Fast Neutron Source Research Articles

An update on (n,charged particle) research at WNR

Neutron-induced reactions producing light charged particles continue to be investigated at the spallation fast-neutron source at the Los Alamos Neutron Science Center (LANSCE). New data on the cross sections for a-particle production for neutrons on 58Ni and 60Ni are presented from threshold to 50 MeV. Recent changes in the experiment now allow protons, deuterons, tritons, 3He and α-particles to be identified.

97/02844 Operational experiences of fast neutron source reactor ‘YAYOI’ as a pulsed fast reactor

97/02844 Operational experiences of fast neutron source reactor ‘YAYOI’ as a pulsed fast reactor

Studies of neutron dosimetry

A high-purity, intense, fast-neutron source has been developed at our laboratory. We describe two inexpensive, straightforward methods of determining the total number of neutrons produced and we review precautions to be taken in applying these methods.

A new fast-neutron source for radiobiological research

A biomedical cyclotron facility primarily dedicated to radionuclide production has been extended by the addition of an experimental fast-neutron source for radiobiological and biophysical studies. Several beams of fast-neutrons with different average energies and LET distributions can now be provided. The neutrons are produced by bombarding berryllium targets with 18 - 32 MeV protons or 8.7 - 15 MeV deuterons from our K = 32 negative-ion cyclotron. Average neutron energies range from approximately 4 to 15 MeV. Doses at maximum build-up vary from 0.24 to at 1 m SSD, i.e. approximately at of proton current at maximum energy. The design of the facility and some dosimetric results are presented.

A rather intense source of fast neutrons for small scale activation

A thick beryllium target has been developed for installation onto a small proton cyclotron (CTI RDS 112), providing an intense source of fast neutrons for activation purposes. Carefull attention was paid to cooling and irradiation geometry, allowing cubic centimeter scale samples to approach the proton beam strike surface to within 5 mm. This proximity assures ≈ 10 11 neutron/s passing through the irradiated sample when 20 μA of 11 MeV protons are incident on the beryllium primary target.

In-Situ Tritium Release Experiment from Molten Li2Bef4 Salt Under Neutron Irradiation at Elevated Temperatures (Intrexflibe)

The 2:1 mixture of LiF and BeF{sub 2} (Flibe) is a potential liquid tritium breeding material for fusion reactors, because of low electric conductivity, high chemical stability in air, etc. We have been studying tritium release behavior from Flibe by in-pile tritium release experiment in the fast neutron source reactor `YAYOI` of the University of Tokyo. About 100g of Flibe was utilized for neutron irradiation to produce tritium, and the tritium released from the specimen was swept by making purge gas flowing over the specimen. Tritium release rate increased with elapsed time after the start of irradiation and a steady state was attained in about four hours at 873K in case of H{sub 2} purge gas. Released chemical forms of tritium were HT and TF, and their proportions depended not on the kind of container and tubing materials, but on the chemical composition of purge gas and the dehumidification time of specimen at high temperatures. Tritium generated as T{sup +} in Flibe was released by way of two routes; (1) direct release to purge gas as TF and (2) release to purge gas after converted to HT by the isotopic exchange reaction with H{sub 2}. The reaction rate was controlledmore » by the H{sub 2} concentration and F{sup -} potential in the system. In case of high hydrogen concentration and low F{sup -} potential in the system, tritium was released as HT with a relatively high rate. In case of high F{sup -} potential in the system, on the other hand, tritium was released as TF with a low rate. 5 refs., 7 figs., 1 tab.« less

