基于Micromegas探测技术的中子CT可行性研究(英文)

在培育新品种过程中能够实时掌握了解土壤中植物根系的形态以及其生长情况将为植物学家提供很多不可或缺的信息。对基于 micromegas 探测技术的中子 CT 在这一新应用的试验环境进行了仿真模拟, 通过三维图像重建后,得到了令人满意的预期结果。首先通过 Geant4 和 Garfield 模拟计算利用聚乙烯薄膜作为中子转换层的 micromegas 中子探测器, 得到了非常理想的位置分辨, 说明基于micromegas 探测技术建立中子 CT 照相系统的可行性。 然后利用MCNP仿真模拟14 MeV 中子CT 的实验环境, 最后由Matlab程序进行图像重建。In this paper, the possibility using micromegas(Micro Mesh Gaseous Structure) as neutron detector in 14 MeV neutron computed tomography(CT) has been simulated. The results show that the micromegas neutron detector has high spatial resolution and is a good candidate for neutron radiography. The three dimensional images of plant roots in soil are successfully and clearly obtained by the 14 MeV neutron CT with micromegas as a neutron detector. In the present simulation, MCNP is employed for 14 MeV neutron transport in the sample and Matlab for the 3 D photograph reconstruction.     

Neutron separation energy for heavy and superheavy nuclei

The neutron separation energy is calculated within a macroscopic-microscopic approach for nuclei with proton and neutron numbers, Z=82?128 and N=126?190, respectively. The results are compared with existing experimental and other theoretical data.     

Neutron diffraction experiments in diamond and sapphire anvil cells

Diamond anvil cells (DAC) provide the highest static pressures ≥1?Mbar. Because of the low intensity of neutron sources, for a long time it was thought impossible to use DAC or other anvil cells in neutron experiments. We describe pressure cells with diamond and sapphire anvils and neutron instrumentation allowing neutron diffraction experiments to be carried out under pressures as high as 50?GPa, temperatures down to 0.1?K, and applied magnetic fields up to 7.5?T.     

Neutron scattering from 208Pb

Elastic scattering of 7, 9, 11, 20 and 26 MeV neutrons from ²⁰⁸Pb has been measured with the Ohio University Tandem Van de Graaff accelerator. Standard pulsed beam time-of-flight techniques were employed. Measurements of the incident flux at 0° were used to normalize the differential cross sections. The measured cross sections were corrected for dead time, detector efficiency, flux attenuation, multiple scattering, finite geometry, neutron source anisotropy and compound elastic contribution. Relative uncertainties are estimated to be between 5%–10% and the uncertainty in the normalization is estimated to be less than 5 %. The data were used to obtain neutron optical potential parameters. A comparison with proton optical parameters is presented, and the (p, n) quasi-elastic cross section is calculated and compared with available data. Deformation parameters for the 3^? state (Q = ?2.615 MeV) and 5ā (Q = ?3.198 MeV) in ²⁰⁸Pb were obtained at incident energies of 11 and 26 MeV.     

