Multiple scattering theory for slow neutrons (from thermal to ultracold)

The general theory of neutron scattering is presented, valid for the whole domain of slow neutrons from thermal to ultracold. Particular attention is given to multiple scattering which is the dominant process for ultracold neutrons (UCN). For thermal and cold neutrons, when the multiple scattering in the target can be neglected, the cross-section is reduced to the known value. A new expression for inelastic scattering cross-section for UCN is proposed. Dynamical processes in the target are taken into account and their influence on inelastic scattering of UCN is analyzed. Received 21 July 1999     

Polarization of neutrons from the d-t reaction and in n-p scattering

The polarization of 32 MeV neutrons from the d-t reaction has been measured using n-He scattering at an angle of 130° (lab). A He gas scintillator was used as a scatterer, and the scattered neutrons were measured by counting the He recoils which were coincident with pulses from the side detector and which were synchronized with the deuteron beam bunch. Polarizations in n-p scattering have been measured using the polarized neutrons at angles of 30°, 40° and 50° (lab).     

On polarized neutron scattering from magnetized samples and the spin echo effect

The scattering of polarized neutrons from magnetized samples is considered in the general case of arbitrary orientation between the incident polarization, the magnetic field of the sample and the polarization of the scattered neutrons. Including both nuclear and magnetic scattering the general expressions for the cross section and the polarization of the scattered beam are derived in the case of small-angle scattering. It is shown that under certain experimental conditions an effect analogous to the Neutron Spin Echo should be observed, which could be used to measure small energy transfers. The contribution arising from the scattering by spin waves is examined in detail.     

质子和中子在硅中位移损伤等效性计算

基于蒙特卡罗软件Geant4,探讨质子与硅的库仑散射和核反应及中子与硅的核反应产生反冲原子沉积非电离能量的过程,建立质子和中子在硅中的非电离能量阻止本领计算方法。在此方法中,描述了原子间库仑散射的物理过程,模拟带电粒子与晶格原子之间的屏蔽库仑散射。计算得到不同能量质子和中子在硅中因库仑散射和核反应产生反冲原子的非电离能量沉积及阻止本领的等效性,计算结果与中子ASTM标准及文献计算得到的质子数据符合很好。     

散射中子对超瞬发临界系统动态行为的影响北大核心CSCD

在点反应堆理论的基础上,建立了研究散射中子影响超瞬发临界系统动态行为的模拟计算方法,利用CFBR-Ⅱ堆的脉冲实验验证了该方法的正确性。通过分析计算表明,在超瞬发临界状态,散射中子的影响包括增加反应性、使脉冲波形后沿展宽和增加脉冲裂变产额。建立的模拟计算方法为预测脉冲堆外各种物体产生的散射中子的动力学效应提供了途径,对提高脉冲堆的核安全有应用价值。     

Larmor labeling development

The first applications of polarized neutrons were to distinguish magnetic from nuclear scattering amplitudes. Much later the first set-ups for measuring the precession phase of the polarization passing a magnetic field were realized. This phase, determined by the field strength and interaction time, could label magnetic fields, wavelength of the neutrons and length over which the field is present. The latter could be used by proper shaping of the magnetic field to label also the direction of transmitted neutrons. The advantage of this labeling is that high precision measurements are possible without strong confinement of the beam by diaphragms. An overview of the use of Larmor labeling of polarized neutrons is given for applications in magnetism, in inelastic neutron scattering and small angle scattering.     

Magnetic scattering of neutrons in magnetic superconductors

The magnetic dipolar interaction responsible for the neutron scattering due to magnetic moments in a crystal is affected by the persistent current in the superconducting state. From this effect, the small angle scattering of neutrons is strongly depressed, and the intensity maximum of the critical scattering appears at a finite scattering wave number even in ferromagnetic superconductors.     

On the possibility of high-energy neutron diffraction in a crystal in the field of laser radiation

For the first time the high-energy neutrons scattering in a crystal is considered under the influence of an external laser wave field. The process is inelastic with respect to the laser wave and at the same time it is elastic with respect to a crystal. The possibility of high-energy neutron diffraction is illustrated—wavelength of neutrons is less than the lattice period. The method is based on the multiphoton interaction of the neutron anomalous magnetic moment with the field of laser radiation by using the Farri representation. The neutron-phonon interaction is considered to be a perturbation described by the Fermi pseudopotential.     

Measurement of the total cross sections of ultracold neutrons with noble gases and search for long-range forces

The comparison of results of cross section measurements by the different methods can provide useful information on existence of long-range interaction. The total neutron cross sections of He, Ne, Ar, Kr, and Xe were measured using a method of ultracold neutrons. Measurements with ultra cold neutrons confirm the discrepancy between coherent cross-section of scattering for He measured by a neutron interferometer and scattering cross-section measured by the transmission method. The discrepancy makes up 5.3 standard deviations.     

