Concept of Small-Angle Diffractometer in Classical Configuration at the Cold Moderator of the IBR-2 Reactor

The possible concepts whereby a time-of-flight small-angle diffractometer optimized for a neutron moderator operating in the cold (at 30 K) and thermal (at 300 K) modes can be implemented at the IBR-2 reactor are studied on the basis of numerical calculations. Under cold conditions, the peak of the neutronbeam energy spectrum is shifted toward low energies (long wavelengths). This extends the sensitivity range of the instrument with respect to the sizes of the objects under study (1–100 nm and higher). A classical scheme enabling the separation of thermal/cold neutrons (E ~10^–3–10^–2 eV) from the background (formed mainly by fast neutrons), which is based on bent neutron-optical devices, is discussed. Due to restrictions imposed by the geometry of the beamline within which the instrument is planned to be located, a configuration with a short multichannel mirror device for beam bending (beam bender) is preferable. Simulation and optimization of the proposed small-angle instrument is carried out taking into account the real beamline geometry and the available space in the experimental reactor hall. A comparison of the setup has been made with the facility based on the curved neutron guide and the facility with direct view of the moderator.     

Cold neutron moderator on an upgraded IBR-2 reactor: The first set of results

The first criticality of a new KZ-202 neutron moderator on the IBR-2M reactor is achieved. The moderator consists of thermal and cold units. The former is a room-temperature comb water moderator; the latter, a moderator using a mixture of aromatic hydrocarbons (mesitylene and m-xylene). The cold moderator is filled with granules of this mixture, which are supplied by a cold helium flow, and operates at 30 K. The combination of two units in one moderator makes it possible to simultaneously take the thermal and cold neutron spectra for extracted-beam spectrometers. The arrangement of the thermal and cold moderators is numerically optimized by the Monte Carlo method. The use of the cold moderator allows a 13-fold increase in the cold neutron intensity from its surface.     

Evaluation of the void fraction in the crescent-shape moderator cell of the CARR-CNS

The mathematical model developed in a previous paper is improved in the present paper for analyzing and evaluating the void fraction profiles in the crescent-shape moderator cell of the Cold Neutron Source (CNS) of the China Advanced Research Reactor (CARR) which is now under construction in the China Institute of Atomic Energy (CIAE). The model is then applied to the case of the CARR-CNS with liquid hydrogen as a moderator and the void fraction in the crescent-shape moderator cell of the CARR-CNS is evaluated. The calculation results show that the void fraction in the crescent-shape moderator cell less than 20%. The model and the calculation results will help to obtain insight of the mechanism that controls the void fraction distribution in the crescent-shape moderator cell, and provide theoretical supports for the moderator cell construction.     

Optimization and Comparing a Different Various of Head Part of a Moderator of “Central” Direction for IBR-2 Reactor

The performance of the upstream moderator for neutron beams 1, 4, 5, 6, and 9 extracted from the IBR-2 reactor is simulated towards selecting its optimum configuration. For different moderator options, gain factors with respect to the existing comb-shaped water moderator are computed. Taking into account the technological issues in manufacturing a combined moderator, the scheme with optimal physical characteristics is selected.     

The production of very cold neutrons

We present a method of calculating the current of very cold neutrons emerging from a cold moderator applicable to any moderator whose spectral density is known. Results are shown for a hydrogenous moderator with a Debye spectrum.     

Sample to moderator volume ratio effects in neutron yield from a PGNAA setup

Performance of a prompt gamma ray neutron activation analysis (PGNAA) setup depends upon thermal neutron yield at the PGNAA sample location. For a moderator, which encloses a sample, thermal neutron intensity depends upon the effective moderator volume excluding the void volume due to sample volume. A rectangular moderator assembly has been designed for the King Fahd University of Petroleum and Minerals (KFUPM) PGNAA setup. The thermal and fast neutron yield has been measured inside the sample cavity as a function of its front moderator thickness using alpha particle tracks density and recoil proton track density inside the CR-39 nuclear track detectors (NTDs). The thermal/fast neutron yield ratio, obtained from the alpha particle tracks density to proton tracks density ratio in the NTDs, shows an inverse correlation with sample to moderator volume ratio. Comparison of the present results with the previously published results of smaller moderators of the KFUPM PGNAA setup confirms the observation.     

On the use of a composite moderator at the IBR-2 reactor: Advantages for the neutron-diffraction texture analysis of rocks

A unique cold moderator based on a mixture of mesitylene and m-xylene is developed for the IBR-2 pulsed reactor. If combined with a standard warm-water moderator, it provides a high neutron flux in a wide wavelength range. The advantages of the use of this composite moderator in neutron-diffraction texture analysis are discussed using the example of a sample of slate (formed from five minerals) studied using the SKAT diffractometer. The diffraction data obtained in experiments with warm-water and composite moderators are compared.     