基于相对论平均场有效对力的中子物质BCS-BEC渡越

基于核物质的相对论Hartree-Bogoliubov（RHB）理论,采用相对论平均场单玻色子交换形式的有效对相互作用,研究了不同对力强度下双中子关联及其中的BCS-BEC渡越现象。通过引入有效衰减因子χ调节对力强度,定量分析费米面处中子对能隙和单粒子动能比值△_Fn/e_Fn及无量纲参数1/（k_Fna）随费米动量k_Fn的演化行为,发现双中子对在χ=0.51时进入BCS-BEC渡越区域,在χ=0.67时可达到幺正极限。进而分析了中子对能隙、中子对波函数及相干长度等物理量在相应临界点处的特征,并给出渡越时双中子短程关联概率的定量判据。发现当配对中子处于平均中子间距以内的概率P（d_n）?0.80时,双中子对从纯BCS耦合过渡到BCS-BEC渡越区域。Based on the relativistic Hartree-Bogoliubov theory in nuclear matter, the dineutron correlations and the crossover from Bardeen-Cooper-Schrieffer (BCS) region of neutron Cooper pairs to Bose-Einstein condensation (BEC) are investigated with the one-boson-exchange type of pairing force generated from the relativistic mean field (RMF) model. By introducing an effective factor χ in the RMF effective pairing interaction, the density dependence of the ratios between neutron pairing gap at Fermi surface and neutron Fermi kinetic energy △_Fn/e_Fn and the dimensionless parameter 1/(k_Fna) are analyzed quantitatively. Then the criteria where dineutron correlations exactly reach the threshold of BCS-BEC crossover or unitary limit are determined to be χ=0.51 or 0.67, respectively. In addition, features of neutron pairing gap, Cooper pair wave function and dineutron coherence length are illustrated, and the value of the probability for partner neutrons correlated within the average inter-neutron distance, namely P (d_n) ?0.80, is obtained as a criterion of BCS-BEC crossover.     

Defect characterization in CeO2−x at elevated temperatures—II: Neutron diffraction

Polycrystalline CeO_2?x(0? x? 0.21) has been examined at 900°C by means of neutron diffraction to characterize the predominant atomic point defects responsible for nonstoichiometry. The stoichiometry was controlled by adjusting the oxygen partial pressure between 1 and 10^?21 atm. For x ? 0.11, good agreement (residual ? 1.0%) is obtained by assuming a random distribution of defects on the oxygen sublattice. Vacancy concentrations obtained from the neutron-data analysis are in agreement with thermogravimetric determinations. Over this region of x, the temperature factors of both the cation and anion increase with an increase in vacancy concentration, which implies greater root-mean-square (r.m.s.) displacement of the atoms. However, for x > 0.11, the analysis of the neutron data indicates a smaller anion vibrational amplitude than in the near-stoichiometric compositions and is accompanied by a shift in the equilibrium position of the oxygen atoms. The effect of anharmonic thermal motion has also been considered, but alone, cannot explain the neutron data. The results of the present study correlate well with the previous X-ray experiment.     

用MCNPx程序计算宽能谱中子雷姆仪的响应曲线

利用MCNPx程序计算了宽能谱中子雷姆仪的响应曲线。计算表明,增加铅层对低能中子的响应没有明显的影响,但在高能区(几百MeV以上)宽能谱中子雷姆仪的响应与铅层的厚度有关。铅层厚度为0．6cm时响应比普通雷姆仪提高约3倍,当铅层厚度增加到1．2cm时响应高约5倍。虽然计算结果与ICRP建议书中的H^*(10)曲线相比还有一定的差别,但改变慢化体的结构对提高高能中子的探测效率是有明显效果的。The responses of two extended neutron rein counters as function of neutron energy was ca1cuated Monte Carlo code,MCNPx,was applied in the calculations．Isotropic neutron incidence was employed．The results show that the neutron response is independent on thickness of lead 1aver at low energies,and it is clearly increased with thickness of lead layer at high energies．Although tt1e shape of the response curve does not completely agree with the H (10) curve in ICRP 74 report ,the resuhs obtained give good bases for the practical use of the new instrument in high—energy neutron fields     

Neutron density distributions deduced from antiprotonic atoms

The differences between neutron and proton density distributions at large nuclear radii in stable nuclei were determined. Two experimental methods were applied: nuclear spectroscopy analysis of the antiproton annihilation residues one mass unit lighter than the target mass and the measurements of strong-interaction effects on antiprotonic x rays. Assuming the validity of two-parameter Fermi neutron and proton distributions at these large radii, the conclusions are that the two experiments are consistent with each other and that for neutron rich nuclei it is mostly the neutron diffuseness which increases and not the half-density radius. The obtained neutron and proton rms radii differences are in agreement with previous results.     

