A comparison of n-γ discrimination by the rise-time and zero-crossing methods

The n-γ discrimination performance of two experimental arrangements based on the rise-time method and the zero-crossing method was compared for a 50.8 mm-diametered and 50.8 mm-high BC501A liquid scintillator coupled to a 50.8 mm-diametered 9807B photomultiplier in this work. The low energy limitation of the detected neutron with different detector high voltages and the figure of merit of the n-γ discrimination in four neutron energy regions (1–2 MeV, 0.75–1 MeV, 0.5–0.75 MeV and below 0.5 MeV) were studied by using the Am-Be neutron source. Under a time statistical model of the photoelectron emission process in scintillation counters, the intrinsic capability of the n-γ discrimination performance under the optimal condition was evaluated. The experimental results of the zero-crossing method demonstrate a better n-γ. discrimination performance than those of the rise-time method, which is consistent with the calculated results.     

Study of n-γ discrimination in low energy range(above 40 keVee) by charge comparison method with a BC501A liquid scintillation detector

A VME-based experiment system for n-y discrimination using the charge comparison method was established.A data acquisition program for controlling the programmable modules and processing data online via VME64X bus was developed through the use of Lab VIEW.The two-dimensional(2D) scatter plots of the charge in the slow component vs.the total charge from ~(241)Am-Be and ~(252)Cf neutron sources are presented.The 2D scatter plots of the energy vs.the ratio of the charge in the slow component to the total charge of the pulses are also presented.The quality of n-γ discrimination was checked by the figure-of-merit,and the results showed good performance of n-γ discrimination at the low energy range.Neutrons and γ-rays were separated above 50 keVee(electron-equivalent energy).The quality of n-γ discrimination has been improved compared with others' results at five energies(150,250,350,450,550 keVee).     

Digital pulse shape discrimination methods for n-γ separation in an EJ-301 liquid scintillation detector

A digital pulse shape discrimination system based on a programmable module NI-5772 has been established and tested with an EJ-301 liquid scintillation detector. The module was operated by running programs developed in Lab VIEW, with a sampling frequency up to 1.6 GS/s. Standard gamma sources22 Na,137Cs and60 Co were used to calibrate the EJ-301 liquid scintillation detector, and the gamma response function was obtained. Digital algorithms for the charge comparison method and zero-crossing method have been developed. The experimental results show that both digital signal processing(DSP) algorithms can discriminate neutrons from γ-rays. Moreover,the zero-crossing method shows better n-γ discrimination at 80 ke Vee and lower, whereas the charge comparison method gives better results at higher thresholds. In addition, the figure-of-merit(FOM) for detectors of two different dimensions were extracted at 9 energy thresholds, and it was found that the smaller detector presented better n-γseparation for fission neutrons.     

Discrimination of neutrons and γ-rays in liquid scintillator based on Elman neural network

In this work,a new neutron and γ(n/γ) discrimination method based on an Elman Neural Network(ENN) is proposed to improve the discrimination performance of liquid scintillator(LS) detectors.Neutron andγ data were acquired from an EJ-335 LS detector,which was exposed in a ~(241)Am-~9Be radiation field.Neutron and γ events were discriminated using two methods of artificial neural network including the ENN and a typical Back Propagation Neural Network(BPNN) as a control.The results show that the two methods have different n/γdiscrimination performances.Compared to the BPNN,the ENN provides an improved of Figure of Merit(FOM)in n/γ discrimination.The FOM increases from 0.907 ± 0.034 to 0.953 ± 0.037 by using the new method of the ENN.The proposed n/γ discrimination method based on ENN provides a new choice of pulse shape discrimination in neutron detection.     

Measurement of the neutron spectrum of a Pu-C source with a liquid scintillator

The neutron response function for a BC501A liquid scintillator （LS） has been measured using a series of monoenergetic neutrons produced by the p-T reaction. The proton energies were chosen such as to produce neutrons in the energy range of 1 to 20 MeV. The principles of the technique of unfolding a neutron energy spectrum by using the measured neutron response function and the measured Pulse Height （PH） spectrum is briefly described. The PH spectrum of neutrons from the Pu-C source, which will be used for the calibration of the reactor antineutrino detectors for the Daya Bay neutrino experiment, was measured and analyzed to get the neutron energy spectrum. Simultaneously the neutron energy spectrum of an Am-Be source was measured and compared with other measurements as a check of the result for the Pu-C source. Finally, an error analysis and a discussion of the results are given.     

