光中子源上吸收片功能研究

15 MeV电子直线加速器驱动的光中子源装置,将用于中国科学院战略性先导科技专项“钍基熔盐堆”中的核数据初步测量工作、中子探测器的研制和反应堆相关材料的辐照研究等。光中子源的中子能谱是连续的,中子能量通过中子飞行时间法测量得到,需要利用吸收片确认中子吸收峰,刻度飞行时间,计算等效飞行距离,扣除实验本底等,而实验本底的扣除对最终总截面计算有很大的影响。因此通过Geant4蒙特卡罗模拟软件构建了包括中子源、吸收片在内的模拟实验环境;研究了不同吸收片的吸收谱和吸收片厚度的关系,同理论计算值进行了比较,给出了推荐的吸收片厚度值;模拟计算了中子飞行时间谱,并和实验测量结果比较,确定中子等效飞行距离为5.70 m。Geant4的理论计算也可以模拟出多吸收片本底函数曲线,可用于实验数据的本底扣除和误差分析。实验测量、模拟分析以及理论公式计算的吸收片厚度和中子飞行时间参数得到了完全一致的结果,验证了实验测量的可靠性。A photo-neutron source driven by a 15 MeV electron LINAC is built for the "Strategic Priority Research Program" of the Chinese Academy of Sciences-the "thorium-based molten salt reactor" project to conduct the nuclear data measurement work,develop neutron detector and carry out reactor material irradiation studies.Since the neutron energy spectrum is continuous,the neutron energy is measured by the time of flight (TOF) method,and neutron filters are needed to confirm absorption peaks,calibrate the TOF,calculate the equivalent flight distance,and remove the experimental background which has great influence on the calculation accuracy of the total cross section.Based on the Monte Carlo simulation tool,Geant4 a simulation environment is set up,including neutron source and neutron filters,to study the energy absorption spectra and thickness of different filters and recommended data for the thickness are provided.The neutron TOF spectra are simulated and compared with experimental measurement,deciding the equivalent TOF distance to be 5.7 m.Geant4 can also simulate the background curve of multiple filters and be used to remove background and analyze errors for the experimental data.All the experiments,simulation and theoretical calculation show consistent results on filter thickness and neutron TOF parameters,indicating the accuracy of the measurement.     

利用252 Cf快裂变室测量BC501A液闪探测器的相对探测效率和响应函数

本文利用²⁵²Cf快裂变室和多参数数据采集系统,逐事例的同时记录了自发裂变中子和瞬发伽马的飞行时间(TOF),脉冲形状甄别(PSD)和反冲能量(RE,裂变中子是通过测量反冲质子;瞬发伽马是通过测量康普顿反冲电子)三维信息.详细介绍了通过离线数据分析完全扣除三维信息中的伽马事例贡献,以获得Φ50.8 mm×50.8 mm的BC501A液闪探测器的相对探测效率和响应函数的方法.在不通过探测器响应函数进行数据转换的条件下,利用中子的能量直接确定了中子的有效测量阈值.得到的中子 关键词： 252Cf快裂变室')" href="#">²⁵²Cf快裂变室 BC501A液闪探测器 相对探测效率 响应函数     

Preliminary investigation of heterogeneity in the microzone of composites by laser ultrasonic probe measurements

We carry out a preliminary investigation of heterogeneity in the microzone of composites by using a laser ultrasonic probe to measure the time-of- flight (TOF) of the laser-excited longitudinal (L-) and transverse (S-) waves that propagate within the samples. The influence of the number of fibers on the inhomogeneity of the TOF is observed. The results show that the heterogeneity of the TOF in a small region within a composite can be observed with a resolution of 2 ns in time and 50 m in space. The S-wave is better than the L-wave for examining the heterogeneity. The change in the TOF for the sample with two fibers is smaller than that for the sample with one fiber. PACS 62.65.+k; 43.35.+d; 42.62.-b; 81.70.Cv; 81.05.-t     

Monte Carlo method for neutron spectrum recovery - a new approach based on the accelerated ions distribution

New method of neutron spectrum recovery described in the paper involves accelerated deuterons (that produce neutrons in DD reaction) and allows getting neutron spectrum in any direction from computed time-velocity characteristics of deuterons. Time of flight signals registered in various distances and directions are used, that makes information involved in the recovery process (Monte-Carlo simulation) more complete than in a one-directional case, although additional assumption about axial symmetry of deuterons motion, is required. In the paper recent results of two standard tests of the proposed method are presented demonstrating its capability to recover neutron spectrum from time-of-flight signals.     

