Distribution Of Fast Neutrons Research Articles

Study on the contribution matrix for EAST radial neutron camera system

The EAST (Experiment Advanced Superconductor Tokamak) Radial Neutron Camera (RNC) is designed to measure the temporal and spatial distribution of fast neutrons. The system primarily consists of detector components, shielding collimators, support structure, and data acquisition system. We have studied on the neutronic transport through MCNP with EAST reference D-D neutron source (EAST #77160), and calculated the plasma neutron source chord integral along the observation sightlines and scattering neutrons from structural facilities. Contribution matrixes, incorporating both uncollided and scattered neutrons determines the response from the neutron source to every detector unit. We evaluate the RNC primarily through the variation of the RNC sightlines to improve detector performance by reducing interference particles, thereby laying the foundation for subsequent upgrades. Benefiting from these measurements, we can better study the detector signals in relation to corresponding plasma configurations in the future.

First measurements of beam backgrounds at SuperKEKB

The high design luminosity of the SuperKEKB electron–positron collider is expected to result in challenging levels of beam-induced backgrounds in the interaction region. Properly simulating and mitigating these backgrounds is critical to the success of the Belle II experiment. We report on measurements performed with a suite of dedicated beam background detectors, collectively known as BEAST II, during the so-called Phase 1 commissioning run of SuperKEKB in 2016, which involved operation of both the high energy ring (HER) of 7 GeV electrons as well as the low energy ring (LER) of 4 GeV positrons. We describe the BEAST II detector systems, the simulation of beam backgrounds, and the measurements performed. The measurements include standard ones of dose rates versus accelerator conditions, and more novel investigations, such as bunch-by-bunch measurements of injection backgrounds and measurements sensitive to the energy spectrum and angular distribution of fast neutrons. We observe beam–gas, Touschek, beam–dust, and injection backgrounds. As there is no final focus of the beams in Phase 1, we do not observe significant synchrotron radiation, as expected. Measured LER beam–gas backgrounds and Touschek backgrounds in both rings are slightly elevated, on average three times larger than the levels predicted by simulation. HER beam–gas backgrounds are on average two orders of magnitude larger than predicted. Systematic uncertainties and channel-to-channel variations are large, so that these excesses constitute only 1–2 sigma level effects. Neutron background rates are higher than predicted and should be studied further. We will measure the remaining beam background processes, due to colliding beams, in the imminent commissioning Phase 2. These backgrounds are expected to be the most critical for Belle II, to the point of necessitating replacement of detector components during the Phase 3 (full-luminosity) operation of SuperKEB.

Estimation method of planetary fast neutron flux by a Ge gamma-ray spectrometer

An intensity map of lunar fast neutrons (LFNs) and their temporal variation has been estimated by fitting “sawtooth” peaks in the energy spectra of lunar gamma rays observed by the Kaguya (SELENE) Gamma Ray Spectrometer (GRS) consisting of a high-purity germanium (HPGe) detector with a BGO scintillator. While an ordinary peak in the spectrum is produced by only gamma ray lines, the sawtooth peak is produced by gamma ray lines and recoil nuclei in the detector by Ge(n,n′γ) reaction. We develop a model for the shape of the sawtooth peak and apply it to fit sawtooth peaks together with ordinary peaks in actual observed spectra on the Moon. The temporal variation of LFNs is synchronous with that of galactic cosmic rays (GCRs), and the global distribution of fast neutrons on the lunar surface agrees well with the past observation reported by the Neutron Spectrometer aboard Lunar Prospector. Based on these results, a new method is established to estimate the flux of fast neutrons by fitting sawtooth peaks on the gamma ray spectrum observed by the HPGe detector.

Mathematical modeling problems connected with pulsed neutron-gamma logging

The work is devoted to direct and inverse problems of the transport equation in the context of a nuclear geophysical technology based on pulsed neutron-gamma logging of inelastic scattering (PNGL-IS). In the first part of the paper we analyze the distribution of fast neutrons from a pulsed source of 14.1 MeV and study distributions of gamma-quanta of inelastic scattering. The particle distributions are computed by the Monte Carlo methods. In the second part of the paper we consider the problem of evaluating the elemental composition of the rock from the PNGL-IS measurement data. In its solution we use the method of successive approximations over characteristic interactions, which can be classified as a simple iteration; at each iteration step we solve the corresponding direct problem for the system of neutron and gamma-quantum transport equations. The main aspects of the employed method and the results of the numerical experiments that prove the convergence to the exact solution are presented.

