Improvement of the minimum detectable energy of a recoil-proton spectrometer based on a silicon telescope

A recoil-proton spectrometer based on a monolithic silicon telescope coupled to a polyethylene converter was recently proposed and discussed in the literature. The device consists of a ΔE and an E-stage detector (about 2 μm and 500 μm in thickness, respectively) made out of a single silicon wafer and separated by a highly-doped layer acting as a common electrode.The detection system allowed continuous neutron spectra to be measured down to about 400 keV by discriminating against the contribution of low-LET radiation generated by photons from the distribution of the energy deposited in the E stage. This discrimination was carried out by selecting detected particles, event-by-event, with a ΔE ? E correlation. At neutron energies lower than 400 keV recoil-protons cannot reach the E stage owing to the thickness of the ΔE stage and therefore the discrimination failed.In order to further reduce the minimum detectable energy, an improved detection system, which also accounts for the energy deposited by recoil-protons in the ΔE stage, was studied and tested. The new set-up permits the total energy deposited in the telescope to be measured directly by collecting the charge carriers, generated in both stages, at the deep common electrode. The capability of reproducing continuous neutron spectra was also verified by irradiating the improved set-up with neutrons generated by protons striking a thick beryllium target at INFN – Laboratori Nazionali di Legnaro (Legnaro, Italy). The agreement of the unfolded spectra with literature data was satisfactory at energies higher than about 200 keV.     

液体闪烁体探测器测量皮秒激光脉冲中子源能谱

在星光Ⅲ实验装置上开展皮秒激光脉冲中子源实验,使用液体闪烁体探测器测得较好的中子信号,利用飞行时间法获得中子的能量/时间分布,通过示波器电压时间积分与阻抗之比得到不同能量段的电荷值。建立液体闪烁体探测器Geant4计算模型,通过实际打靶情况与标定情况下液体闪烁体探测器出光口收集到的可见光光子数之比,结合标定的灵敏度数据,获得液体闪烁体探测器对不同能量中子的灵敏度。计算得到源发射的中子能谱,能量在1 MeV以上的液体闪烁体探测器方向测得的中子产额为1.04108 sr-1。     

ST-401薄塑料闪烁体中子能量响应测量技术研究

利用中国原子能科学研究院核工业放射性计量测试中心的5SDH-2串列加速器进行了ST-401薄塑料闪烁体的能量响应实验.选用T(p,n)³He反应和D(d,n)³He反应作为中子源，子源，中子束流采用复合屏蔽体进行准直，源强采用正比计数管和半导体望远镜进行监测，实验测量了厚度从0.16mm到2.00mm的八种规格薄闪烁体的能量响应曲线，对实验的结果的不确定度进行了分析.结果表明探测器的灵敏度随着晶体厚度的增加而增加，对于一定厚度的薄闪烁体，随着中子能量的增加，探测器的能量响应曲线坡度不大. 关键词： 塑料闪烁体 能量响应 正比计数器 半导体反冲质子望远镜     

Neutron field mapping and dosimetry by CR-39 for radiography and other applications

A neutron telescope based on the n–p elastic scattering on H is suggested to investigate the inhomogeneity of neutron fields used for different applications like neutron radiography, biological investigations or dosimetry where the number of neutrons with high energy (above 8 MeV) is negligible. The telescope consists of a thin polyethylene foil and a track etch detector divided by an air gap and placed inside a glass tube which can be evacuated by a simple vacuum pump. The angular and energy resolution of the telescope mostly depend on the diameter of the tube, the length and pressure of the air gap, on the thickness and H content of the radiator and on the type of the track detector. The response of the telescope is characterized by several parameters: the incident angle distribution of protons, the length and the minor diameter distributions of the etched tracks. The response was calculated for several assemblies by a Monte Carlo code and based on the results, three different experimental telescopes were constructed and tested. The most suitable one was selected for routine measurements.     