Structures of rotating traditional neutron stars and hyperon stars in the relativistic $\sigma-\omega$ model

The influence of rotation on the total masses and radii of neutron stars is calculated by Hartles slow-rotation formalism, while the equation of state is considered in a relativistic - model. As the changes of the mass and radius of a real neutron star caused by rotation are very small in comparison with the total mass and radius, one can see that Hartles approximate method is rational to deal with the rotating neutron stars. If three property values, mass, radius and period, are observed for the same neutron star, then the EOS of this neutron star could be decided entirely.Received: 7 November 2003, Revised: 17 February 2004, Published online: 31 August 2004PACS: 04.40.Dg Relativistic stars: structure, stability, and oscillations - 95.30.Sf Relativity and gravitation - 97.10.Kc Stellar rotation - 97.60.Jd Neutron stars     

In-situ SEOP polarizer and initial tests on a high flux neutron beam

Polarized ³He has shown its unique characteristics in many areas of polarized neutron scattering, its ability to polarize neutrons at short wavelengths, accept wide-angle and divergent beams and low backgrounds enable new classes of experiments. While polarized ³He is not a steady state solution as commonly applied, the benefits have been shown to offset the drawbacks of polarizing and refreshing the polarization in the neutron spin filter cells. As an extension of this work, in-situ polarization using the spin-exchange optical pumping (SEOP) method was explored as a means to construct a system which could be used to polarize ³He in the state used for an effective neutron spin filter to constant polarization while on the neutron beam. An in-situ SEOP polarizer was constructed. This device utilized many devices and principles developed for neutron spin filters which are polarized off the beam line using either SEOP or metastability exchange optical pumping (MEOP) under the same research program. As a collimation of this work effects of extremely high neutron capture flux density incident on the in-situ polarizer were explored.     

Time-of-flight distributions of HD molecules abstracted at a Si(1 0 0) surface

We study the dynamics of D abstraction by 0.05 eV H atoms on a Si(1 0 0) surface. Time-of-flight (TOF) distributions of the abstracted HD molecules are measured using a quasi-random chopper/cross-correlation method. The measured TOF distribution is found to be broad and fast. The distribution is decomposed into two components being related to direct abstraction (ABS) and adsorption-induced-desorption (AID), which were revealed in the kinetics studies. The best curve fits yield mean kinetic energies of 1.15 ± 0.20 eV and 0.33 ± 0.05 eV for the ABS and AID components, respectively. Dynamics and kinetics of hydrogen abstraction at Si(1 0 0) surfaces are consistently understood.     

Band structure effects for dripped neutrons in neutron star crust

The outer layers of a neutron star are supposed to be formed of a solid Coulomb lattice of neutron rich nuclei. At densities above neutron drip density (about one thousandth of nuclear saturation density), this lattice is immersed in a neutron fluid. Bragg scattering of those dripped neutrons by the nuclei which has been usually neglected is investigated, within a simple mean field model with Bloch type boundary conditions. The main purpose of this work is to provide some estimates for the entrainment coefficients, as required for hydrodynamical two fluid simulations of neutron star crust [nucl-th/0402057, astro-ph/0408083], which relate the momentum of one fluid to the particle currents of the other two fluids [Sov. Phys. JETP 42 (1976) 164]. The implications for the equilibrium neutron star crust structure are also briefly discussed.     

Prompt fission neutron spectra of <Superscript>238 </Superscript>U(n,f) above emissive fission threshold

Model calculations were performed to interpret prompt fission neutron spectra (PFNS) of the ²³⁸ U(n, f) reaction for incident neutron energies -18 MeV. Pre-fission (pre-saddle) reaction neutron spectra were calculated with Hauser-Feshbach statistical model, ²³⁸ U fission and reaction cross-section data being described consistently. The increase of the cut-off energy of (n, nf) reaction neutron spectra with excitation energy of fissioning nucleus is described. For -9 MeV the low-energy PFNS component, which is due to the contribution of pre-fission (n, nf) neutrons, is compatible with measured data. Average energy of prefission (n, nf) neutrons is shown to be rather dependent on . For -18 MeV, a decrease of measured PFNS average neutron energies is interpreted. Spectra of neutrons, evaporated from fission fragments, were approximated as a sum of two Watt distributions. The reduced fission fragment velocity is assumed for the neutron emission during fragment acceleration. Several interpretations of observed soft neutron excess are investigated, i.e., possible uncertainties of emissive fission contributions and additional neutron source. We claim the soft neutron excess cannot be attributed to the pre-saddle neutrons contribution.Received: 3 February 2003, Revised: 24 April 2003, Published online: 9 October 2003PACS: 25.85.Ec Neutron-induced fission     