Fast Neutron Detection With ${\rm Cs}_{2} {\rm LiYCl}_{6}$

This paper discusses our initial investigation of fast neutron detection with a Cs2LiYCl6 (CLYC) scintillator. CLYC has been already developed for dual mode detectors (thermal neutrons and gamma rays). Described are results collected under mono-energetic irradiation from a Van de Graaff generator and a continuous irradiation from a 252Cf source. There are two reactions in which fast neutrons are captured 35Cl(n,p)35S and 6Li(n,t)α; both have been observed. They produce a proton and a pair of α/t particles, respectively. The response to mono-energetic fast neutrons due to the 35Cl(n,p) reaction produces a peak and due to the 6Li(n,t) reaction a continuum in the energy spectra. The relation between the peak (continuum) position and the excitation energy is linear within evaluated energy range. This allows for fast neutron spectroscopy. The α/β ratios for both reactions were found to be different and somewhat dependent on the excitation energy. The decay times under the proton excitation slightly differ from these under the α/t excitation, while both are significantly different from the gamma ray excited curves. This is important for pulse shape discrimination. Using continuous excitation from 252Cf initial efficiency was estimated for the 35Cl(n,p) reaction. For a 1-inch right cylinder crystal the measured intrinsic efficiency is at least 0.06%, while calculated is 0.5%. The discrepancy is due to experimental inaccuracies. The relative detection efficiency scales linearly with the volume. A convolution of the theoretical 252Cf fast neutron distribution and the 35Cl(n,p) cross-section curve was found to match the experimental data.

Measurement of fast neutron by using imaging plate

Measurements of yield and spatial distribution of fast neutrons are of extreme importance in the inertial confinement fusion research to estimate the spatial distribution of temperature in core plasma. In experiments using an ultra intense laser light such as fast ignition research, strong electromagnetic pulses generated by the laser irradiation make it very difficult to use a detector with an electric circuit. In order to avoid this problem, we have developed several detectors to measure fast electrons, X-ray, and so on under these environments using an imaging plate (IP). Here, we demonstrate the simple fast neutron detection method by using a stack of IPs. In the experiment using a monochromatic 14MeV neutron source, the IP has linear response to quantity of incident neutrons within the fluence from 104 to 109. The minimum neutron yield measureable by the detector is estimated to 103 or less depending on detection efficiency.

Experimental and Monte-Carlo studies of the spatial distribution of neutrons around extended Pb-spallation target

The spatial distribution of thermal and fast neutrons on the surface of a paraffin moderator surrounding a cylindrical lead target, irradiated with 1 GeV protons was studied. The lead target had 8 cm diameter and 20 cm length. The thickness of the paraffin around the target was 6 cm . The slow and fast neutron distributions were determined using LR 115 2B and CR-39 detectors via the 10 B( n,α) reaction and neutron induced nuclear recoils, respectively. The observed slow and fast neutron distributions on the surface of the paraffin were compared with Monte-Carlo simulations using the MCNPX-2.1.5 code.

Fast-neutron profiling with an imaging plate

A method for taking the fast-neutron profile has been developed using an imaging plate combined with a polypropylene converter. By this method, we can obtain the spatial distribution of fast neutrons without any γ-ray contamination by taking the difference between the data with and without the converter. We have studied properties of this method concerning the neutron energy, converter thickness, fading, dynamic range and spatial resolution. Under appropriate conditions, the method has more than a 10 2 dynamic range with linearity and less than 1.0 mm spatial resolution for 5 and 15 MeV neutrons. By using this method, we successfully obtained a fast-neutron profile after a collimator and radiography of a 5 cm thick iron block.

Measurement of accelerator-based neutron distributions using nuclear track detectors

Nuclear track detectors were used to measure the longitudinal and transverse distributions of slow neutrons in a moderated neutron field as well as the longitudinal and transverse distributions of fast neutrons produced on the 0° beam line of the KFUPM 350 keV ion accelerator. The neutrons were first produced from the T(d,n) 4He reaction with a neutron energy of approximately 14 MeV and were then moderated in a cylindrical polyethylene moderator placed at the end of the 0° beam line. The optimal transverse slow neutron distribution was found to be uniform within ±4.5% at a 3 cm depth inside the moderator. The fast neutron distribution component along the moderator central axis exhibited an exponential-like drop in intensity with depth. Linearity checks of alpha and proton recoil track density with irradiation time for the nuclear track detectors were verified for both slow and fast neutrons.

