Burst Of Fast Neutrons Research Articles

Rapid Annealing in Advanced Bipolar Microcircuits

Several new device technologies were characterized for rapid-annealing following a burst of fast neutrons. The annealing responses for the breakout devices were found to be consistent with those predicted from existing models. An analytical expression derived from the model is presented. Several of the breakout devices studied indicated that digital integrated circuits produced from some of the technologies studied could have neutron hardness levels in excess of 1×1015 nvt (E > 10 keV). In addition the neutron response data for FAST and ALS nand gates are presented.

The U. K. Spallation Neutron Source

Pulsed neutron sources offer an attractive route for the realization of effective fluxes greater than those currently available from high flux reactors. The spallation neutron source now under construction at the Rutherford Laboratory will produce intense bursts of fast neutrons through interactions of 800 MeV protons with a heavy metal target. The fast neutrons are slowed down in nearby hydrogenous moderators viewed by some 20 time-of-flight neutron scattering instruments. The spectrum of the moderated neutrons is strongly enhanced in the high velocity region compared with that from a reactor. The new source will be comparable with the best beam reactors for experiments with neutrons of mid-thermal energy, and will provide unrivalled potential for use of the epithermal neutrons. Areas of science that will benefit immediately are the study of liquids and amorphous materials, high energy excitations in crystalline materials, molecular spectroscopy, surface phenomena and kinetic processes, as well as a range of crystallographic applications.

Measurement of the 2200 m/sec neutron-proton capture cross section

The neutron-proton capture cross section at the neutron laboratory velocity of 2200 m/sec has been determined from the time decay of the thermalized neutron population following short bursts of fast neutrons in water samples of widely varying sizes. Use of an intense pulsed neutron beam enables elimination of many of the problems encountered in earlier experiments. The present result for this cross section is 332.6\ifmmode\pm\else\textpm\fi{}0.7 mb, which is the most precise of any result obtained by this method and is comparable in accuracy and consistent with the most accurate values determined by any method.NUCLEAR REACTIONS $p(n, \ensuremath{\gamma})d$, $E=0.0253$ eV; measured lifetimes of thermal neutrons in water samples of various sizes; deduced ${\ensuremath{\sigma}}_{a}$.

PULSED NEUTRON METHOD FOR URANIUM WELL LOGGING

The physical principles of a new method for uranium detection based on use of a pulsed source of neutrons are presented. The method employs measurement of the neutron time distributions in rock media following a burst of fast neutrons. A time‐ and energy‐dependent (but space‐independent) theory is developed for the four principal time distributions that follow production of fission neutrons by each of the two possible reactions: 1. Prompt thermal fission of [Formula: see text]: a. Epithermal neutron time distribution, b. Thermal neutron time distribution. 2. Delayed fission of uranium: a. Neutron time distribution due to delayed fission in [Formula: see text], b. Neutron time distribution due to delayed fast fission in [Formula: see text]. Theoretical analysis of these phenomena leads to the conclusion that the detection of uranium in rocks is possible using either the epithermal neutron time distribution from the prompt fission of [Formula: see text] by thermal neutrons or the delayed neutron distribution from fast neutron fission of [Formula: see text] or thermal neutron fission of [Formula: see text]. Experiments performed on laboratory models of uranium ore show the feasibility of detecting fission neutrons and the validity of the theory presented in the paper. For normal borehole‐logging measurements, a pulsed neutron source with an average output of about [Formula: see text] n/sec is needed.

Neutron thermalization measurements in graphite by the reaction rates method, using a Ge(Li) gamma-ray detector

The thermalization of a burst of fast neutrons after injection into a graphite stack of dimensions 101 × 101 × 81 cm 3 has been studied by the reaction rates method. The Cd/Hg reaction ratio was measured in the centre and near one face of the stack over the period 90–1290 μus after neutron injection. Characteristic prompt gamma rays from both indicator materials were detected simultaneously using a small Ge(Li) detector placed within the stack. The results compare favourably with previous theoretical determinations of the time-dependent Cd/Hg ratio and with ratios calculated from time-dependent differential neutron spectra.

Measurements of Neutron Slowing Down Time in Graphite by Capture Gamma-Ray Detection

The neutron slowing down time in graphite was measured by means of capture γ-ray detection. Bursts of fast neutrons produced by D-T reaction were injected into a graphite pile. Detection was by Nal(Tl) scintillator covered by metallic foil of either In, Cd or Gd, serving as resonance absorber. The slowing down time was determined from the maximum point of the reaction rate registered by this counter and presented in the form of time spectrum. The values thus determined were 19.2±0.6, 39±3, 43.0±0.7, 46.1±1.0, 47.5±0.8, 55±2 and 35±6 μs, for 1.58, 0.48, 0.44, 0.40, 0.36, 0.30 and 0.17eV respectively. These values agree well with calculated based on crystal model. The time response of the resonance detector also was in reasonable agreement with calculation. The effect of the medium being finite and that of detector position on the observed slowing down time were estimated. From this experiment, it became clear that the process of the neutron slowing down shows some definite influence of the crystal effect o...