Measurement of the response function and the detection effciency of an organic liquid scintillator for neutrons between 1 and 30 MeV

The light output function of a φ50.8 mm×50.8 mm BC501A scintillation detector was measured in the neutron energy region of 1 to 30 MeV by fitting the pulse height （PH） spectra for neutrons with the simulations from the NRESP code at the edge range. Using the new light output function, the neutron detection effciency was determined with two Monte-Carlo codes, NEFF and SCINFUL. The calculated effciency was corrected by comparing the simulated PH spectra with the measured ones. The determined effciency was verified at the near threshold region and normalized with a Proton-Recoil-Telescope （PRT） at the 8-14 MeV energy region.     

D-D 中子发生器n-γ 密度测井的蒙特卡罗模拟研究

根据D-D 反应中子的能谱和角分布数据,建立了中子源模型;根据石灰岩地层标准刻度井群数据,建立了井模型。采用MCNP 程序模拟了井中中子和γ 射线的输运,得到了不同地层密度和不同源距处NaI 探测器中的混合γ 射线能谱和非弹γ 射线能谱。在混合γ 射线能谱2.5~5.0 MeV 能区开窗,研究了开窗区混合γ 射线相对计数随源距的变化关系,确定源距应选择在30~80 cm 范围,给出了密度与混合γ 射线计数之间的非线性关系。研究表明,可以利用D-D中子源的混合γ 射线能谱来实现n-γ 密度测井。A D-D neutron source model was developed according to the neutron spectrum and the neutron angular distribution of D-D reaction. A standard calibration well model was built. The transports of the D-D neutrons and γ-rays in the well were simulated using MCNP code. The mixed γ-spectra and inelastic γ-spectra in the NaI detector were obtained for both different distances from a neutron source and different densities. Mixed γ count in the energy range of 2.5 to 5.0 MeV as a function of distance shows that the NaI detector should be located at a distance of 3080 cm from the neutron source. The nonlinear relationships between the density and the mixed γ count were presented in this paper.It was demonstrated that the energy spectrum of mixed γ rays can realize the n-γ density logging.     

Improvement of algorithms for digital real-time n-γ discrimination

Three algorithms(the Charge Comparison Method,n-γ Model Analysis and the Centroid Algorithm)have been revised to improve their accuracy and broaden the scope of applications to real-time digital n-γ discrimination.To evaluate the feasibility of the revised algorithms,a comparison between the improved and original versions of each is presented.To select an optimal real-time discrimination algorithm from these six algorithms(improved and original),the figure-of-merit(FOM),Peak-Threshold Ratio(PTR),Error Probability(EP) and Simulation Time(ST) for each were calculated to obtain a quantitatively comprehensive assessment of their performance.The results demonstrate that the improved algorithms have a higher accuracy,with an average improvement of 10%in FOM,95%in PTR and 25%in EP,but all the STs are increased.Finally,the Adjustable Centroid Algorithm(ACA) is selected as the optimal algorithm for real-time digital n-γ discrimination.     

Study of neutron spectra in a water bath from a Pb target irradiated by 250 MeV protons

Spallation neutrons were produced by the irradiation of Pb with 250 MeV protons. The Pb target was surrounded by water which was used to slow down the emitted neutrons. The moderated neutrons in the water bath were measured by using the resonance detectors of Au, Mn and In with a cadmium (Cd) cover. According to the measured activities of the foils, the neutron flux at different resonance energies were deduced and the epithermal neutron spectra were proposed. Corresponding results calculated with the Monte Carlo code MCNPX were compared with the experimental data to check the validity of the code. The comparison showed that the simulation could give a good prediction for the neutron spectra above 50 eV, while the finite thickness of the foils greatly effected the experimental data in low energy. It was also found that the resonance detectors themselves had great impact on the simulated energy spectra.     