Spin-flip probability in the inelastic scattering of 16.9 MeV neutrons from 12C

The neutron spin-flip probability S(θ) for inelastic scattering of unpolarized 16.9 MeV neutrons to the 4.44 MeV state of ¹²C has been determined by measuring the absolute directional correlation between the scattered neutrons and the subsequent E2 deexcitation γ- radiation emitted perpendicular to the scattering plane. Time-of-flight techniques with carbon recoil detection in a plastic scintillator were used to separate elastically and inelastically scattered neutrons. The neutron spin-flip data were found to be in close agreement with 20.0 MeV proton spin-flip results. Comparison of the measurement is made with the predictions of a microscopic antisymmetrized distorted wave calculation in which the direct reaction mechanism is supplemented by a two step resonance contribution. This theoretical analysis reveals the presence of a strong quadrupole resonance at 20.5 MeV excitation in ¹²C. The results are also influenced to a lesser extent by the E1 giant dipole resonance.     

Elastic neutron transfer in the scattering of Si isotopes

Angular distributions of the elastic scattering of ²⁸Si on ²⁹Si and ³⁰Si have been measured for incident beam energies at E = 65 and 70 MeV with a time-of-flight spectrometer for heavy ions. At 70 MeV the neutron transfer ³⁰Si(²⁸Si, ²⁹Si)²⁹Si was observed in addition to the elastic channel. The pronounced oscillations in the elastic scattering distributions are interpreted as being due to an elastic transfer of neutrons between the colliding nuclei during the scattering process. This assumption is in accordance with some general features of the data and allows for the extraction of spectroscopic factors of the transferred neutrons.     

Parity non-conservation effects in neutron elastic scattering reactions

Energy dependence of parity non-conservation effects is derived for neutron elastic scattering on nuclei: emission asymmetry and the rotation of the polarization plane for the polarized neutron beam and longitudinal polarization for unpolarized neutrons. Both potential scattering and scattering through the compound-nucleus resonances (multi-level approximation) are taken into account. The expressions obtained are compared with experimental data on thermal neutron scattering.     

Magnetic form factor determination by inelastic scattering of neutrons as compared to elastic scattering measures

The difference between the magnetic form factor for elastic and inelastic scattering of neutrons is derived to interpret recent neutron scattering experiment.     

Klassische Ableitung des effektiven differentiellen Streuquerschnittes für Neutronen in einem einatomigen Gas

The paper gives a simple, classical derivation on the effective differential scattering cross section concerning the scattering of neutrons in a monatomic gas.     

Theory of inelastic neutron scattering of metallic electrons in one-electron approximation

The cross section is studied for the inelastic magnetic scattering of epithermal neutrons by electrons in a metal where the states for the electrons are described by band theory. If the ratio between the wave number of the electrons and the wave number of the incoming neutrons is small compared with unity but not smaller than the mass ratio between electron and neutron, it is possible to calculate the cross section in the vicinity of certain scattering angles more closely. By such experiments one could obtain some information about the diameter of the Fermi surface and the mass tensor of the electrons.     

Symmetry properties of the lattice vibrations of cadmium diarsenide crystals

The symmetry of the normal vibrations of crystals with the CdAs₂ lattice is established. Selection rules are derived for the infrared absorption and Raman scattering of light; also derived are the selection rules for the inelastic scattering of slow neutrons.     

Temperature shift of the maximum of neutron critical scattering as a result of scattering of neutrons on spin waves in a paramagnetic phase

A new physical mechanism which causes the temperature shift of the maximum of neutron critical scattering has been proposed. It has been assumed that a dominant role is played by the scattering of neutrons on spin-wave-like excitations of magnetization fluctuations in the paramagnetic phase. The relationships between the temperature shift and a scattering vector (or observation angle) have been obtained. The theoretical results obtained are in agreement with the experimental data.     

Application of generalized matrix method to neutrons in magnetically non-collinear stratified media

The generalized matrix method [1] has been applied to neutrons in magnetically non-collinear stratified media. The smoothness of transition of the magnetization from one domain to the other has been shown to be essential for transmission of neutrons through a Bloch wall. The problem of the scattering of neutrons at a crystal with a helical magnetic structure having distortions at its surface has been solved in one-beam approximation of the dynamic diffraction theory.     

241Am-Be中子参考辐射的蒙特卡罗模拟研究

研究用于校准场所中子剂量监测仪表的241Am-Be中子参考辐射场计量特性。采用蒙特卡罗方法模拟了空气自由中子参考辐射(FRNR),GB/T 14055规定的最小尺寸中子参考辐射(SRNR)和实际中子参考辐射(ARNR)中不同检验点处中子周围剂量当量率、散射中子占比和能谱分布特征。研究结果表明,空气对FRNR中的剂量率和能谱分布影响小,近似为理想中子参考辐射;采用5%含硼聚乙烯作屏蔽的最小尺寸SRNR可减少热中子,降低散射中子占比,影锥法不适用于小尺寸中子参考辐射中对散射中子的修正;ARNR中的散射中子更少、占比更低,影锥法所得散射中子占比与理论值基本一致。     

强辐射环境下高能脉冲裂变中子探测方法

在研究通道衰减、探测方法分离和探测器中子/射线本征分辨的基础上,研究了测量高能脉冲裂变中子数目的探测技术。基于电流型Si-PIN探测器,设计了减本底的背靠背探测结构,给出了测量强射线和低能散射中子干扰信号及有效扣除强辐射本底的实现方法,最终实现了高n/n和n/分辨测量和强裂变中子、射线混合场中的高能脉冲裂变中子数目探测,探测系统的信号/辐射本底比可达到10倍以上。