Capabilities of the Gamma-400 Gamma-ray Telescope for Observation of Electrons and Positrons in the TeV Energy Range

The space-based GAMMA-400 gamma-ray telescope will measure the fluxes of gamma rays in the energy range from ∼20 MeV to several TeV and cosmic-ray electrons and positrons in the energy range from several GeV to several TeV to investigate the origin of gamma-ray sources, sources and propagation of the Galactic cosmic rays and signatures of dark matter. The instrument consists of an anticoincidence system, a converter-tracker (thickness one radiation length, 1 X₀), a time-of-flight system, an imaging calorimeter (2 X₀) with tracker, a top shower scintillator detector, an electromagnetic calorimeter from CsI(Tl) crystals (16 X₀) with four lateral scintillation detectors and a bottom shower scintillator detector. In this paper, the capability of the GAMMA-400 gamma-ray telescope for electron and positron measurements is analyzed. The bulk of cosmic rays are protons, whereas the contribution of the leptonic component to the total flux is ∼10⁻³ at high energy. The special methods for Monte Carlo simulations are proposed to distinguish electrons and positrons from proton background in the GAMMA-400 gamma-ray telescope. The contribution to the proton rejection from each detector system of the instrument is studied separately. The use of the combined information from all detectors allows us to reach a proton rejection of up to ∼1 × 10⁴.

     

用于n,γ混合场的新型脉冲中子探测器研究

新型脉冲中子探测器采用特殊工艺将两个PIN半导体组合而成.利用脉冲γ辐射研究了探测器对γ的响应；利用脉冲中子源研究了探测器对DT中了的响应，并与闪烁探测器进行了比较 .结果表明：脉冲中子探测器对脉冲γ辐射基本不灵敏，对脉冲中子辐射的灵敏程度依赖于中子辐射体，是一种用于n，γ混合脉冲辐射场中子测量的新型探测器. 关键词： 硅半导体 差分补偿 脉冲中子探测器 n γ混合场     

Low-background EN-detector for the investigation of the neutron component of EASs

Thermal neutrons are detected using a scintillator compound based on ZnS(Ag) with B₂O₃ added to it. The pulse height generated by single charged particles in a scintillator of small thickness (50 mg/cm²) is not larger than that of a noise signal due to a low sensitivity to single charged particles. As a result, the detector satisfies the requirements of a low-background one in the context of cosmic-ray experiments. The detector is to be used for neutron detection in extensive air showers.     

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.     

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.     

A dose meter for pulsed neutron fields

Many active radiation detectors exhibit severe limitations in pulsed radiation fields, which cannot be easily overcome without the design of new technology. Therefore a new area dose monitor for the measurement of pulsed and continuous neutron and gamma radiation fields was developed. The measurement of pulsed radiation is based on activation of short-lived nuclides in the detector materials or in the surroundings. The instrument comprises a thermal neutron detector in a moderator for neutron detection and a plastic scintillator for beta and gamma detection. These detectors are measuring direct radiation and also decay particles from the instable activated nuclei. The data are time-resolved acquired by a fast FADC module and the delayed decay products are identified by analyzing their timing characteristics. Measurements with pulsed and with continuous radiation are reported.     

Digital approaches to field neutron spectrometry

Miniature digitising electronics have been coupled to a detector based on an organic liquid scintillator to form a digital neutron and gamma spectrometer. The electronics used are small enough to be packaged with a detector to form an easily portable unit. This miniaturisation has some impact on the achievable sampling rate and resolution, in comparison with the bulkier devices often used for such an application, so it is not clear which of the many pulse shape discrimination algorithms reported in the literature is best suited for use with this device. To explore this, the spectrometer was exposed to a selection of the radiation fields available at NPL, and three different neutron/gamma discrimination algorithms were compared on an equal footing. The energy resolution of the device, and its ease of use, were compared with those of a conventional analogue system.     

Application of CsI(T1) Scintillator With Photodiode Readout Detector in Heavy Ion Reaction Experiments

The applications of CsI(T1) scintillator with photodiode read out in heavy ion-induced reaction experiment are reported.This kind of detector has been used as E detector in a ΔE-E telescope with silicon as ΔE detector,and has been used to measure the light charged particles.Time of crossing zero point method has been employed in the experiment with a new kind of electronics,and has been used to indentify the light charged particles.     