Transient neutron spectra and the slowing down relaxation time in beryllium

The Boltzmann equation in the diffusion approximation has been numerically solved by the time-step method to determine transient neutron spectra, following a burst of fast neutrons inside beryllium assemblies of various sizes. Such a study also gives information about the decay constant of the fundamental mode and the slowing down relaxation time. We find that for B2 < 4 × 10 −2 cm 2̄, the decay constant as well as the neutron energy distribution at long,times agrees well with the corresponding quantities calculated by an iteration procedure. However, for larger B 2 up to the time investigated (600 μsec after the initial burst) equilibrium is not established. Timedependent neutron spectra have been compared with the measurements of GAERTTNER et al. The slowing down relaxation time for an infinite assembly comes out to be 114 μsec. This result is compared with the measurements of ZHEZHERUN et al. who report a considerably higher value: 185 ± 20 μsec. The possible causes of this difference are discussed.

Harmonic free determination of the thermal neutron absorption cross section of hydrogen by a pulsed source technique

A method, based on Fourier analysis of harmonics, has been developed for measuring the rate of decay of the fundamental mode of the thermal neutron density distribution which is established in a cylinder of water after thermalization of an injected burst of fast neutrons. Measurement of this decay for a range of water depths enables determination, by extrapolation, of the decay constant vΣa of an infinitely large system. The 2200 m sec-1 thermal neutron microscopic absorption cross section of hydrogen has thus been measured. The value obtained, 326 ± 3 mbn, is in good agreement with that obtained by most previous workers by a variety of other methods. The thermal neutron diffusion coefficient was also measured, although the experiment was not optimized for this. The value obtained, D = 35 200 ± 400 at 22 °C, also agrees very well with previous results.

A Nearly-Four-Pi Liquid Scintillation Counter for Bursts of Fast Neutrons

This liquid scintillation counter detects 14 mev neutrons by both proton-recoil and neutron-proton capture processes. It is designed to measure fast neutrons which occur in a single burst of a few microseconds duration, a burst which may contain anywhere from 100 to 100,000 neutrons. The counter is 36 inches in diameter, 36 inches long, and has an axial access cylinder 12 inches in diameter. It contains 475 liters of triethylbenzene, 2.5 grams per liter of p-terphenyl and 0.02 grams per liter of POPOP. Scintillations are detected by 20 Dumont Type 6364 multiplier phototubes. Four tubes are used as proton-recoil detectors and 16 are used as neutron-proton capture detectors. The counter was completed in January 1959, and has shown high reliability and only a relatively small change in sensitivity since it was first calibrated.

Time Dependent, Space Independent Neutron Thermalization

The time dependent thermalization following a burst of fast neutrons is analyzed in the space independent case. The spectrum is decomposed in an asymptotic and a transient part, and the latter is expanded in a series of Laguerre polynomials. The argument of the polynomials is proportional to the time after the burst and the coeffi cients of the expansion are linear combinations of the transient time moments. (auth)

An Efficient Counting System for the Detection of Neutrons from Low-Yield Pulsed Neutron Sources

ABS>This system uses a large organic scintillator as a moderator for a burst of fast neutrons, many of which are subsequently captured by the hydrogen in the scintillator. The pulses produced by the 2.2-Mev capture gamma rays are observed by four photomultiplier tubes whose anodes are paralleled. The output pulses are amplified and counted by a 10 Mc scaler. The scaler is gated to count for 300 mu sec after the pulse, during which interval background is very small. Statistically significant information on total neutron output may be obtained for as few as lO/sup 3/ neutrons per pulse, with practically no upper limit. Relative calibration of the system is simple, and absolute calibrations are stable and reproducible. (auth)

The Time-Dependent Thermal Neutron Flux from a Pulsed Subcritical Assembly

A two-group analysis has been applied to the transient behavior of the thermal flux in a subcritical assembly which has been given a short burst of fast neutrons. This analysis has been applied to ...

Theory of Pulsed Neutron Experiments in Multiplying Media

AbstractThe spatial and temporal distribution of thermal neutrons in a multiplying assembly following the introduction of a short burst of fast neutrons is investigated by means of an extension of the so-called “asymptotic reactor theory” to the time-dependent case. It is shown that the solution for an nth mode fast neutron source can be reduced to that for an nth mode thermal neutron source, so that only the latter need be considered. A formal solution to the time-dependent thermal diffusion equation with an nth mode thermal source is found for an arbitrary slowing-down kernel. The asymptotic behavior of the flux in the long-time limit is shown to be exponential, with a decay constant satisfying a generalized material buckling equation. The asymptotic behavior following a burst of fast neutrons is also found to be exponential with the same time constant. In a continuous slowing-down model, all neutrons slow down in the same time, implying that the time-dependent part of the time-dependent slowing-down ke...

An Apparatus for the Direct Determination of Slow Neutron Velocity Distributions

Apparatus is described by means of which a modulated source of slow neutrons is produced and their intensity determined as a function of time. The deuteron beam of a high voltage tube is deflected by a modulated electrostatic field in such a way as to produce square bursts of fast neutrons of any desired length. After being slowed by hydrogenous material a selected beam of neutrons falls on a suitable detector at a measured distance from the source. Pulses from the detector are impressed on an oscilloscope screen, together with a sharply defined time scale and photographic records made. The time analysis of the neutron distribution is then made by a numerical count of the neutron pulses as a function of the time interval in which they occur.