Exploring nuclear symmetry energy with isospin dependence in neutron skin thickness of nuclei

We propose an alternative way to constrain the density dependence of the symmetry energy from the neutron skin thickness of nuclei which shows a linear relation to both the isospin asymmetry and the nuclear charge with a form of Z^2/3. The relation of the neutron skin thickness to the nuclear charge and isospin asymmetry is systematically studied with the data from antiprotonic atom measurement, and with the extended Thomas-Fermi approach incorporating the Skyrme energy density functional. An obviously linear relationship between the slope parameter L of the nuclear symmetry energy and the isospin asymmetry dependent parameter of the neutron skin thickness can be found, by adopting 70 Skyrme interactions in the calculations. Combining the available experimental data, the constraint of -20 MeV L 82 MeV on the slope parameter of the symmetry energy is obtained. The Skyrme interactions satisfying the constraint are selected.     

The β Decay of 21N

The β-delayed neutron and γ energy spectra taken from the decay of neutron-rich nucleus ²¹N were measured by using the β-γ and β-n coincidence detection method. Thirteen new neutron groups ranging from 0.28MeV to 4.98MeV and with a total branching ratio of 88.7±4.2% were observed andpresented. One γ transition with an energy of 1222keV emitted from the excited state of ²¹O, and four γ transitions with energies of 1674, 2397, 2780, and 3175keV emitted from the excited states of ²⁰O were identified in the β decay chain of ²¹N. The β decay half-life for ²¹N is determined to be 82.9±1.9ms. The uncertainty of half-life is much smaller than the previous result.     

Monitoring the energy variation of an electron linac using a Cerenkov detector

A new method to monitor the energy variation of a multi-energy electron linac by combining a Cerenkov detector and a CsI（Tl） detector is reported. The signals in the Cerenkov detector show an appreciable but different dependence on the energy of the electron linac from the traditional CsI（Tl） detector due to the particular response of the former to charged electrons with high velocity above threshold. The method is more convenient than the HVL （half-value layer） method which is commonly employed to calibrate the energy of an electron linac for real time monitoring. The preliminary validity of the method is verified in a dual-energy electron linac with 6 MeV and 3 MeV gears. Moreover, the method combining the Cerenkov detector and the CsI（Tl） detector is applicable to probe the X-ray spectrum hardened by the inspected material and may serve as a novel tool for material discrimination with effective atomic number in radiation imaging.     

Measurement of neutron cross sections and resonance parameters of ^169Tm below 100 eV

The neutron total cross-sections of thulium （^169Tm） were measured in the neutron energy region from 0.01 eV to 100 eV by using the time-of-flight method at the Pohang Neutron Facility, which consists of an electron linac, a water-cooled tantalum target with a water moderator, and a 12 m time of flight path, Two thulium plates with different thicknesses were used for the neutron transmission measurement. The background level was determined by using a notch-filter of Co, In, and Cd sheets. The present measurement was compared with the previous ones, and a new set of resonance parameters of ^169Tm isotope was obtained from the transmission rate by using the SAMMY code, with a comparison with the recommended parameters by Mughabghab.     

Study on gamma response function of E J301 organic liquid scintillator with GEANT4 and FLUKA

The gamma response function is required for energy calibration of EJ301 （5 cm in diameter and 20 cm in height） organic liquid scintillator detector by means of gamma sources. The GEANT4 and FLUKA Monte Carlo simulation packages were used to simulate the response function of the detector for standard 22Na, 60Co, 137Cs gamma sources. The simulated results showed a good agreement with experimental data by incorporating the energy resolution function to simulation codes. The energy resolution and the position of the maximum Compton electron energy were obtained by comparing measured light output distribution with simulated one. The energy resolution of the detector varied from 21.2% to 12.4% for electrons in the energy region from 0.341 MeV to 1.12 MeV. The accurate position of the maximum Compton electron energy was determined at the position 81% of maximum height of Compton edges distribution. In addition, the relation of the electron energy calibration and the effective neutron detection thresholds were described in detail. The present results indicated that both packages were suited for studying the gamma response function of EJ301 detector.     

