A comparative study of promising filter materials for neutron imaging facilities

The effect of sapphire and bismuth single-crystal filters and their combinations on the quality of neutron radiographic images and neutron tomography data has been studied. The parameters of the contrast of the neutron image were analyzed depending on the monocrystalline filters. Neutron transmission spectra were obtained for sapphire and bismuth single crystals. Additionally, the effect of filters on the overall intensity of the thermal neutron beam and the background of gamma-rays was investigated. Based on the obtained data, we assume that a single-crystal sapphire filter can be most effectively used for radiographic and tomographic installations using thermal neutrons.

Collimated neutron beam design for TRR thermal column

Tehran Research Reactor (TRR) is the main neutron source in Iran which can be used for different applications of neutrons such as neutron radiography and neutron therapy. TRR has a thermal column which can provide high intensity flux of thermal neutrons for users. The aim of this study is to design a neutron collimator for TRR thermal column to produce parallel neutron beam with suitable intensity of thermal neutrons. To achieve this goal, Monte Carlo code of MCNX has been used to evaluate different configurations, geometries and materials of neutron collimator. The results show that the final selected configuration can provide a uniform thermal neutron beam with a flux of 1.21E+13 (cm-2·s-1) which is suitable for many different neutron applications.

NEUTRON DEPTH PROFILE CALCULATIONS USING ARTIFICIAL NEURAL NETWORKS

Neutron depth profiling (NDP) is a non-destructive technique used for identifying the concentration of impurity isotopes below the sample surface. NDP is carried out by detection of the emitted charged particles resulting from bombarding the sample with neutrons. NDP specifies the isotopic concentration versus the sample depth for a few micrometers below the surface. The sample is bombarded inside a research reactor using a thermal neutron beam. Charged particles like alpha particles or protons are produced from the neutron induced reactions in the sample. Each neutron isotopic interaction produces a certain Q, indicating a specific kinetic energy for the emitted charged particle. As the charged particle travels through the sample to eject the surface, it loses energy to atoms (electrons) on its path. The charged particle energy loss holds information regarding the number of atoms by which the emitted particle passed, thus indicating its original depth. The purpose of this work is to check the capability of Artificial Neural Networks (ANNs) in predicting the boron concentration profile across a boro-silicate sample of thickness 3.5 μm divided into 10 layers. Each layer included different boron concentration than the other. Also, the boron concentration had the values {0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1}. Training, validation, and test data were generated synthetically using MCNP6 in which the boron concentrations varied in the layer number from one sample to another. MCNP6 model consisted of a silicon barrier detector, boro-silicate sample, chamber body and an idealized thermal neutron source. The detector, sample, and the source were located in a voided chamber. The samples were irradiated with a 0.025 eV monoenergetic thermal neutron beam from a monodirectional disk source. To cover the whole area of the samples, the thermal neutron beam had a radius of 3 cm. The silicon detector active volume was modelled as a 100 μm thick and 3 cm radius facing the sample directly. The sample, beam, and the detector were placed on the same axis. Ten ANN regression models were developed, one for each layer boron concentration prediction where the input for each model was the alpha spectrum read by the detector, while the output was the boron concentration for each layer. Results showed regression values higher than 0.94 for all of the developed models. ANNs proved its capability of predicting the boron profile form the alpha spectrum read by the detector regarding neutron depth profiling in a boro-silicate samples.

SIMULATION OF NEUTRON ENERGY SPECTRA OF FILTERED THERMAL NEUTRON BEAM

Trong bài báo, các tính toán mô phỏng về phân bố phổ năng lượng của các neutron nhiệt truyền qua phin lọc của tinh thể sapphire và bistmut đã được thực hiện. Kỹ thuật nơtron phin lọc sử dụng bismuth, sapphire đã được áp dụng. Mô phỏng PHITS (Particle and Heavy Ion Transport System) đã được áp dụng để mô tả đặc điểm của phổ năng lượng neutron dựa trên các tham số thiết kế vật liệu, cấu trúc hình học, và độ dày lớp che chắn.

Low counting rate measurement on thermal neutron induced fission using cross-correlation technique

The measurement procedure based on the continuous thermal neutron beam modulation with a mechanical chopper was developed for delayed neutron yield measurement of the thermal neutron induced fission of [Formula: see text]Np. The idea of the procedure is similar to that widely used in modern computer communications to prevent unauthorized data access. The data is modulated with a predefined pattern before transmission to the public network, and only the recipient that has the modulation pattern is able to demodulate it upon receipt. For thermal neutron induced reaction applications, the thermal neutron beam modulation pattern was used to demodulate the measured delayed neutron intensity signals on the detector output, resulting in nonzero output only for the detector signals correlated with the beam modulation. The comparison of the method with the conventional measurement procedure was provided, and it was demonstrated that the cross-correlation procedure has special features making it superior to the conventional one, especially when the measured value is extremely small in comparison with the background. Due to the strong sensitivity of the measurement procedure on the modulation pattern of the neutron beam, one can implement the modulation pattern of a specific shape to separate the effect of the thermal part of the beam from the higher energy part in the most confident way in the particular experiment. The remarkable property of our method is related to the unique possibility of separation of the effects caused exclusively by thermal neutrons using the neutron time-of-flight measurement available on the IBR-2 pulsed reactor.