Neutron Matter Properties Using Skyrme Interaction

The purpose of the present work is to extend earlier nuclear matter calculations to study the properties of neutron matter. The binding energy per particle, symmetry energy, single particle potential, effective mass, and magnetic susceptibility are calculated using a modified Skyrme interaction. These are calculated as a function of the Fermi momentum k_f in the range 0 < k_f < 2 fm^?1. Two sets of the interaction parameters are obtained by fitting the interaction parameters using the available information on neutron matter. Relativistic corrections to the order 1/c² are also calculated. The relativistic corrections are very small and they increase as k_f is increased.     

Neutron spectroscopy of molecular nanomagnets

This short overview gives an account on the use of neutron spectroscopy for the examination of molecular nanomagnets, systems constructed by crystalline arrangement of finite size clusters (usually with regular form) of interacting moment carrying atoms ?C magnetic molecules. Opposed to extended magnetic systems with bands of collective excitations such as spin-waves the molecular nanomagnets are entities with local properties, each magnetic molecule possessing a finite number of energy levels that can be solved exactly for small enough systems. In essence, the number of states remains finite despite growing rapidly with increasing number of magnetic centers and the value of the spin quantum number. Increasingly large numbers of states and more complex exchange networks lead to the need for approximative treatments, the validity of which can be checked with neutron spectroscopy. Molecular nanomagnets provide interesting examples of physics and magnetochemistry, illustrated here with a few examples that highlight the power of neutron spectroscopy for precise investigation of the energy level structure and spatial configuration of the magnetic exchange parameters.     

核物质的对称能与中子皮厚度(英文)

在现有的平均场模型中引入同位旋相关的高阶修正项．研究了核物质对称能的密度依赖性和。^208Pb的中子皮厚度。采用新提出的PK1相互作用以及NL3．S271和Z271相互作用．得到核物质饱和点对称能的范围为29-38MeV以及相应的^208Pb中子皮厚度为0．17—0.28fm．在所有相互作用中，核物质饱和点的对称能与^208Pb的中子皮厚度近似呈线性关系。After adding isospin dependent high order correction terms to existing relativistic mean field models (RMF), the density dependence of symmetry energy and the neutron skin thickness S for 208Pb are studied. Using the new effective interaction PK1, together with NL3, S271 and Z271, a range of 29—38 MeV for the symmetry energy for nuclear matter at saturation point and the corresponding neutron skin thickness S = 0.14—0.28 fm for 208Pb are obtained. For all effective interactions, a linear relation between the symmetry energy at saturation point and the neutron skin thickness for 208Pb is observed.     

Neutron Star Matter Including Delta Isobars

In this paper a new phase structure of neutron star matter including nucleons and delta isobars is presented. Particle fractions populated and pion condensations in neutron star matter are investigated in this model. The existence of the pion condensations can postpone the appearance of delta isobars. We found that both the pion condensation and reduce of the ratio of delta isobar to nucleons couplings can soften the corresponding equation of state. The maximum masses and their corresponding radii of neutron stars are calculated, and the obtained values are in observational region.     

Neutron Star Matter Including Delta Isobars

In this paper a new phase structure of neutron star matter including nucleons and delta isobars is presented.Particle fractions populated and pion condensations in neutron star matter are investigated in this model. The existence of the pion condensations can postpone the appearance of delta isobars. We found that both the pion condensation and reduce of the ratio of delta isobar to nucleons couplings can soften the corresponding equation of state. The maximum masses and their corresponding radii of neutron stars are calculated, and the obtained values are in observational region.     

厚靶D（d,n）^3He反应加速器中子源的产额、能谱和角分布

给出一种计算氘钛厚靶D(d,n)3He反应加速器中子源的产额、能谱和角分布的方法,并发展了一个计算机模拟程序,程序能够计算氘束流能量小于1.0 MeV的中子源的产额、能谱和角分布.计算时使用推荐的D(d,n)3He反应截面数据和来自SRIM-2003程序的氘在氘钛靶中的阻止本领数据.给出一些典型计算结果,包括中子积分产额、中子能谱和角分布.     

