Spatial Distribution Of Thermal Neutrons Research Articles

Flat-response neutron detector using spatial distribution of thermal neutrons in a moderator

We propose a novel neutron detector that realized flat-response using information of a spatial distribution of thermal neutrons in a moderator. The proposed detector consists of a 3He position sensitive proportional counter (PSPC) and a cylindrical moderator surrounding the 3He PSPC. The cylindrical detector is irradiated by neutrons along the cylinder axis. The spatial response of the 3He PSPC is used to correct the detector response into flat-response. We adopt a weighting method to achieve flat-response, in which detected neutrons weighted depending on their detected positions are accumulated as the detector response. Through Monte Carlo simulation studies, we confirm that the flat-response neutron detector can be realized by correcting the response of the proposed detector using the weights determined by a multiple least square method (MLSM). Additionally, fundamental property of the 3He PSPC is experimentally investigated to check applicability to the proposed flat-response neutron detector. We conclude that we should take account of the end effect when determining the weights and correcting the detector response.

Estimation of Argon-41 concentrations in the vicinity of a high-energy medical accelerator

This study presents the estimation of 41Ar concentrations using the neutron activation method. The distribution of thermal neutron flux in a 15 MV medical accelerator (linac) treatment room was determined and contoured by measuring the radioactivities of indium foils irradiated by thermal neutrons. The 41Ar concentrations were calculated based on the spatial distribution of thermal neutrons. The evolution of 41Ar concentration with time in the treatment room was predicted and the corresponding radiation dose associated with 41Ar was derived and shown to be insignificant for both patients and workers, being below the regulatory level. Indium foil activation method showed high detection sensitivity for estimating the low-level 41Ar in the vicinity of medical accelerators, yielding a minimum detectable concentration of less than 10 Bq m - 3 .

Use of neutron generators for monitoring the increase in petroleum recovery from wells due to ultrasonic action

The decrease in petroleum output from a well due to degradation of the permeability in the extraction zone of the productive fluid is a frequent negative phenomenon which occurs when a deposit is being developed. This effect is inevitable, since the extraction zone of the productive fluid operates as a filter, retaining and accumulating during well operation all pos sible impurities in the pores of the formation. One of the most effective methods for cleaning the extraction zone is to expose it to a longitudinal ultrasonic pressure wave. In so doing, nonstationary oscillating microflows appear in the fluid-filled pores in the formation. If the acoustic field is strong enough, these microflows can promote the removal of these impurities from the fluid extraction zone. In this case, the petroleum output from the well should increase [1]. In addition, ultrasonic action on the formation and the space around the well is very likely to cause water to flow into the well from behind the main shaft of the well, resulting in the dis placement of the water-petroleum contact and other undesirable and difficult-to-control effects. Thus, there are conditionally three states of the porous medium subjected to acoustic action: 1 ‐ pores are filled with impurities (initial state), 2 ‐ pores are filled with products of hydrocarbon fluid, and 3 ‐ pores are filled with water. These states and the processes resulting in a transition of one state into another must be followed by remote-controlled means through a casing pipe using transparent monitoring methods. Neutron methods are most effective [2]. Examples of such methods are the integral and pulsed neutron‐neutron methods with thermal-neutron detection, integral and pulsed neutron γ-logging with detection of radiative-capture γ-rays, and neutron activation of oxygen. These methods can be implemented using large in-well neutron generators based on sealed accelerator tubes with flux exceeding 10 8 sec ‐1 in a full solid angle [3]. Such generators make monitoring much safer, because a generator in the switched-off state does not create radiation fields and, consequently, serious ecological problems do arise in an accident, which cannot be completely prevented. A block diagram of a neutron monitoring system is presented in Fig. 1. SNM-Type 3 He-filled detectors are used to detect thermal neutrons. The nuclear reaction 3 He(n, p)T occurs in the interior volume of these detectors. γ Rays are detected with a NaI(Tl) or CsI(Na) single crystal scintillation detector with a combined filter for the direct neutron and γ radiation of the in-well generator. The integral neutron‐neutron and neutron γ-logging are based on changing the spatial distribution of thermal neutrons and radiative-capture γ rays depending on the moderating, diffusion, and absorbing properties of the medium being studied. They make it posssible to record the change in the count rate of thermal neutrons or γ rays as a result of a transition from state 1 into state 2 or 3, which are characterized by a substantial concentration of hydrogen atoms in the pores. The hydro