Neutron Moisture Meter Measurement in Waste Packages

A neutronic method based on neutron therinalization by hydrogen nuclei is used to measure the moisture content in packages of radioactive waste embedded in hydraulic binders. The two steps of the measurement are (a) acquisition of the neutron characteristics of the embedded waste considered (or of a chemically similar material) and (b) the measurement itself obtained with a neutron moisture meter. The neutron characteristics required are the adsorption and diffusion cross sections Σa and Σd for thermal neutrons of the dried material. These two parameters are used to calculate the calibration curve (valid only for the material considered) as follows: N = (α. Ds + β) Hv + γ. Ds + δ, which allows the counting N of the neutron moisture meter to be converted into free-water content Hv (α, β, γ, and δ are deduced from Σa and Σd,- Ds is the dry density of the material). The neutron moisture meter (containing a fast neutron source and a thermalized neutron detector) is portable. Measurements are taken at various depths in a core hole made in the package to draw a water profile. The measurements are taken in materials used for waste solidification and in active or inactive packages. The results obtained (free-water content) are in good agreement with those obtained by determining the weight loss at 120° C (the differences between these two measurements are generally ∼ 10% when the free-water content is ∼20 to 25 %). The water profiles allow one to detect the presence of excessive free water.

Characterization of the YAYOI fast neutron radiography field

Fast Neutron Radiography (FNR) has been studied with the use of the fast neutron source reactor YAYOI of the University of Tokyo. Three kinds of imaging techniques have been applied to the FNR. A CR39 nuclear track detector called the track-etch method has been successfully used in the first FNR experiments at YAYOI. The etched out images on the CR39 sheet usually have a low optical density, and took about 24 hours to get images. In order to perform an imaging technique with a high resolving power and high contrast, a thin type of luminescent converter and X-ray film method (film method) has been developed for the FNR. The converter is made of a mixture of PMMA and ZnS(Ag). the thickness of which is 40 μm. The system could observe a gap of 100 μm, and took about 3 hours to get results. To make the required imaging time shorter, a television system of fast neutron radiography (FNR-TV) was proposed. In the system, the same technique as a current thermal neutron television system was applied while using a different luminescent converter. The prototype converter was made of a mixture of polypropylene resin and ZnS(Ag). A method of determining L and D was proposed in this paper. Experimental result of L agreed with the geometric dimension of L, while the value of D will be discussed in another paper.

Correction of fast neutron scattered components from fast neutron radiography images

Fast Neutron Radiography (FNR) results have been successfully obtained by the use of the fast neutron source reactor YAYOI of the University of Tokyo. However, an effect of contrast distortion in the FNR images was observed. The intensity of scattered fast neutrons at the center of an object image was analyzed with a simple model of an iron cylindrical sample. Analytical results agreed well with experimental results.

Designs and experiments for studies of fast neutron fields at the RB reactor

Abstract The RB reactor is a heavy water critical assembly that has been in operation since 1958 using, at different times, natural metal uranium, 2% enriched metal uranium, and 80% enriched uranium dioxide fuel of Soviet origin. A feasibility study of the RB reactor as a fast neutron source began in 1976, and four versions of fast neutron fields around or in the reactor were designed through 1990: an external neutron converter (ENC) in 1976; an experimental fuel channel (EFC) in 1982, an internal neutron converter (INC) in 1983, and a coupled fast-thermal core (HERBE) in 1990. This paper presents an overview of the characteristics and experimental applications of each particular fast neutron field mentioned above, including available irradiation space, neutron spectra, and equivalent neutron and gamma dose rates. Control and safety-related implications of these modifications are emphasized. The computer codes and nuclear data libraries used in calculations are described briefly.

