Study of anomalous subbarrier fission of Th-232 in the lead body of the SVZ-100 lead slowing-down neutron spectrometer

Anomalous fission of Th-232 was twice studied at the SVZ-100, in 2003 and after the formation of hydroxide films on the surface of lead blocks, in 2005. After the formation of these films, the Th-232 fission cross section in the region of short slowing down times decreased by a factor of 140 to 0.3 mbarn which is 200 times larger than the values determined using neutron time-of-flight spectrometers (TFSs) to energies of 700 keV. The explanation of previously unknown features of neutron properties in Lead Slowing-down Spectrometers, previously published by A. A. Bergman, is presented.     

Effect of the size of experimental channels of the lead slowing-down spectrometer SVZ-100 (Institute for Nuclear Research, Moscow) on the moderation constant

Lead slowing-down (LSD) spectrometers have a low energy resolution (about 30%), but their luminosity is 10³ to 10⁴ times higher than that of time-of-flight (TOF) spectrometers. A high luminosity of LSD spectrometers makes it possible to use them to measure neutron cross section for samples of mass about several micrograms. These features specify a niche for the application of LSD spectrometers in measuring neutron cross sections for elements hardly available in macroscopic amounts—in particular, for actinides. A mathematical simulation of the parameters of SVZ-100 LSD spectrometer of the Institute for Nuclear Research (INR, Moscow) is performed in the present study on the basis of the MCNPX code. It is found that the moderation constant, which is the main parameter of LSD spectrometers, is highly sensitive to the size and shape of detecting volumes in calculations and, hence, to the real size of experimental channels of the LSD spectrometer.     

Simulation of the response functions of an extended range neutron multisphere spectrometer using FLUKA

In this paper, the distribution of radiation field in the CSNS spectrometer hall at Dongguan, China, was simulated by the FLUKA program. The results show that the radiation field of the high energy proton accelerator is dominated by neutron radiation, with a broad range of neutron energies, spanning about eleven orders of magnitude.Simulation and calculation of the response functions of four Bonner spheres with a simplified model is done with FLUKA and MCNPX codes respectively, proving the feasibility of the FLUKA program for this application and the correctness of the calculation method. Using the actual model, we simulate and calculate the energy response functions of Bonner sphere detectors with polyethylene layers of different diameters, including detectors with lead layers, using the FLUKA code. Based on the simulation results, we select eleven detectors as the basic structure for an Extended Range Neutron Multisphere Spectrometer(ERNMS).     

Experimental study of some important characteristics of the thermal neutron induced fission of 237Np

Fission fragment mass and kinetic energy distributions and their correlations have been studied for the thermal neutron induced fission of ²³⁷Np. The global mass distribution is rather smooth, apart from a weak shoulder at μ_H = 140–141. When low excitation events are selected, fine structures associated with the charge of the fragments are observed. Furthermore, there is a sudden increase in E_k for μ_H > 155, which is probably due to a spherical shell N = 50 in the light fragment and the corresponding deformed (but stable) heavy fragments with masses in the rare earth region. For the average (pre-neutron emission) total fragment kinetic energy, a value of 176.4 ± 0.6 MeV has been obtained, in agreement with the systematics.Also the prompt neutron emission curve $\text{v(m}{}^1\text{)}$ has been calculated, which shows the well-known saw-tooth shape. Finally, the energy distribution and the emission probability of the ternary α-particles have been determined.     

Search for solar neutrons using the data of neutron detector of the PAMELA spectrometer

The study is devoted to the development of the technique for searching for neutrons of solar origin during solar flares using the neutron detector of the PAMELA magnetic spectrometer onboard the RESURS-DK1 satellite. The study of the neutron detector counting rates showed that it operates properly and background conditions are stable in the geomagnetic equator region. 27 solar events from December 2006 to September 2014 were analyzed. Some events contain indications of solar neutron detection. The final conclusion on the nature of these neutrons requires an increase in statistics; to this end, background conditions in polar regions should be analyzed to increase the observation interval.     

Measurement of fast neutron induced fission cross section of thorium using Lexan plastic track detector

Fission-track registration characteristics of Lexan solid state nuclear track detectors have been used to measure the fast neutron induced fission cross section of²³²Th. The fast neutrons (?14.2MeV) were produced with the help of an AN-400 model Van-de-Graaff accelerator at Banaras Hindu University laboratory using³H(²H,n)⁴He reaction and were used to irradiate the fissile target deposited on the plastic detector. The track densityT, registered on the plastic detector is related to the fission cross sectionσ _f, through the relationT=knσ _føt wheren is the number of fissile atoms per cm² in the deposit, ø is the neutron flux,k is fission track registration efficiency andt is the time of irradiation. The fission cross sectionσ _f of²³²Th, relative to the well measured fission cross section of²³⁸U, was found to be 0.36±0.04 barn.     