Twin position sensitive ionization chamber,utilizing backgammon and time projection method

The anomalous dependence of average prompt fission neutron (PFN) multiplicity on the fission fragment total kinetic energy of ²⁵²Cf(sf) was investigated using digital signal processing (DSP). A twin Frisch-grid ionization chamber (TGIC) was used for fission fragment (FF) spectroscopy and PFN was registered with help of fast neutron detector (FND) on liquid scintillator base. The DSP apparatus was build up around the 8-channel fast synchronously sampling waveform digitizers (WFD). It was found that the main limitation of the experimental method, developed in ref. [1] for investigation of the PFN properties in neutron induced fission reaction ²³⁵U(n,f), ²³⁹Pu(n,f) etc is related to the FF energy loses of in the target backing. A modification of the method was suggested to overcome such a limitation.     

Uncertainties induced by statistical fluctuations in fast monoenergetic neutron yield measurements

A plastic scintillation detector was used to measure the yield of deuterium-deuterium (DD) neutrons or deuterium-tritium (DT) neutrons. Collisions of fast neutrons with hydrogen nucleus in a scintillator generated recoil protons, the energies of which were fully deposited in the scintillator. The statistical fluctuation of the protons' number and that of the protons' total energy were two sources of measurement uncertainty. Based on DT neutrons, this paper represents the algorithms of computing the probability density functions of the two sources. Uncertainties of the measurement induced by statistical fluctuations were finally computed by constructing the probability density functions of the proton number and that of the neutron number.     

Determinations of niobium and palladium resonance cross-sections by time-of-flight transmission experiment

The neutron total cross-sections and resonance parameters of Niobium (Nb) and Palladium (Pd) were measured in the neutron energy regions from 0.1 eV to 100 eV by using the time-of-flight method at the Pohang Neutron Facility, Korea, which consists of an electron linear accelerator, a water-cooled Tantalum (Ta) target with a water moderator, and a 12.06 m long time-of-flight path. A ⁶Li-ZnS(Ag) scintillator with a diameter of 12.5 cm and a thickness of 1.6 cm was used as a neutron detector, and high purity natural Nb and Pd metallic sheets with thickness of 15 mm (0.0833 atoms/barn) and 1 mm (0.0068 atoms/barn) respectively, were used for the neutron transmission measurements. The notch filters composed of Cobalt (Co), Indium (In), and Cadmium (Cd) were used to estimate the background level and also to calibrate energy. In order to reduce the gamma-ray background from Bremsstrahlung and from neutron capture, we employed a neutron-gamma separation system based on their different pulse shapes. The resonance parameters of ^natNb and ^natPd were obtained from the transmission ratio by using the SAMMY code, which utilizes both Doppler and resolution broadening effects and Bayes’ generalized least squares technique. The present measurements were compared with the existing experimental and the evaluated reactions data files.     

Energy deposition distributions caused by fast neutrons in converter type silicon detectors

Polyethylene converter silicon detectors have been developed as part of a dosimetry system for the application in neutron fields. Signals have been investigated which are mainly caused by reactions occurring when neutrons hit the PE converter or the silicon diode directly. In this work, neutron interactions in the components of a PE converter silicon sensor have been calculated to determine energy deposition distributions for neutron energies from 1.2 v MeV to 14.8 v MeV. Experiments in quasi-monoenergetic neutron fields have been performed. For the simulation of the neutron interactions with the detector layers, the GSF neutron interaction code NISD has been applied. The transport of ions has been calculated separately by means of the program TRIM. It has been shown that by the use of these concepts and models, a rather good agreement of measured and computed pulse height distributions can be obtained and, consequently, the response of converter type detectors to neutron radiation can be determined.     