Effects of Heavy-Ion Irradiation on Microstructure of V-4Cr-4Ti Alloy at Moderate Temperatures

AbstractV-4 wt.% Cr-4 wt.% Ti alloy is a promising candidate material for first-wall and structural applications in magnetic fusion reactors. In the past, fast neutron sources were used to evaluate postirradiation properties of fusion candidate materials. The recent shutdowns of the Fast Flux Test Facility (FFTF) and the Experimental Breeder Reactor (EBR-II) left U.S. researchers without local experimental facilities for such tests. Under such circumstances, ease of experimental control, availability, and relatively low cost make heavy-ion irradiation an attractive alternative, provided its limitations are appreciated. We selected 3-MeV V+ and 4.5-MeV Ni++ ions to investigate the effects of irradiation on the microstructure of V-4Cr-4Ti alloy in the temperature range of 200 – 420°C. The main interest is in the evaluation of this alloy's dimensional stability and susceptibility to irradiation embrittlement. In this paper, we report results of ion irradiation experiments and compare them with available data on fast-neutron irradiation. From transmission electron microscopy (TEM) analysis of ion-irradiated specimens, we found that the dominant feature of the postirradiation microstructure was a high density of dislocation loops and pointdefect clusters. Density and defect size depended on irradiation dose and temperature. Precipitates and voids/bubbles were not observed, even in specimens that were simultaneously injected with He and exposed to heavy-ion irradiation. Increased transport of point defects to internal interfaces was observed, as manifested by defect denuded zones along grain boundaries, Defect denuded zones along grain boundaries could lead to segregation of impurities and solutes and formation of precipitates on grain boundaries.

A study on the mechanism model of luminescence in fast neutron television converter

For a real time imaging technique using a fast neutron radiography (FNR), a fast neutron television system (FNR-TV) was studied using the fast neutron source reactor YAYOI at the University of Tokyo. In order to obtain a good quality image with the FNR-TV, it is essential to develop a high sensitive converter to detect fast neutrons. A sensitive prototype converter made of polypropylene (PP) resin and ZnS(Ag) mixture was recently developed. It was called a PP converter. In this paper, the mechanism models of luminescence for fast neutrons and gamma-rays in the PP converter are presented as well as the comparison with the experimental results. The analytical results agree well with the experimental results.

A study on the development of a fast neutron television converter

Experiments on fast neutron radiography (FNR) were carried out using the fast neutron source reactor YAYOI of the University of Tokyo. For a real-time imaging technique using FNR, for a fast neutron television system (FNR-TV), it is essential to develop a highly sensitive luminescent converter in order to obtain good quality images. A new converter was recently developed and tested. It showed an about six times higher sensitivity compared with the first prototype converter. In this paper, the characteristics of the converter are discussed as well as the clearer images of a FNR-TV when using the converter. The FNR-TV images were obtained with 500 frames integrated. A small hole (1.0 mm in diameter and 10 mm in depth) in an acrylic plate was well imaged through a 50 mm thick iron block. A 0.15 mm thick aluminum spacer in the calibrated fuel pin was also discernible.

Television Imaging System for Fast Neutron Radiography Using Baby Cyclotron.

Abstract A television imaging system for fast neutron radiography (FNR-TV) developed using the fast neutron source reactor YAYOI was applied to the baby-cyclotron based fast neutron source to get images of thick objects quickly. In the system the same technique as a current television imaging system of thermal neutron radiography was applied, while the luminescent converter was used to detect fast neutrons. Using the CR39 track etch method it took about 7 h to get an image, while the FNR-TV only 20 s enough for taking the same object. However the FNR-TV imaging result of the simulation model of a large explosive device for the space launch vehicle of H-2 type was not so good as the image taken with the CR39 track etch method. The reason was that the luminescence intensity of the FNR-TV converter was a quarter of that in the YAYOI.

A theoretical model of neutron thermalization in a medium

A simple theoretical model is developed to explain neutron thermalization properties. Assuming that a fast neutron source is located at the origin, the neutron distribution is described as a function of distance from the source. Based on the continuous slowing-down model, it is shown that the thermal neutron flux is a simple function of the diffusion constant D, the reciprocal length κ, and the Fermi age τ of a moderator material. Several points are noteworthy from calculation of the thermal neutron flux. First, theoretical results for polyethylene moderator agree remarkably well with simulation data obtained from a Monte Carlo simulation. Second, the thermal neutron flux at the origin is proportional to the square of moderator density. Third, the volume of the large thermal neutron flux increases drastically as the Fermi age increases. Finally, we also note that theoretical results agree reasonably well with experimental data for a broad range of physical parameters. A simple analytical expression of the thermalization factor of a moderator material is derived and it is found that its theoretical values agree rather closely with measured values.