Study of the structure of neutron halo using neutron transfer reaction

To estimate the probability of two-neutron configurations in halo nuclei, we propose an experimental method of studying neutron-neutron correlations at periphery of such nuclei by measuring two-neutron transfer reaction. The experimental study of ⁶He + A ↦ ⁴He + B for various targets is performed using ⁶He beam of Flerov Laboratory of Nuclear Reactions (JINR, Dubna) at energy of about 15 MeV/u and technique of nuclear photoemulsions. Searching for events of two-nucleon transfer reaction and their processing is performed using the PAVICOM-setup at P.N. Lebedev Physical Institute.     

Response matrix of an extended range Bonner sphere spectrometer for the characterization of collimated neutron beams

Accelerator-based neutron beams are becoming popular tools for material testing, radiation hardness and soft errors studies. The characterization of these beams in terms of dosimetric and spectrometric quantities is a challenging task, mainly due to their wide energy interval (from thermal up to hundreds MeV) and, in certain facilities like VESUVIO @ ISIS (RAL, UK), to their small dimension (few cm in radius). Extended Range Bonner Sphere Spectrometers (ERBSS) would be a valuable tool, due to their wide energy range, good photon discrimination and possibility to choose among different central detectors according to the intensity, photon component and time structure of the field. Nevertheless, the non-uniform irradiation of the spheres could lead to important systematic errors. With the aim of bringing the advantages of ERBSS into the characterization of collimated beams, a dedicated study was performed using the VESUVIO spallation-based collimated beam at ISIS (Rutherford Appleton Laboratory, Oxford). Here a 3.21 cm radius collimated beam was characterized using a Dysprosium activation foil-based ERBSS whose response matrix was recalculated for this specific beam diameter. Besides the results of the experimental campaign, this paper presents the calculation of the response matrix and its dependence on the beam dimension.     

Analysis of prompt fission neutron spectrum and multiplicity for 237Np（n,f） in the frame of multi-modal Los Alamos model

The improved version of Los Alamos model with the multi-modal fission approach is used to analyse the prompt fission neutron spectrum and multiplicity for the neutron-induced fission of 237Np. The spectra of neutrons emitted from fragments for the three most dominant fission modes (standard Ⅰ, standard Ⅱ and superlong) are calculated separately and the total spectrum is synthesized. The multi-modal parameters contained in the spectrum model are determined on the basis of experimental data of fission fragment mass distributions. The calculated total prompt fission neutron spectrum and multiplicity are better agreement with the experimental data than those obtained from the conventional treatment of the Los Alamos model.     

Half-lives,neutron emission probabilities and fission yields of neutron-rich rubidium isotopes in the mass regionA=96 toA=100

Neutron-rich rubidium isotopes were produced by thermal-neutron induced fission of²³⁵U and were isolated by surface ionization and mass separation. The new isotope (51 ±17) ms¹⁰⁰Rb was identified. The half-lives of the rubidium isotopes with mass numberA=96 toA=100 were measured using neutron multiscaling or gamma multispectrum analysis. Measurements of the delayed-neutron emission probabilityP _n of⁹⁹Rb and of the fission yields of⁹⁷Rb,⁹⁸Rb, and⁹⁹Rb were performed.     

Determination of the emission rate of an Am–Be neutron source with a Bonner sphere spectrometer

The Neutron Measurements Laboratory of the Technical University of Madrid (LMN) has an automated panoramic irradiator with a 111 GBq ²⁴¹Am–Be neutron source installed in a bunker-type large room. This facility is going to be used for calibration purposes. Recently, a spectrometry campaign involving four research groups working with different Bonner Sphere Spectrometers (BSS) and using different spectral unfolding codes was carried out. As part of these measurements the emission rate, B(t), was estimated. The application of the generalized fitting method to account for the scattering contribution is difficult due to specific characteristics of the neutron installation. A reduced fitting method, which includes room-return and in-scatter, has instead been used to overcome this problem.Detailed Monte Carlo simulations (with MCNPX code) were also performed to estimate the fluence rate using the measured source strength value. This was performed at different points. Results were then compared with measurements.Finally, the ambient dose equivalent rate measured with a neutron monitor (LB6411) was compared with results using the BSS.     