低本底液闪中子探测器的材料测试研究

为测量中国锦屏地下实验室（China JinPing underground Laboratory,CJPL）极低通量的中子本底,需要建造大型快中子液体闪烁体探测器。建造探测器需要选用低放射性本底材料,并且化学性质稳定,不与液体闪烁体发生反应而影响液体闪烁体（EJ-335）的性能。为此,建造了小型液闪探测器,在其中分别放入聚四氟乙烯、无氧铜、氟橡胶、全氟醚橡胶这4种材料进行长期浸泡,并分别进行了如下测试:液闪长时间浸泡各种材料前后液闪的吸收光谱测试;用60Co γ源检测小型液闪探测器光产额随时间的变化情况;用Am-Be中子源检验探测器在浸泡前后的n-γ甄别性能。最终实验结果表明,聚四氟乙烯和全氟醚这2种材料与液闪有很好的兼容性,可作为大型快中子液闪探测器的容器和密封圈的备选材料。In order to measure the flux and spectrum of neutron background which is extremely low in China JinPing underground Laboratory (CJPL), liquid scintillator detector with large scale used to detect fast neutron need to be fabricated. Surrounding materials used in detector require the excellent performance, such as low radioactivity as well as stable chemical properties, which do not react with liquid scintillator, so as to avoid affecting the performance of detector. Polytetrafluoroethylene(PTFE), highly purified no-oxygen copper, fluororubber and perfluoroelastomer had been put in a small detector and soaked for a long time. The change of absorption spectrum and light yield along with time as well as the performance of n-γ discrimination after soaked were tested. The results show that PTFE and Perfluoroelastomer are compatible with liquid scintillator, which offers a reference for the selection of material used in container and seal of liquid scintillator detector used to detect fast neutron.     

Neutron flux measurement using activated radioactive isotopes at the Baksan underground scintillation telescope

Preliminary results of a neutron background measurement at the Baksan underground scintillation telescope (BUST) are presented. The external planes of the BUST are fully covered with standard scintillation detectors shielding the internal planes and suppressing thus background events due to cosmogenic and local radioactivity. The shielded internal planes were used as target for the neutron flux registration. The experimental method is based on the delayed coincidences between signals from any of the BUST counters. It is assumed that the first signal is due to inelastic interaction of a neutron with the organic scintillator, while the second signal comes from the decay of an unstable radioactive isotope formed when the fast neutron interacts with the ¹²C nuclei. Using the Monte-Carlo method (GEANT4) we also simulated propagation of neutrons through a layer of scintillator. The experimentally found muon induced neutron flux is j =1.3_-0.3^+0.7 ×10^-10cm^-2s^-1 for neutron energies E ≥ 22MeV, which is in a qualitative agreement with similar measurements of other underground laboratories as well as with predictions of the GEANT4.     

An apparatus of the NBC type and the physics results obtained

A nonbubble chamber (NBC) setup, especially designed to perform a series of experiments, is described. The setup consists of a neutron missing-mass spectrometer coupled with a system of large electromagnetic shower detectors. In spite of its large dimensions (the sensitive surface and volume are 2.16 × 10⁴ cm² and 7.8 × 10⁵ cm³, respectively), the neutron spectrometer has a time resolution of ±0.70 nsec FWHM, with ∼25% detection efficiency in the range (70–390 MeV) neutron kinetic energy. The time equalization between the various components of the neutron spectrometer has been established to within ±0.1 nsec. At present this is the most powerful and the most accurate high-energy neutron detector. The electromagnetic shower detector is based on the principle of simultaneous measurements of the spatial development of the electromagnetic cascade and of its energy release. This is obtained with nine elements of lead foil — spark chamber — plastic scintillator, all sandwiched together. The sensitive surface and volume of the electromagnetic shower detector are 1.45 × 10⁴ cm² and 7.2 × 10⁵ cm³, respectively.     

4He闪烁裂变中子探测器的中子灵敏度

对4He闪烁裂变中子探测器的中子灵敏度进行了理论和实验研究。采用蒙特卡罗方法模拟了不同能量中子和不同厚度裂变靶产生的裂变碎片在4He中的能量沉积,计算结果表明:中子在4He气中的能量沉积曲线和裂变碎片的能量沉积曲线能够互补,从而使探测器对中子的能量响应变得更平坦;探测器的中子灵敏度为10-15 Ccm2量级。并对探测器的中子灵敏度进行了实验标定,实验结果与理论计算结果较为一致。