Investigation of the neutron detection properties of the fast liquid scintillator BC-505

The energy resolution, the absolute detection efficiency, and the pulse-shape discrimination capability for fast neutrons by the fast liquid scintillator BC-505 have been investigated. The response to fast neutrons was investigated using the proton-tagged neutrons from the d-d reaction generated by the Colorado School of Mines low-energy charged particle accelerator. Using a collimated source of gamma rays from a 60Co source, the spatial resolution of the scintillator was measured by comparing the arrival times of the scintillation signals at the PMTs at the ends of the scintillator cell. A spatial resolution of 2.7 cm was measured corresponding to a total time resolution of about 150 ps. The results of our investigation will be applied to the conceptual design of a ‘‘pin-hole’’ neutron camera which may be used to provide real-time radial distributions of fast neutron sources in fusion plasmas.

Advantages and limits of 14-MeV neutron radiography

The paper evaluates the potentials of fast-neutron radiography (FNR) for the inspection of bulky, solid objects. Data for both a fast ( E n = 14.7 MeV) and a slow ( E n = 0.1 eV) neutron source are compared. The reproduction of images consists of Monte Carlo simulations of (a) the neutron random walk in a slab (iron, SiC ceramic, and polyethelene (CH 2) n plastic) with a void, (b) the process of neutron recording in a detector, and (c) a print-out of images. For a general analysis, 3D-MCSC-RWR software operates without simplification of either the FNR design or the nuclear data files. The results first show the feasibility of the use of 14-MeV neutron radiography, then the superiority of FNR over slow-neutron radiography in-the-field when the thickness of the full slab is over 1 cm and requires a resolution better than 0.1 mm. Examples of some numerically simulated images as well as FNR scaling functions are shown. A review of the available fast-neutron sources reveals that only plasma-focus machines would simultaneously meet all FNR requirements: Y n ≥ 10 13 n/pulse, small-source dimensions and mobility.

Neutron diagnostics of the premises of the destroyed unit at the Chernobyl nuclear power plant by use of CR-39 track detectors and boron converters

Abstract Neutron detectors containing boron as a converter, CR-39 plastic as a track detector and water as a neutron moderator have been applied for the neutron diagnostics of the premises of the destroyed unit at the Chernobyl nuclear power plant. Theoretical (3.8 × 10 -2 tracks per neutron) and experimental (5.1 × 10 -3 tracks per neutron) assessments of the detector sensitivity to thermal neutrons were carried out. Values of the neutron flux density within a range of 1–92 neutrons cm -2 s -1 were detected on the premises of the destroyed unit. It is assumed that the (α, n) reactions for oxygen and spontaneous fission are the most effective fast neutron sources within the unit.

Investigation of the Radiation Shielding Capacity of Asphalt and Sand for Fast Neutron Sources

The internal wall of the cavity of experimental assemblies, involved with high energy fusion neutrons, can be covered by an asphalt-sand layer for radiation protection purposes. The calculations have demonstrated that asphalt has a radiation protection capability superior to that of concrete, on equivalent weight basis.For an experimental cavity using a 1012 neutrons/sec neutron generator, the best structure has been found to be a 10% asphalt - 90% red sand homogeneous mixture, against neutron + gamma energies, with a protection capability of more than 200% compared to the same thickness of a concrete structure.This work has proved that the local and cheap asphalt and sand can replace the relatively expensive concrete in constructing biological radiation shielding of experimental assemblies on fusion neutronics.

Neutron-activation logging for aluminum and silicon with a fast-neutron source

Neutron-activation logging for aluminum and silicon with a fast-neutron source