Concept of a high-resolution miniature spectrometer using an integrated filter array

A high-resolution miniature spectrometer has been demonstrated by utilizing a 128-channel integrated filter array, fabricated by using the combinatorial deposition technique, as a dispersive component whose passbands range from 722.0 to 880.0 nm with a bandwidth (or spectral resolution) from 1.7 to 3.8 nm and an average channel interval of 1.2 nm. The miniature spectrometer is smaller than 1 cm3 without any moving parts. This kind of miniature spectrometer has the advantages of very low payload, high resolution, and high reliability simultaneously, which are especially urgently needed for space applications.     

Study of an unfolding algorithm for D-T neutron energy spectra measurement using a recoil proton method

A proton recoil method for measuring D-T neutron energy spectra using polyethylene film and a Si(Au) surface barrier detector is presented. An iteration algorithm for unfolding the recoil proton energy spectrum to the neutron energy spectrum is investigated. The response matrices R of the polyethylene film at angles of 0° and 45° were obtained by simulating the recoil proton energy spectra from mono-energetic neutrons using the MCNPX code. With an assumed D-T neutron spectrum, the recoil proton spectra from the polyethylene film at angles of 0° and 45° were also simulated using the MCNPX code. Based on the response matrices R and the simulated recoil proton spectra at 0° and 45°, the respective unfolded neutron spectra were obtained using the iteration algorithm, and compared with the assumed neutron spectrum. The results show that the iteration algorithm method can be applied to unfold the recoil proton energy spectrum to the neutron energy spectrum for D-T neutron energy spectra measurement using the recoil proton method.     

Simulation of the response functions of an extended rangeneutron multisphere spectrometer using FLUKA

In this paper, the distribution of radiation field in the CSNS spectrometer hall at Dongguan, China, was simulated by the FLUKA program. The results show that the radiation field of the high energy proton accelerator is dominated by neutron radiation, with a broad range of neutron energies, spanning about eleven orders of magnitude. Simulation and calculation of the response functions of four Bonner spheres with a simplified model is done with FLUKA and MCNPX codes respectively, proving the feasibility of the FLUKA program for this application and the correctness of the calculation method. Using the actual model, we simulate and calculate the energy response functions of Bonner sphere detectors with polyethylene layers of different diameters, including detectors with lead layers, using the FLUKA code. Based on the simulation results, we select eleven detectors as the basic structure for an Extended Range Neutron Multisphere Spectrometer (ERNMS).     

漫反射板法定标成像光谱仪特性研究

采用漫反射板法定标成像光谱仪是近年来发展起来的一种辐射定标新技术,它具有实现容易、定标精度较高的特点。文章根据漫反射板定标法的原理,对于成像光谱仪狭缝平行和垂直纸面两种特殊定标状况,推导出相应的光谱辐射信号电子数采集表达式,数值分析了在此两种定标状况下成像光谱仪光谱辐射定标的特性。结果表明,成像光谱仪狭缝取向对光谱辐射定标结果有明显不同的影响,对于狭缝平行纸面情况下,不同景物像元相同光谱通道采集到的信号电子数是不同的,越靠近辐照度标准灯,采集到的信号电子数越多,反之亦然;对于狭缝垂直纸面情况下,不同景物像元相同光谱通道采集到的信号电子数是相同的。     

3004铝合金“反常”锯齿屈服现象的研究

在应变速率为5.56×10^-5s^-1—5.56×10^-3s^-1的范围内，在不同温度下(从223K至773K)，对3004铝合金进行系列拉伸试验，探索其锯齿屈服规律；通过激活能的计算、内耗研究、微观组织观察和能谱分析，探讨锯齿屈服的机理与物理本质.结果表明，3004铝合金在形变过程中会出现动态应变时效现象；发现了一种“反常”的锯齿屈服现象：在出现锯齿屈服的温区内，存在锯齿屈服临界应变量转变温度T_t， 关键词： 动态应变时效 锯齿屈服 铝合金 内耗     

Study of the excited 0+ states through conversion electron spectroscopy of neutron rich Zr and Mo fission products

A windowless Si(Li) electron detector has been used in conjunction with the gas filled fission product separator JOSEF to study totally converted transitions in Zr and Mo isotopes. The existence of twoβ-decay modes in⁹⁶Y,⁹⁸Y and¹⁰⁰Nb is confirmed. The half-lives of theβ-decaying levels feeding O ₂ ⁺ states in⁹⁶Zr and⁹⁸Zr are 6.0 ± 0.4 sec and 0.60±0.05 sec respectively. A search for other first excited O ₂ ⁺ states shows that no such states are apparent in¹⁰⁰Zr and¹⁰⁴Mo.