2.8MeV中子诱发238U裂变的瞬发中子谱高能段实验测量

在中国原子能科学研究院的高压倍加器上利用中子飞行时间方法测量了2.8 MeV中子引起238U裂变的瞬发中子能谱,通过增大样品量和设计合适的屏蔽体提高了效应/本底比,使得测量数据的不确定度满足预期目标（在5.5～14 MeV能区内,能量间隔为0.5 MeV条件下能谱的不确定度小于10%）。将实验几何、中子源能量分布及角分布、探测效率、束流的时间结构等实验条件输入到MCNP程序里,模拟了出射的中子谱,模拟结果与测量结果在不确定度范围内一致,验证了在入射中子能量较低时238>U的裂变瞬发中子谱评价数据的可靠性。The Prompt Fission Neutron Spectrum (PFNS) of 238U induced by 2.8 MeV neutron was measured using the Cockcroft-Walton accelerator in China Institute of Atomic Energy (CIAE). The signal/background ratio was improved by increasing the amount of sample mass and using an appropriate shielding system. The final uncertainty of neutron energy spectrum in 5.5～14 MeV region is less than 10% with a bin size of 0.5 MeV which has reached this project's anticipation. The experimental geometry, the angular distribution and energy distribution of neutron source, the detection efficiency and time structure of deuteron beam were inputted into the MCNP code to simulate the outgoing neutron spectrum. The simulated results agree with the experimental ones within the uncertainty. The result indicates that the evaluated PFNS for 238U at low neutron energy is reliable.     

Multi-modal analysis for low energy neutron-induced fission of 235U

Low energy neutron induced fission of 235U is studied in the framework of the multi-modal fission model. The fission fragment properties, such as the yields, the average total kinetic energy distribution and the average neutron separation energy, are investigated for incident neutron energies from thermal to 6.0 MeV. The multi-modal fission approach is also used to evaluate the prompt fission neutron multiplicity and spectra for the neutron-induced fission of 235U with an improved version of the Los Alamos model for incident neutrons below the （n, nf） threshold. The three most dominant fission modes are taken into account. The model parameters are determined on the basis of experimental data. The calculated results are in good agreement with the experimental data.     

First neutron spectrometry measurement at the HL-2A Tokamak

A compact neutron spectrometer based on the liquid scintillator is presented for neutron energy spectrum measurements at the HL-2A Tokamak. The spectrometer was well characterized and a fast digital pulse shape discrimination software was developed using the charge comparison method. A digitizer data acquisition system with a maximum frequency of 1 MHz can work under an environment with a high count rate at HL-2A Tokamak. Specific radiation and magnetic shielding for the spectrometer were designed for the neutron spectrum measurement at the HL-2A Tokamak. For pulse height spectrum analysis, dedicated numerical simulation utilizing NUBEAM combined with GENESIS was performed to obtain the neutron energy spectrum. Subsequently, the transportation process from the plasma to the detector was evaluated with Monte Carlo calculations. The distorted neutron energy spectrum was folded with the response matrix of the liquid scintillation spectrometer, and good consistency was found between the simulated and measured pulse height spectra. This neutron spectrometer based on a digital acquisition system could be well adopted for the investigation of the auxiliary heating behavior and the fast-ion related phenomenon on different tokamak devices.     

Calculation of prompt fission neutron spectrum for 233U（n,f） reaction by the semi-empirical method

The prompt fission neutron spectra for the neutron-induced fission of 233U for low energy neutrons （below 6 MeV） are calculated using nuclear evaporation theory with a semi-empirical method, in which the partition of the total excitation energy between the fission fragments for the nth＋233U fission reactions is determined by the available experimental and evaluation data. The calculated prompt fission neutron spectra agree well with the experimental data. The proportions of high-energy neutrons of prompt fission neutron spectrum versus incident neutron energies are investigated with the theoretical spectra, and the results are consistent with the systematics. The semi-empirical method could be a useful tool for the prompt evaluation of fission neutron spectra.     

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.     

Research on the prompt neutron multiplicity of fission fragments for 235U（n,f）

According to some experimental and evaluated data,the total excitation energy partitioning way between both of the fission fragments was given with a semi-empirical method. With the calculated energy partitioning way,the prompt neutron multiplicity as a function of fragment mass,（A） ,for neutron-induced fission of 235U at En=0.0253 eV,3 MeV,and 5 MeV was calculated. The results are checked with the total average prompt neutron multiplicities  and compared with the experimental and evaluated data.