High Energy Neutron Flux Research Articles

Energy spectrum and flux of high-energy neutrons at balloon altitudes

The energy spectrum and flux of neutrons from En ≃ 2 MeV to En ≃ 75 MeV were measured with organic liquid scintillators near the top of the atmosphere during a balloon flight in 1973 at 4.2–2.9 g/cm² over Palestine, Texas (λ = 42°N). Two cylindrical liquid scintillators of dimensions 5 cm × 5 cm and 12.5 cm × 12.5 cm filled with NE 213 were used to cover the neutron energy range from 2 to 75 MeV. Pulse shape discrimination on each detector separated the protons and alphas produced within the scintillator by neutron interactions from the electrons produced by gamma rays. The response functions for monoenergetic neutrons up to En = 75 MeV were determined from acclerator calibrations. The neutron leakage current from 2 to 10 MeV is given by R = (0.25 ± 0.05)E−1.8±0.1 neutron/cm² s MeV, and at 20, 30, 50, and 75 MeV the leakage rates are (1.6 ± 0.4) × 10−3, (1.15 +0.20−0.25) × 10−3, (8.0+2.0−1.4) × 10−4, and (6.3 ± 1.3) × 10−4 neutron/cm² s MeV, respectively.

Thorium-uranium fission radiography

The high energy secondary neutron flux, produced by stopping the 650 MeV proton beam of the Los Alamos Meson Physics Facility (LAMPF), has been tested for use in measurements of the microscale distribution of Th in heterogeneous samples, particularly rocks. Polished samples are covered with mica which serves as a detector for neutron-induced fission tracks. The sample is also irradiated in a nuclear reactor in order to measure the U distribution and to determine the correction required for U fission in the high energy exposure. The relative Th/U fission rates range from 0.14 to 0.31, depending on the amount on internal neutron moderation within the samples. The relative 232Th/ 238U fission rate, as calculated from the theoretical beam stop spectrum is somewhat higher (0.5), probably reflecting a contribution from low energy 235U fission in the observed ratio. The measured relative 209Bi/ 232Th fission rates are higher by a factor of 2–7 than those calculated from the theoretical neutron spectrum indicating that the flux of high energy (200 MeV) neutrons has been underestimated in the theoretical spectrum. Th fission track production rates of (0.5–2) × 10 4/cm 2·mA·h·ppm Th were measured, with the lower values being more in accord with those calculated from the theoretical spectrum. The technique is limited by background from recoil tracks in the mica. The relative rate of fission to recoil is ∼3 × 10 −6 fission/recoil·ppm Th. However, by differential annealing of the recoil tracks, we are able to count fission tracks in the presence of a large background of recoil tracks. In order to measure the Th concentration in individual grains to ±10% in 10 h exposures, the product of the Th concentration in ppm and the area of the grain in cm 2 is ∼2 × 10 −3, i.e., for 1 ppm Th roughly 400 μm grains are required.

Production of strong, high-energy neutron fluxes in a cyclotron by irradiating thick lithium and beryllium targets with 22-MeV deuterons

Production of strong, high-energy neutron fluxes in a cyclotron by irradiating thick lithium and beryllium targets with 22-MeV deuterons

Neutron irradiation effects in LSA oscillators

Four high peak power LSA diode oscillators were exposed to high-energy neutron flux in successive steps. Oscillator failure resulted from a decrease in the diode breakdown voltage to the point that breakdown occurred at threshold for fluences of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> neutrons/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The reduction in breakdown voltage and an observed frequency increase at a fixed bias can be related to an increase in the diode low field resistance which results primarily from neutron induced carrier removal.

Weakening of the flux of high-energy neutrons in a cylindrical channel

Weakening of the flux of high-energy neutrons in a cylindrical channel

Solar neutron flux in the energy range 20–160 MeV

An attempt to measure the flux of high-energy solar neutrons was made by measuring the difference in flux from the direction of the sun and from the symmetrical direction about the zenith, using oriented nuclear emulsions flown by balloon on July 30, 1966 from Fort Churchill, Manitoba.An excess of (2.2 ± 2.5) × 10−2 neutrons cm−2 s−1 was observed from the direction of the sun in the energy region of 20–160 MeV. On the day of the flight the sunspot number was 63, and no major solar flares were reported.

