Isotropic Neutron Source Research Articles

The flux and source of energetic protons in Saturn's inner magnetosphere

The flux of energetic protons in Saturn's inner magnetosphere was observed in two channels from 48–63 and 63–160 MeV. Absorption features caused by the G ring and the satellites Enceladus and Mimas were easily identifiable. The flux observed in the absorption slot of Mimas can be maintained by the decay of a cosmic ray albedo neutron flux of 7×10−3 cm−2 s−1 sr−1. This flux is entirely consistent with calculations of the neutron flux produced by galactic cosmic ray interactions with the rings of Saturn. The omnidirectional proton flux of 8.2×10³ cm−2 s−1 at 2.734 Rs requires a residence time of 30 years. Both the residence time and the energy spectrum are comparable to those found in the inner radiation belt of the earth. The angular distribution is nearly isotropic in the Mimas slot and beyond 4 Rs. Otherwise the pitch angle distribution is pancake and has been approximated by sinn θ with n in the range 2–7. This distribution is consistent with an isotropic neutron source in the ring plane.

First-Flight Neutron Collision Probabilities in an Annular Region Containing Fuel Rods

A method for calculating the first-flight collision probabilities in an annular region containing identical cylindrical fuel rods is described. Based on the assumptions of flat isotropic neutron sources and isotropic incidence fluxes on boundary surfaces, 5 of the 15 probabilities in the annular region are calculated. By neutron conservation and the reciprocity theorem, the other 10 probabilities are obtained in terms of the first 5. The results are satisfactory in comparison to other existing theories. The method can be used in the treatment of a cluster-type fuel assembly.

Neutron propagation and 2.2 MeV gamma-ray line production in the solar atmosphere

We have calculated the 2.2 MeV gamma-ray line intensity from the Sun using a Monte Carlo method for neutron propagation in the solar atmosphere. We provide detailed results on the total gamma-ray yield per neutron and on the time profile of the 2.2 MeV line from an instantaneous and monoenergetic neutron source. The parameters which have the most significant effects on the line intensity are the energies of the neutrons, the position of the neutron source on the Sun, and the abundance of He3 in the photosphere. For an isotropic neutron source which is not too close to the limb of the Sun, the gamma-ray yield is between about 0.02 to 0.2 photons per neutron, provided that the neutron energies are in the range 1 to 100 MeV and the ratio He3/H is less than about 5 × 10−5.

Méthode expérimentale de mesure de l'évolution d'une population de neutrons rapides dans des ensembles non multipliants

A method is described which allows the measurement of neutron time and energy spectra. Neutron bursts are produced by a the pulsed beam of a 4 MeV Van de Graaff using the T(p, n) and V(p, n) reactions. The time and energy behaviour of these neutrons in ion multiplying assemblies is followed by means of a stilbene detector placed at the surface of the assemblies, the energy spectrum being obtained by unscrambling the proton recoil spectra. The response function to monoenergetic neutrons has been recorded experimentally between 0.7 and 1.6 MeV; these spectra are considered as basis vectors, the components of a neutron spectrum into this basis are easily obtained with the help of a PDP 9 computer. Different geometries have been experimentally realized in graphite and in lead, the bombarding energy has been fixed to 1.66 MeV. The T(p, n) reaction is utilised for producing a plane wave of neutrons incident on cubic assemblies, the V(p, n) reaction is utilised as an isotropic neutron source located at the centre of spherical assemblies. These simple experimental conditions will make further calculations easier. Experimentally it can be seen that the neutron modes decay monotonically with time, however, in the case of lead the inelastic scattering of neutrons is clearly dominating the low energy groups.

Neutron energy spectra and angular distributions from targets bombarded by 45 mev electrons

The time-of-flight method has been used to measure the energy spectra of neutrons emitted by targets of natural uranium, lead, bismuth and tungsten when bombarded by 45 MeV electrons from the Harwell linear accelerator. The spectra were measured at 90 to the electron beam and covered the energy range 40 keV-6 MeV. For targets 3 cascade shower radiation lengths in thickness, the mean energy of the neutrons was found to be 1·92 MeV for lead and 1·37 MeV for uranium and the relative yield from lead to that for uranium was 0·47 ± 0·05. For targets of lead, bismuth and tungsten the observed spectra could be represented over a large energy range by Maxwellian distributions at temperatures of 0·98, 0·82 and 0·44 MeV respectively. The angular distributions of the neutrons from lead and uranium targets were determined from foil activation measurements using the threshold reactions 31P(n, p) 31Si, 27Al(n, p) 27Mg and 27Al(n, α) 24Na. For a small uranium target forward peaking of the emitted neutrons relative to the electron beam direction was observed. The development of larger targets of uranium and lead which act as isotropic neutron sources for use in reactor physics measurements is described.

Neutron Flux Spectra in Air

Application of the moment method of L. V. Spencer and U. Fano is made to the penetration and slowing down of neutrons in infinite media. General considerations are restricted mainly to those modifications of the method necessary for the neutron problem; in particular, a special technique is given for reducing the integral term of the neutron transport equation. Calculations are carried out for point isotropic monoenergic neutron sources of 1, 100, 500 kev, 1, 2, 5, and 14 Mev in an infinite atmosphere of air. Best available numerical values of the cross sections are used, and the effects of absorption, anisotropic angular distributions, and inelastic scattering are included. Distances out to 10 mean free paths are considered.