High-Resolution Neutron Diffractometer Based on the IN-06 Pulsed Neutron Source at the Institute for Nuclear Research,Russian Academy of Sciences

A high-resolution neutron diffractometer has been fabricated on the basis of an IN-06 pulsed neutron source at the Institute for Nuclear Research, Russian Academy of Sciences. The diffractometer incorporates two blocks of detectors with helium neutron counters and time-of-flight (TOF) focusing at scattering angles of 156°–165°. A block of new-type high-efficiency scintillation detectors of thermal neutrons based on a ZnS(Ag)/LiF scintillator and silicon photomultipliers with TOF focusing is developed and tested. Test measurements are performed, and the diffractometer resolution is estimated. The diffraction pattern of nonmagnetic NiCrAl alloy is measured and used to determine the phase composition by means of the Rietveld method. It is demonstrated that the given setup can be applied to phase analysis.     

Spatial modulation of a neutron beam by Larmor precession

We modulated the intensity of a neutron beam using Larmor precession techniques. We simulated triangular coils by using magnetised foils in electromagnets with rectangular pole shoes. Reversing the orientation of two of the magnetised foils in the spin-echo small-angle neutron scattering (SESANS)-instrument in Delft and changing the field settings was sufficient to obtain a modulation with a period in the order of 1 mm. We expect to be able to go to a modulation with a period of .This technique can be used to measure small-angle neutron scattering, analogous to the method used in SESANS, but with the advantage that all the manipulation of the neutron spin occurs before the sample. This means that the technique is insensitive to magnetic perturbations at the sample position. By positioning several of these devices after each other it will be possible to obtain a sharper modulation, or a modulation in two directions.     

Neutron polarization for P- and T-violation experiments

The polarization of neutron spin, target nucleus spin and He nuclear spin was developed at KEK for measuring the T-odd term in polarized neutron transmission through a polarized nuclear target. A method to measure the neutron spin rotation was developed for the T-violation experiment. This apparatus has been found to be quite useful for the P-violating neutron spin rotation experiment. The angular distribution of neutron-capture -rays was measured for the study of the enhancement mechanism of the P-violation. The results are also discussed.On leave from Tohoku University.     

CdZnTe <Emphasis Type="BoldItalic">γ</Emphasis> detector for deep inelastic neutron scattering on the VESUVIO spectrometer

In this paper it is shown that solid-state cadmium-zinc-telluride (CZT) is a promising photon detector for neutron spectroscopy in a wide energy interval, ranging from thermal (25 meV) to epithermal (70 eV) neutron energies. In the present study two CZT detectors were tested as part of the inverse-geometry neutron spectrometer VESUVIO operating at the ISIS pulsed neutron source. The response of the CZT detector to photon emission from radiative neutron capture in ²³⁸U was determined by biparametric measurements of neutron time of flight and photon energy. The scattering response function F(y) from a Pb sample has been derived using both CZT and conventional ⁶Li-glass scintillator detectors. The former showed both an improved signal to background ratio and higher efficiency as compared to ⁶Li glass, allowing us to measure F(y) up to the fourth ²³⁸U absorption energy (E_r=66.02 eV). Due to the small size of CZT detectors, their use is envisaged in arrays, with high spatial resolution, for neutron-scattering studies at high energy (>1 eV) and low wavevector (q <10 Å^-1) transfers. PACS 29.30.Hs; 29.30.Kv; 29.40.Kw; 61.12.Ek     

Neutron polarization analysis on the multiferroic TbMn2O5

Polycrystalline TbMn₂O₅ was prepared by the standard solid-state reaction method and characterized by powder X-ray diffraction and magnetization to assure it is of single phase. Heat capacity measurements on the compound reveal an antiferromagnetic phase transition at 45 K. A broad peak below 6 K in the heat capacity measurements corresponds to the crossover transition of Tb³⁺ ordering. To confirm these magnetic orderings, neutron powder diffractions on TbMn₂O₅ with XYZ neutron polarization analysis were performed at the diffuse neutron scattering (DNS) spectrometer, FRJ-II, by using neutron wavelength of 4.8 Å in the temperature range of 1.8–250 K. Magnetic scattering was separated from nuclear coherent and spin incoherent scattering contributions. Long-range ordered magnetic peaks were observed below 39 K which is consistent with the heat capacity results. The drastic increasing intensities below 6 K indicate the ferromagnetic transition in Tb³⁺ orderings.     