High energy neutron flux detection with dielectric plastics

Abstract Dielectric plastics were irradiated with high energy neutrons to investigate their usefulness as flux detectors. Shields thick to heavy particles and of varying composition were placed in front of the plastics to test their effectiveness as radiators. Track producing particles resulted from (n, 4He) reactions and from recoils of light nuclei. Elements possessing high (n, 4He) cross sections were chosen as shields along with other heavy elements chosen to screen the surface of the plastic from all external track-producing particles. Carbon and aluminum satisfied the first condition while gold and lead satisfied the second. After etching, the track densities in cellulose nitrate were counted. The sensitivity under graphite varied from about 2.7 × 10−5 to 3.4 × 10−5 tracks/neutron for neutrons between 13.1 and 17.1 MeV. Useful track densities are produced by fluxes in the range of about 2 × 108 to 5 × 1010 n 0/cm2. The 12C (n, n'3 4He) reaction also took place in the carbon nuclei within the plastic...

A search for high-energy solar neutrons

Two stacks of G-5 emulsions on a directional device were exposed to cosmic rays at a ceiling altitude of 114 000 ft on July 30, 1966, at Fort Churchill (geographic latitude 58.8 °N and longitude 94.1° W). An attempt to measure the flux of high-energy solar neutrons has been made by inspecting the difference in flux from the direction of the sun and from the symmetrical direction about the zenith. The flux of neutrons has been determined by making observations on the tracks of protons recoiling from elastic collisions of neutrons with hydrogen nuclei present in the emulsions. The preliminary result of the experiment has shown that in the energy region of 20–100 MeV an excess of 1.1 × 10−2neutrons cm−2 s−1from the direction of the sun was observed. On that day the sunspot number was 63, and no major solar flare was reported.

Moderation of a high-energy neutron flux by heterogeneous shielding

Moderation of a high-energy neutron flux by heterogeneous shielding

The attenuation of a high-energy neutron flux in shielding

The attenuation of a high-energy neutron flux in shielding

Beam-tube design for the high-flux beam reactor

The Brookhaven High-Flux Beam Reactor (HFBR) is being constructed primarily to satisfy a need for neutron beams that are more intense than those available from other research reactors. The design principles chosen have also made it possible to provide neutron beams that are tailored to the requirements of individual experiments. That is, each beam will emphasize some region of the neutron energy spectrum, with an associated reduction of emission of neutrons of unwanted energies, and also of γ-rays. Special discrimination is provided against the very high-energy neutrons which are health hazards, and which add to background problems. The reactor is not designed solely for neutron beam users. Two irradiation thimbles will make possible the exposure of small samples (such as single crystals) to an intense and relatively pure flux of high-energy neutrons. These facilities will permit radiation damage studies at temperatures near that of liquid helium. Two irradiation thimbles in a region of somewhat poorer quality neutron flux can also be used in experiments of this kind. Three isotope irradiation facilities will be placed in regions where the thermal neutron flux is large and the fast flux contamination is small. To a greater extent than has been common in the past, the design of this reactor is based on experiment. Critical experiments have not only established the necessary core size, but they also have been relied on for much additional information normally found by theoretical methods. Experiments have been especially valuable in setting the design of the experimental facilities. The combination of unusual neutronic properties and of complex geometry made it necessary to make this departure from customary design practice.

Dosimeter for High-Energy Neutrons

A neutron detector with a threshold at 20.6 Mev is described which is based on the C12(n,2n)C11 reaction in liquid organic scintillators. The efficiency of the detector is almost independent of neutron energy from ∼40 Mev to ∼400 Mev. The detector can be used to measure with an error of 6% high-energy neutron fluxes in the tolerance range (20–50 n/cm2 sec).