Multiplier amplification)-TPC(Time Projection Chamber), named the neutron-TPC, has been under development at Tsinghua University. It is designed to have a high energy resolution,high detection efficiency, easy access to the medium material, an outstanding n/γ suppression ratio, and a wide range of applications. This paper presents the design, test, and experimental study of the neutron-TPC. Based on the experimental results, the energy resolution(FWHM) of the neutron-TPC can reach 15.7%, 10.3% and 7.0% with detection efficiency higher than 10~(-5) for 1.2 Me V, 1.81 Me V and 2.5 Me V neutrons respectively.     

Study on the novel neutron-to-proton convertor for improving the detection efficiency of a triple GEM based fast neutron detector

A high-efficiency fast neutron detector prototype based on a triple Gas Electron Multiplier(GEM) detector, which, coupled with a novel multi-layered high-density polyethylene(HDPE) as a neutron-to-proton converter for improving the neutron detection efficiency, is introduced and tested with the Am-Be neutron source in the Institute of Modern Physics(IMP) at Lanzhou in the present work. First, the developed triple GEM detector is tested by measuring its effective gain and energy resolution with55 Fe X-ray source to ensure that it has a good performance.The effective gain and obtained energy resolution is 5.0×104and around 19.2%, respectively. Secondly, the novel multi-layered HDPE converter is coupled with the cathode of the triple GEM detector making it a high-efficiency fast neutron detector. Its effective neutron response is four times higher than that of the traditional single-layered conversion technique when the converter layer number is 38.     

Study of Borromean halo nuclei by the neutron wall with simulation

The model of three-body Borromean halo nuclei breakup was described by using standard phase space distributions and the Monte Carlo simulation method was established to resolve the detection problem of two neutrons produced from breakup reaction on the neutron wall detector. For ⁶He case,overall resolution σE_k for the Gaussian part of the detector response and the detection efficiency including solid angle acceptance with regard to the excitation energy E_k are obtained by the simulation of two neutrons from ⁶He breakup into the neutron wall. The effects of the algorithm on the angular and energy correlations of the fragments are briefly discussed.     

Physical design of target station and neutron instruments for China Spallation Neutron Source

The China Spallation Neutron Source(CSNS)is the first accelerator-based multidiscipline user facility to produce pulsed neutrons by tungsten target under collision of a pulsed proton beam with a beam power of 100 kW at a repetition rate of 25 Hz.In this paper,we focus on the physical design of CSNS target station and neutron instruments.Under optimized design,the flat tungsten target and the compact target-moderator-reflector coupling enhance effective cold and thermal neutron output from moderators.Three wing-type moderators supply four different characteristics of neutrons to 19 beamlines primarily for neutron scattering applications.Layout of neutron instruments are conceptually planned for total 20 beamlines,the configuration and specification have been determined for three day-one neutron instruments.All designs are optimized for the Phase I of 100 kW with a upgradable capacity to 500 kW.     

Neutron time-of-flight spectroscopy measurement using a waveform digitizer

The photoneutron source(PNS, phase 1), an electron linear accelerator(linac)-based pulsed neutron facility that uses the time-of-flight(TOF) technique, was constructed for the acquisition of nuclear data from the Thorium Molten Salt Reactor(TMSR) at the Shanghai Institute of Applied Physics(SINAP). The neutron detector signal used for TOF calculation, with information on the pulse arrival time, pulse shape, and pulse height, was recorded by using a waveform digitizer(WFD). By using the pulse height and pulse-shape discrimination(PSD)analysis to identify neutrons and γ-rays, the neutron TOF spectrum was obtained by employing a simple electronic design, and a new WFD-based DAQ system was developed and tested in this commissioning experiment. The DAQ system developed is characterized by a very high efficiency with respect to millisecond neutron TOF spectroscopy.     

锂玻璃探测器中子探测效率的刻度

为了精确测量keV能区的中子俘获截面,中国原子能科学研究院正在建造一台4π 全吸收型γ 探测装置---GTAF,锂玻璃探测器将会作为中子束流监视器测量中子能谱。利用5SDH-2 加速器刻度了锂玻璃探测器在两个入射中子单能点(250 和565 keV) 的探测效率,并使用EANT4 和MCNP 程序模拟计算了锂玻璃探测器的相对探测效率。通过归一化实验数据和模拟结果,得到了锂玻璃探测器在10keV~1 MeV 能区的中子探测效率曲线。对于把锂玻璃探测器测量得到的飞行时间谱转化为中子束流能谱,是一项非常重要的工作,同时为探测器效率刻度提供了新方法。In order to accurately measure the neutron capture cross section in the energy range of keVMeV, 4 πgamma-ray total absorption facility (GTAF) is being constructed at China Institute of Atomic Energy (CIAE). The lithium glass detector will be used as a neutron beam monitor for GTAF. The detection efficiency of the lithium glass detector at two incident neutron energy points (250, 565 keV) was calibrated in 5SDH-2 accelerator, and the relative detection efficiency was simulated by GEANT4 and MCNP code. By the normalization of the experimental data and simulation result, the neutron detection efficiency curve of the lithium glass detector between 10 keV and 1 MeV was obtained. This work will be important to convert the Time-of-flight spectrum that be measured by Li-glass detector to the energy spectrum of neutron beam, and provide the new method for calibration of detection efficiency.     

Neutron-induced single event upset simulation in Geant4 for three-dimensional die-stacked SRAM

Three-dimensional integrated circuits(3D ICs)have entered into the mainstream due to their high performance,high integration,and low power consumption.When used in atmospheric environments,3D ICs are irradiated inevitably by neutrons.In this paper,a 3D die-stacked SRAM device is constructed based on a real planar SRAM device.Then,the single event upsets(SEUs)caused by neutrons with different energies are studied by the Monte Carlo method.The SEU cross-sections for each die and for the whole three-layer die-stacked SRAM device is obtained for neutrons with energy ranging from 1 MeV to 1000 MeV.The results indicate that the variation trend of the SEU cross-section for every single die and for the entire die-stacked device is consistent,but the specific values are different.The SEU cross-section is shown to be dependent on the threshold of linear energy transfer(LETth)and thickness of the sensitive volume(Tsv).The secondary particle distribution and energy deposition are analyzed,and the internal mechanism that is responsible for this difference is illustrated.Besides,the ratio and patterns of multiple bit upset(MBU)caused by neutrons with different energies are also presented.This work is helpful for the aerospace IC designers to understand the SEU mechanism of 3D ICs caused by neutrons irradiation.     

Simulation study on the boron-coated GEM neutron beam monitor

A Monte Carlo method of the 3D GEM simulation is introduced.The physical process of the neutron detection is described with the Geant4 code and the Garfield code.The results of the optimized electric-fields,the emitted ions spectrum,the electrons transverse diffusion and the signal width are presented.Moreover,the preliminary result with a CF-252 neutron source is reported.These are useful in designing detector structures and to provide an optimized option for the development of the boron-coated GEM neutron beam monitor.