Neutron experiments with cryogenic methane hydrate and mesitylene moderators

In this article we describe the experimental results of a methane hydrate moderator as well as as mesitylene moderator operated at a temperature around 20K at the JESSICA (Jülich Experimental Spallation Target Set-up In COSY Area) experiment at the Jülich cooler synchrotron COSY. For the first time the cold neutron spectrum of a methane hydrate moderator was experimentally investigated. A comparison with a solid methane and an ice moderator atT = 20 K will be shown. MCNPX simulations with new developedS(α,β) scattering kernels will be compared with experimental data. The applicability of mesitylene and methane hydrate as cold moderators at spallation neutron sources will be discussed.     

Frictional accumulation of negative muons and its application

A novel method is proposed for the efficient conversion of intermediate energy negative muons into a low-energy muon beam. It is based on using an electric field to eject muons from a moderator consisting of a large number of thin carbon foils placed perpendicularly to the axis of a high-field solenoid. High-energy muons are made to slow down within the moderator to an energy where further slowing down is inhibited by the electric field acceleration between the foils. The muons accumulate at low energy within the moderator hopping from one foil to the next until they come out as a low-energy muon beam. The resulting phase compression factor exceeds 1000. Efficient initial injection of the muons into the moderator is obtained either by letting the muons enter it in a direction opposite to the acceleration force or by producing the muons within a magnetic trap containing the moderator. A practical configuration based on the second scheme is presented. By implementing the method into the most intense muon production configurations a new pathway is opened that may ultimately compete with other schemes in the selection of the optimal source for high-energy muon colliders.     

正电子慢化体的研究和进展

正电子湮没技术是一种研究材料的微观缺陷和相变的灵敏工具,在通常的正电子谱仪中,正电子能量为MeV量级,在样品中注入深度比较学（－100μm）,主要研究材料体内的平均缺陷密度,慢正电子束方法把正电子的能量降低为keV量级（而且可以调节）,注入比较浅（－μm）,所以是研究表面缺陷的探测手段,正电子慢化体是产生慢正电子的关键设备,对其研究有重要意义,文章综述了慢化体研究的历史和现状,从物理概念出发介绍使正电子慢化的四种可能方法和当今慢化体的五种几何排列方法,其中应用最广泛的是钨慢化体和百叶窗式的排列方式,效率最高的是惰性气体固体慢化体,而加电场慢化体是有待开发的高效慢化体。     

Analysis of neutron spectra and fluxes obtained with cold and thermal moderators at IBR-2 reactor: Experimental and computer-modeling studies

The results of experimental and computer-modeling investigations of neutron spectra and fluxes obtained with cold and thermal moderators at the IBR-2 reactor (Joint Institute for Nuclear Research (JINR), Dubna) are presented. These studies are for the YuMO small-angle neutron scattering (SANS) spectrometer (IBR-2 beamline 4). The neutron spectra have been measured for two methane cold moderators for the standard configuration of the SANS instrument. The data from both moderators under different conditions of their operation are compared. The ratio of experimentally determined neutron fluxes of cold and thermal moderators is shown at different wavelengths. Monte Carlo simulations have been carried out to determine the spectra for cold-methane and thermal moderators. The results of calculations of the ratio of neutron fluxes of cold and thermal moderators at different wavelengths are demonstrated. In addition, the absorption of neutrons in the air gaps on the way from the moderator to the investigated sample is presented. SANS with the protein apoferritin was done with both cold methane and a thermal moderator and the data were compared. The prospects for the use of a cold moderator for a SANS spectrometer at IBR-2 are discussed. The advantages of using the YuMO spectrometer with a thermal moderator with respect to the tested cold moderator are shown.     

冷中子源冷包截面含汽率数理模型探讨

文章推导出一种计算冷包截面含汽率的数学模型;将模型计算结果与已知文献相关实验结果进行对比,验证了模型的正确性.利用该模型探讨了冷包截面含汽率的影响因素如气泡直径,工质液体的密度、粘性,加热功率等.模型计算表明:气泡直径越大,在同一截面高度处的截面含汽率越低;工质粘性越大,在同一截面高度处的截面含汽率越高.本文模型的计算结果能对冷包设计提供理论支持.     