同质异能靶核的中子核反应计算

基于光学模型和带宽度涨落修正的Hauser-Feshbach理论编写了程序OMHFISO, 该程序适用于计算靶核处于同质异能态并且入射粒子能量小于5MeV的核反应, 入射粒子和出射粒子均可以是n,p,d,t,³He和⁴He 6种.迄今还很少有人计算过同质异能靶核的全套核数据.利用程序UNF, OMHFISO以及DWUCK4计算了不稳定同位素⁹⁰Rb和^{91,93,96,97,98}Y的同质异能靶核在入射粒子能量低于20.0MeV的全套中子反应数据, 并且得到了合理的结果.     

R-mode Instability of Neutron Star with Non-Newtonian Gravity

The Chandrasekhar-Friedmann-Schutz(CFS) instabilities of r-modes for canonical neutron stars(1.4 M ⊙) with rigid crusts are investigated by using an equation of state of asymmetric nuclear matter with super-soft symmetry energy,where the non-Newtonian gravity proposed in the grand unification theories is also considered.Constrained by the observations of the masses and the spin frequencies for neutron stars,the boundary of the r-mode instability window for a canonical neutron star is obtained,and the results show that the observed neutron stars are all outside the instability window,which is consistent with the theoretical expectation.In addition,an upper limit of the non-Newtonian gravity parameters is also given.     

Neutron reflectometry with vector polarization analysis: First steps

The most detailed and reliable information about the magnetic state (magnetization depth profiles) of layers can be obtained by neutron reflectometry with vector polarization analysis. Two schemes of realization of this technique are considered. Precession coils designed to manipulate the polarization vector of monochromatic beams are used in scheme I. This scheme was tested at the neutron reflectometer NR-4M (PNPI, Gatchina). The earliest experimental data on the polarization vector rotation are reported, giving direct evidence of the wave function phase shift of a massive particle, the neutron, under total reflection. The basic elements for scheme II are remanent supermirrors. This scheme is designed for use with a white beam and is advantageous for pulse neutron sources. The effect of stray fields produced by remanent supermirrors on the neutron polarization has been theoretically and experimentally evaluated; efficient ways of compensating the stray fields are proposed.     

Neutron fluence spectrometry using disk activation

A simple and robust detector for spectrometry of environmental neutrons has been developed. The technique is based on neutron activation of a series of different metal disks followed by low-level gamma-ray spectrometry of the activated disks and subsequent neutron spectrum unfolding. The technique is similar to foil activation but here the applied neutron fluence rates are much lower than usually in the case of foil activation. The detector has been tested in quasi mono-energetic neutron fields with fluence rates in the order of 1000–10000 cm⁻² s⁻¹, where the obtained spectra showed good agreement with spectra measured using a Bonner sphere spectrometer. The detector has also been tested using an AmBe source and at a neutron fluence rate of about 40 cm⁻² s⁻¹, again, a good agreement with the assumed spectrum was achieved.     

Neutron elastic scattering from 116,118,120,122,124Sn

Accurate measurements of neutron differential elastic cross sections have been obtained from even isotopes of Sn. Data are presented for the elastic scattering of 11 MeV neutrons from ^{116, 118, 120, 122, 124}Sn, the elastic scattering for 24 MeV neutrons from ^{116, 118, 124}Sn and the neutron total cross section from ^{118, 120, 122, 124}Sn in the energy ranges 5.0–10.6 MeV and 20.0–26.0 MeV. The elastic scattering data are analyzed in terms of an empirical optical-model potential. The obtained optical-potential parameters are analyzed in terms of energy and isospin dependence and compared with those obtained from proton elastic scattering on even Sn isotopes.