基于中子飞行时间法的ICF内爆热斑离子温度诊断技术北大核心CSCD

在惯性约束聚变研究中,内爆热斑离子温度反映了热斑能量的高低,对内爆对称性和内爆速度等物理量十分敏感,是理解内爆物理过程不可或缺的重要参数。介绍了一种基于中子飞行时间法的ICF内爆热斑离子温度诊断技术。建立了一种采用塑料闪烁探测器作为中子测量器件的快时间响应中子飞行时间谱仪。谱仪输出时间波形的半高全宽小于1.1ns,上升时间约为0.5ns。描述了基于反卷积运算和低通滤波的飞行时间谱解谱方法。在神光Ⅲ原型装置较低的中子产额和离子温度条件下通过这种诊断技术成功获得了内爆热斑离子温度。     

Experimental study of the neutron emission mechanisms in a Filippov-type plasma focus device

The results of an experimental investigation of neutron emission characteristics in the Filippov-type plasma focus facility Dena (90 kJ, 25 kV, 288 µF) with D₂ + %1 Kr as working gas are presented. From the experimental results, one can conclude that both thermonuclear and nonthermonuclear mechanisms are always present in neutron production, but their contributions to the total neutron yield are strongly dependent on the initial pressure and discharge voltage. It has been found that at constant discharge voltages and low pressures the beam—target interaction mechanism plays an important role in the neutron production and with increasing the pressure, its role decreases and the importance of thermonuclear mechanism increases. Also at constant pressures, the contribution of beam—target interaction mechanism in neutron production decreases with increasing the discharge energy. The value of the index in the empirical neutron yield scaling law Y_n I E^/2 was found to be about 3.82.     

Measurement of 248Cm/252Cf Cf Spontaneous Prompt Fission Neutron Spectrum

In order to treat the waste material of nuclear power and develop new type of clean nuclear power,it is necessary to measure the neutron adta of long half life nuclei existed in the waste material.The prompt spontaneous neutron spectrum is one of the most important unclear data for new type nuclear power facilities as well as for understanding the mechanism of fission neutron emission.The measurements of ²⁴⁸Cm/²⁵²Cf spontaneous prompt fission neutron spectrum in the neutron energy range form 200keV to 12MeV wer performed by using TOF method.A micro-ionization chamber aws used as fission fragment detector and stibene crystal as neutron detector.The flying paths of neutrons for the measurements were 30cm,50cm and 100cm respectively.The spontaneous prompt fission neutron spectrum of ²⁴⁸Cm was fitted by the Maxwellian distribution and the temperature was determined as (1.401±0.006)MeV in the corresponding neutron energy range.     

Huang diffuse scattering of neutrons

Huang diffuse neutron scattering was measured for the first time on niobium with interstitially dissolved deuterium as well as on MgO after neutron irradiation and Li⁷F after-irradiation. With Huang diffuse scattering the strength and symmetry of the distortion field around lattice defects can be determined. Our results clearly demonstrate that this method is feasible with neutrons.The present results are compared with X-ray experiments and the advantages of using neutrons is discussed in some detail.     

Measurements of quadrupole interaction frequencies of long-lived isomers with the level mixing spectroscopy (LEMS) method

The level mixing spectroscopy (LEMS) method has proven to be a very useful method to determine the quadrupole interaction frequency of an isomer in a solid host. Especially in the difficult cases, e.g. when the isomeric lifetime is very long or its spin is very high, the method yields valuable information which is not accessible with other methods (such as TDPAD). Since the development of the method some years ago, many experiments have been performed on high spin isomers in the lead region. The static quadrupole moment of isomers with lifetimes ranging from 20 ns up to 13 ms andspins up to 65/2 have been determined in neutron deficient isotopes of Bi, At, Fr andRa.