慢正电子束产生实验

本文介绍一台慢正电子束装置。在该装置上我们用W慢化体产生慢正电子,测量了慢正电子能谱,并和在该装置上用Ni+Mgo慢化体所得结果进行了比较。 关键词：     

PGNAA煤质在线测量中同时提高C、O和其它元素测量精度的慢化体模拟

提出两种慢化体设计方案,第一种方案直接在原有慢化体中引入孔道,第二种方案在慢化体中增加一层铅之后再引入孔道.利用蒙特卡罗方法对两种方案进行研究.计算结果表明,采用第一种方案可以有效地提升C、O元素的测量精度,但同时会降低H、Si等元素的测量精度;采用第二种方案可提高C元素和O元素的测量精度,同时也可以提高H、Si等其它元素的测量精度.     

Possibility of loading the chamber of the “central” pelletized cold moderator for IBR-2 reactor beams 1, 4–6, and 9

This paper presents the results of experiments performed at the cold moderator test stand with beams 1, 4–6, and 9 for studying the possibility of loading the moderator chamber with frozen balls (pellets) 3.5 to 3.8 mm in diameter through the cryogenic pipeline of complex configuration with a 4-m-long section ascending at an angle of 50°. The optimum parameters for the successful loading of the moderator chamber are found to be the following: mass flow rate of the carrier gas (helium), 2 g/s; temperature of the pipeline walls and helium, 79–85 K; angular velocity of the disc in the ball dispenser, 3.6°/0.5 s; ball ejection rate from the dispenser to the pipeline, 8 balls/s; and maximum chamber loading time, 6 h. During the construction of the test stand, the cryostat is replaced with a new one with two gas blowers, which is necessary for maintaining a low temperature in two independent cooling circuits (for two cold moderators or for a cold moderator and the test stand) and the software for the control of the operation parameters is modified.     

Application of a cryogenic moderator in the REMUR neutron reflectometer

Examples of the application of a cryogenic moderator in the REMUR time-of-flight neutron reflectometer of the IBR-2 pulsed reactor (Dubna) are given. The results of two experiments are presented: spatial beam splitting upon reflection from a magnetically noncollinear film and the recording of a microbeam formed by a layered waveguide. A conclusion concerning the efficiency of the cryogenic moderator for experiments of this type is made.     

Nuclear chain reaction in intercalated C60 fullerite

The paper discusses a chain nuclear reaction in a homogeneous mixture of U²³⁵–C₆₀ fullerenes. The critical mass is calculated, and is shown to drop considerably when the fullerite moderator is transformed into a diamond-like form under pressure. A possibility to produce a homogeneous nuclear reactor subcritical when fullerenes are used as the moderator, but becoming supercritical in their transformation into a diamond-like structure has been considered.     

一种宽能谱多球中子谱仪能量响应的MC模拟

利用蒙特卡罗程序FLUKA模拟计算了聚乙烯慢化球和辅助材料慢化球对低能中子到高能中子的响应函数曲线。结果表明,对纯聚乙烯球来说,随着聚乙烯层厚度的增加,响应曲线峰逐步右移,峰值在高能区有所下降,对20 Me V以上的中子,无论纯聚乙烯球的尺寸有多大,其响应均下降到很低的程度;对辅助材料慢化球来说,中子能量小于1 Me V时,辅助材料慢化球与聚乙烯慢化球的响应曲线相似,但当中子能量大于20 Me V时,中子与辅助材料层发生(n,xn)反应,慢化球的响应呈显著上升趋势。分析计算结果,最终能够确定宽能谱多球中子谱仪的尺寸组合。     

钍基熔盐堆新型栅格设计与优化

利用上海核工院自主开发的SONG/TANG-MSR程序系统,通过大量的方案筛选,在熔盐燃料成分、慢化剂材料以及栅格结构等方面对钍基熔盐堆（TMSR）栅格进行设计优化。提出采用无铍熔盐作为燃料,以提高重核（ThF4, UF4）溶解度;采用氧化铍（BeO）作为慢化剂,从而提高中子经济性;创新性地采用熔盐燃料与慢化剂隔离的SiC包壳管设计,保持堆芯结构的稳定性和抗辐照性能。对优化后的栅格设计计算结果表明:新型的熔盐堆栅格设计具有很高的增殖比并保持负功率系数,从而满足下一代核能系统可持续性和安全性要求。     

Compact positron beam for measurement of transmission moderator efficiencies and positron yields of encapsulated sources

We have constructed a simple positron beam for measuring both the efficiencies of positron moderator foils and the positron yields of encapsulated positron beam sources. The slow-positron emission rate from the moderator is determined from the positron annihilation radiation. This technique is also applied to measure source yields. In addition to the coincidence technique, the source yield is determined by measuring electric currents generated by the annihilating positrons. The performance of the system is demonstrated with different tungsten moderator foils before and after heat treatments, and several sources with primary activities in the range 1-50 mCi.