Investigation of radiations penetration in aluminum used in reactor technology

Measurements have been carried out to determine the spatial and energy distributions of fast neutrons, total gamma rays and secondary gamma rays in aluminum as a reactor wide structural material. Experimental work was performed using one of the horizontal channels of the ET-RR-1 reactor. For secondary gamma ray investigations, measurements were carried out for bare, cadmium filtered and boron carbide filtered reactor beams. Aluminum cylinders with a diameter of 100 mm and of different thicknesses have been used. Fast neutrons and gamma rays were measured using a neutron-gamma spectrometer with a stilbene scintillator. Discrimination against undesired pulses of neutrons and gamma rays was achieved by a pulse shape discrimination based on zero cross over technique. The measured pulse amplitude distributions of fast neutrons or gamma rays were transformed to neutron or gamma ray energy distributions using the differentiation method and the matrix correction method, respectively. The obtained data were displayed in the form of spatial fluxes and energy distributions and attenuation curves. Total fast neutron macroscopic cross sections, gamma rays linear attenuation coefficients and the relaxation lengths were obtained for the whole energy range and at different energies.

Radiation attenuation in light materials with spherical geometry

The spatial fluxes and energy distributions of fast neutrons and total gamma rays behind spherical shields of polyethylene and graphite have been carried out. A Pu-α-Be source has been used in spherical geometry. Measurements and calculations of the fast neutron spectra behind different thickness of polyethylene and graphite have been carried out. Also the total gamma ray spectra behind different thicknesses of polyethylene and graphite have been measured experimentally. A fast neutron and gamma ray spectrometer with a stilbene crystal was used in the present measurements. The undesired pulses of neutrons or gamma rays were prevented by a pulse shape discrimination technique based on the zero crossing method. The calculations of the fast neutron fluxes were performed by using the ANISN Westinghouse version (ANISN-W) code with the EUR LiB 15 5 cross section library. The obtained data were displayed in the form of spectra and flux distributions and attenuation curves. Comparison between the measured and calculated fast neutron spectra for polyethylene and graphite shows a reasonable agreement for all energies and for all investigated thicknesses. Also comparison between polyethylene, graphite and water from the radiation moderation and attenuation point of view have been carried out.

Measurement of fast neutrons and secondary gamma rays in graphite

The spatial fluxes and energy distributions of fast neutrons, total gamma rays and secondary gamma rays transmitted through different thicknesses of graphite have been measured. The graphite samples were arranged in front of one of the horizontal channels of the ET-RR-1 reactor. Gamma ray measurements were carried out for bare, cadmium filtered and boron carbide filtered reactor beams. A fast neutron and gamma ray spectrometer with a stilbene crystal was used to measure the spectrum of fast neutrons and gamma rays. Pulse shape discrimination using the zero cross over technique was used to distinguish the proton pulses from the electron pulses. The total fast neutrons macroscopic cross section and the linear attenuation coefficient for gamma rays were derived both for the whole energy range and at different energies. The obtained values were used to calculate the relaxation lengths for fast neutrons and gamma rays.

Monte Carlo calculations of fast neutrons distribution inside graphite

Both energy and spatial distributions of fast neutrons inside infinite slabs of graphite (density = 1.72 gm · cm −3) are calculated. The blocks were irradiated (on the surface) with parallel and normal incident neutrons of 15 MeV with a beam diameter of 6 cm. Using Monte-Carlo technique, fluxes at different locations inside the medium, along and away from the incident beam's axis (radial position of the detector) were calculated. The calculations were performed for graphite slabs of different thicknesses ranging from 5 to 90 cm. Comparisons with some measurements are presented. The variation of the fast neutron build up factor with the depth inside the graphite block was investigated. Its dependence on the block thickness was also studied.

Nuclear halos: Structure and reactions

This contribution reports recent results on reactions of 11 Be and 11 Li with light and heavy target nuclei. The angular distributions of fast neutrons in coincidence with selected nuclear fragments illustrate the interplay between halo structure and reaction mechanism It is shown that the unbound nucleus 10 Li has an l =0 ground state characterized by a large scattering length, of the order of −20 fm. The neutron halo of 11 Li can best be interpreted in terms of a strongly correlated state with appreciable (2s 1 2 ) 2 and (1p 1 2 ) 2 components.