Studies on thermal neutron perturbation factor needed for bulk sample activation analysis

The spatial distribution of thermal neutrons produced by an Am–Be source in a graphite pile was measured via the activation foil method. The results obtained agree well with calculated data using the MCNP-4B code. A previous method used for the determination of the average neutron flux within thin absorbing samples has been improved and extended for a graphite moderator. A procedure developed for the determination of the flux perturbation factor renders the thermal neutron activation analysis of bulky samples of unknown composition possible both in hydrogenous and graphite moderators.

Analysis of neutron flux distributions generated by sources of fast neutrons in a range of bituminous coals

Monte Carlo calculations of fast neutron flux distributions have been made for 252Cf, 241Am/Be and 14 MeV monoenergetic neutron sources in a range of bituminous coals. The configurations examined were sources at the centre of a sphere, at the midpoint of the plane surface of a hemisphere and at or near the surface of a parallel-sided slab. The neutron fluxes were analysed in terms of the resultant yield of γ-rays from inelastic scattering on silicon, carbon and oxygen, which are the key reactions for determining the oxygen content of coal by processes yielding prompt γ-rays. The fast neutron spectra were found to undergo significant changes in shape as a function of distance from the source, in contrast with the assumptions made in the model developed by Wormald and Clayton. (1) However, the ratios of the response functions for inelastic scattering by the critical elements were found to be very insensitive to the type of coal, indicating that oxygen analyses could be performed for a given configuration and neutron source by using calibration factors derived from a single calculation or benchmark experiment on a coal of intermediate composition. The calculations described also provided information on the yield and spatial distribution of thermal neutrons from the selected sources.

水中における高速中性子の減速

The migration area of thermal neutrons and the age of In-resonance neutrons in water have been obtained with T-d neutrons. Taking into account the angular dependence of the emitted neutron energy and yield in the T-d reaction, the average source energy was taken to be 14.2 MeV. By means of “Cd difference” method and a small (13mmφ×50mm) BF3 counter, the spatial distribution of thermal neutrons in a large water tank was measured at the 0°, ±45° and ±80° direction along the radial distance, from the source to the point of 110cm. Then, the migration area M2 was obtained from the spatial distribution of thermal neutrons in each direction. The spatial distribution of In-resonance neutrons also was measured by In foils with Cd covers to the point of 55cm at the same directions. And the neutron age τln of In-resonance energy (1.44 eV) was obtained, using the same relaxation length in large distance as the thermal neutron distribution. The values of migration area M2 and neutron age τln are determined as 176±5cm2 and 167±8cm2, respectively. The slowing down length Ls (=τln1/2) to In-resonance energy is 12.9±0.3cm, and in agreement with Volkin's curve, which was calculatad assuming the anisotropic nature of the elastic scattering by oxygen.

Spatial Distribution of Thermal Neutrons from an N17 Source in Water

The thermal neutron flux in a water medium from a source disk containing water irradiated in the Materials Testing Reactor has been measured using a BF3 detector. Neutrons from N17 and photoneutron...

Thermal Flux Distribution from a Fast Neutron Line Source

The spatial distribution of thermal neutrons originating from a fast neutron line source, with slowing down described by age theory, and thermal percolation through an infinite, isotropic medium described by diffusion theory, is presented.

Spatial Distribution of Thermal Neutrons from a Polonium-Beryllium Source in Water-Zirconium Mixtures

The spatial distribution of neutrons emitted by a Po–Be source and moderated by water-zirconium mixtures has been measured for four water-zirconium volume ratios. From these data, migration areas of source neutrons to energies below the cadmium cutoff have been determined with an uncertainty of ±2%. The experimental values of the migration area M2 are: Zr/H2O ratioM2(cm2)0.0069.60.2581.30.5089.71.00112.8