Assuming that the thermal neutron flux through a flat slab can be expressed as the sum of exponential terms, the heating effects due to capture γ-ray absorption are calculated. Curves are presented for total heat absorbed, peak temperature and peak thermal stress for a simple attenuation of neutrons through the slab. The results are applicable to the heating effects in pressure vessel shells.
A method is suggested whereby the energy liberated by the thermonuclear reactions in a stabilized, pinched discharge may be converted directly into electrical power. Physical criteria are derived which must be satisfied by, the discharge. A comparison is made between the direct conversion method and one in which an external heat cycle is used.
The resonance absorption integral of infinitely diluted 238U is calculated between the first resolved level (6·7 eV) and I MeV. The energy-independent parameters Γn0/D and Γγ/Γn0 of the average absorption cross-section are determined from the region of resolved resonances and the nuclear model theories, The effect of l > 0 neutrons is shown to be small but significant. The total value of 277 ± 10 per cent barns is in good agreement with experiment.
The branching ratio, β−/EC = 5·1 ± 0·1, for 242mAm decay has been measured by determining the relative amounts of the decay products, 242Cm and 242Pu formed. The 16 hour 242mAm was produced by neutron irradiation of 241Am in the Materials Testing Reactor. A pile neutron cross-section of 620 ± 65 barns for the reaction 241Am(n, γ)242mAm was measured.
The gamma radiation which usually accompanies a flux of fast neutrons makes it difficult to measure the fast neutron dose. Hurst has proposed a small tissueequivalent chamber for this purpose which makes use of a great difference in ionizing power between Compton electrons and recoil protons. Small voltage pulses from the former are rejected by a discriminator, and the total ionization represented by the larger pulses from proton knock-ons is measured by a special pulseamplitude integrator. A calculation of the integrator loss is given. (A.C.)
The preparation of a very uniform fine-grained photographic emulsion is described. Good reproducibility of the emulsions is obtained by applying a potentiometer method of control during the emulsification process. The emulsion sensitivity can be varied so that charged particles from heavy fragments to relativistic particles can be recorded with a grain density of 60 grains per 100 μ of track.
The degree of mutual screening by resonance absorbing slugs or layers of fuel is calculated for the case where the thickness of moderator between adjacent elements is comparable with or less than the neutron mean free path in the moderating material. It is assumed that the fuel elements are small and that the absorption cross-section follows the Breit-Wigner law. Computations are made for a system of parallel plates; they provide a check on the accuracy of Petrov's earlier formulae, and lead to an expression for the screening coefficient of a nearly homogeneous system. The normally justifiable approximation of replacing the true cell in a lattice of cylindrical elements by a supposedly equivalent cell of circular section is shown to introduce substantial errors in problems of this type. An explicit formula is derived for the resonance self-absorption in elements of annular cross-section.
The dosage attenuation in water of γ-rays from 198Au, 60Co and 24Na has been measured as a function of the source-detector distance over a range from some 3–4 to about 8–12 scattering mean free paths. The primary radiation was collimated in such a direction that none fell on the detector, down to an angle which was varied between 30° and 80°. Theoretical intensities have been calculated by an approximate procedure that replaces the direct solution of the kinetic equation by the reduction of a triple integral and are in satisfactory agreement with observation.
Neutrons from photonuclear reactions in either heavy water or beryllium were slowed down in paraffin, Beta counters were used to determine the beta activity induced by slow neutrons in targets of rhodium or silver. The number of 2.5-Mev gamma rays emitted by La/sup 140/ per second was determined by comparing the photonuclear effects induced in heavy water by samples of La/sup 140/ and RdTh. (A.C.)
The energy dependence of the amplitude of synchrotron oscillations caused by fluctuations in radiation is investigated. A simple stationary solution for the phase equation is obtained which, in practice, is valid with sufficient accuracy for all large synchrotrons. It is shown that if electrons are accelerated up to several BeV the admissible frequency is subject to a sharp upper limit determined by the maximum amplitude of the accelerating voltage.
Specimens of an alloy containing 9 wt per cent of molybdenum were irradiated. The electrical resistance was measured and the structure examined by X-ray diffraction and metallographic techniques. The specimens were prepared by suitable heat treatment to have eutectoid α + γ′ structures with various degrees of dispersion. It was established that the rate of homogenization of these two-phase structures during irradiation was inversely proportional to the square of the thickness of the laminae in the eutectoid. In the homogeneous γ-phase changes in structure and properties under irradiation are essentially complete in 2–4 hr. The ordered γ′-phase was found to be disordered after a few hours of exposure to neutrons.
A systematic study was made of the brightness of light flashes induced in helium and mixtures of helium with other gases by alpha particles emitted from polonium. The dependence of the light output on pressure and on the concentration of different impurities in the helium was determined. (A.C.)
In this review, a method of determining effective boundary conditions (EBC) is described which guarantees the asymptotic agreement of the solution of the neutron diffusion equation with the solution of the corresponding kinetic equation. For monoenergetic neutrons, the EBC are considered for plane and cylindrical surfaces of “black” and “gray” bodies. Results are given for the EBC for the case of cylinders of arbitrary section.
The cross-sections for neutron capture in 238U and 232Th have been measured by an activation method for mono-kinetic incident neutrons in the energy range 30–1000 keV.