Fast-neutron flux distribution in ducted light and heavy concrete shields

This paper reports on a study of the effect of cylindrical air-filled ducts of different lengths and diameters on the distribution of fast neutrons in ordinary concrete ( ϱ = 2.3 g/cm 3) and ilmenite concrete ( ϱ = 4.6 g/cm 3). The source of neutrons was a collimated beam of reactor neutrons emitted from one of the horizontal channels of the ET-RR-1 reactor. The measurements were performed using phosphorus activation detectors. The data show that the presence of air-filled ducts tends to increase the neutron flux in the medium surrounding the duct, as compared with the corresponding values for solid concrete. A marked peak was observed at about 10 cm after the end of the duct. The value of the flux at these peaks is dependent on the duct length and diameter. Also, it was shown that the neutron flux at any point along the duct axis passing through ilmenite concrete is much lower than the corresponding value in ordinary concrete. In addition, solid ilmenite concrete is much better for fast-neutron attenuation than solid ordinary concrete, especially for large thicknesses. However, for small thicknesses (< 30 cm) ordinary concrete is the best. Semiempirical formulae have been derived to calculate the fast-neutron fluxes at different thicknesses along the axis of air-filled ducts of different diameters inserted in ordinary and ilmenite concretes and to calculate the fast-neutron flux distribution in solid concrete media. The derived formulae give results which are in good agreement with the measured values.

Observation of forward neutrons from the break-up of the 11Li neutron halo

The cross-sections and forward angular distributions of fast neutrons have been studied for the reaction ( 11Li, 9Li) at 29 MeV/u on targets of Be, Ni and Au. The cross-sections are large and arise essentially from peripheral reactions. The angular distributions of the neutrons are very narrow and indicate the existence of a neutron halo with radius of about 12 fm.

E1-E2interference in(γ159,n) and(γ209,n) reactions

Angular distributions of fast neutrons from the (\ensuremath{\gamma},n) reactions on $^{159}\mathrm{Tb}$ and $^{209}\mathrm{Bi}$ were measured. Gamma sources in the 7--11.4 MeV range were obtained from (n,\ensuremath{\gamma}) reactions using thermal neu- trons. Pronounced asymmetries around 90\ifmmode^\circ\else\textdegree\fi{} were observed for the angular distributions of photoneu- trons leading to the ground and a few excited states, in both the spherical $^{209}\mathrm{Bi}$ and the deformed $^{159}\mathrm{Tb}$ nuclei. These asymmetries indicate strong and similar E1-E2 interference effects in the two nuclei. The direct-semidirect model was modified to be used for target nuclei with one proton outside an even-even core. From a detailed comparison of the data with calculations using the modified direct-semidirect model, the contributions of different partial waves are estimated. Contributions of the compound nucleus process are included, and are shown to affect to some extent the calculated angular distribution's energy dependence.

Energy distribution of fast neutrons in the F-1 uranium-graphite reactor

Energy distribution of fast neutrons in the F-1 uranium-graphite reactor

Attenuation of fast neutrons in ilmenite concretes.

Energy distributions of fast neutrons (> 0.5 MeV) transmitted through slabs of normal and high density ilmenite concretes, obtained by time of flight methods, are presented for concrete thicknesses increasing in steps of 7.6 cm from 0 to 46 cm. The incident spectrum consisted of a photoneutron continuum from aluminium irradiated with 35 MeV bremsstrahlung, and was thus of somewhat higher mean neutron energy than a typical fission spectrum. The measured distributions are in satisfactory agreement with the results of a Monte Carlo simulation, and results from the latter are used to extrapolate the measured data to greater thicknesses and other ilmenite concrete densities.

Spatial-angular distributions of fast neutrons in iron

The capabilities of the method of reducing the Boltzmann equation to a system of integral equations are inadequate when solving problems in the physics of radiation protection. Attempts to use this method for such purposes havebeen limited by the transport approximation in the representation of the scattering indicatrix [i, 2] although the system of integral equations for any indicatrix was obtained long ago [2]. There is reason to suppose that the method of integral equations can be used to study the detailed characteristics of neutron and T fields which, in particular, makes it possible to obtain the fine structure of angular distribution since it enables an exact solution of the transport equation to be found for the given representation of the scattering indicatrix.