Table of contents

Absorption experiments using lead, gold and tungsten have shown that, in addition to the known γ-radiation of 0.77 MeV. energy, polonium emits other (soft) radiations of which the most intense has a quantum energy of 84±4 keV. The intensity of this radiation is of the same order of magnitude as that of the hard radiation (roughly one quantum per 10⁵ disintegrations). Experiments by the recoil method indicate that the emission of the polonium γ-radiation is not delayed by more than 10^-1 sec. The results of Chang (1946) concerning the fine-structure of the α-particles of polonium remain uncorrelated with all other experimental evidence. Whilst present information regarding γ-ray energies and intensities may be reasonably explained, satisfactory explanation of the α-particle fine structure "data" appears as remote as ever.

The experimental data on the daily variation in cosmic-ray intensity are first surveyed. It appears to be established by Rau's experiments that the energetic meson component at sea level shows a marked semi-diurnal variation which is in phase with the semi-diurnal variation shown by the barometric pressure. Several authors have suggested that this phenomenon is explicable in terms of the Pekeris theory of atmospheric oscillations. The implications of this explanation on the process of meson formation are here examined quantitatively. It is concluded that the explanation is only possible if mesons arise mainly at about 60 or 70 km. above sea level, which is highly unlikely since the corresponding cross-section for meson production would be much larger than appears plausible.

In addition the experimental data on the diurnal variation are briefly discussed. This variation seems to be more complicated in character than is usually assumed and not attributable to factors such as a heliomagnetic field and fluctuations in the geomagnetic field, which are often held responsible.

The photographic plate method has been used in a study of the neutrons produced in the disintegration of nitrogen by deuterons.

A single group of neutrons, corresponding to an energy release of (5.15±0.10) MeV., was detected, but the existence of the low-energy neutron group reported by Stephens, Djanab and Bonner (1937) was not confirmed. The new results show that, if this group does exist, its intensity is not more than 15% of that of the principal neutron group, and that no excited levels of the ¹⁵O nucleus less than 5 MeV. above the ground state are effective in this reaction.

The neutron flux from the N+D reaction was compared with that of D+D neutrons produced by disintegration of deuterium present in the surface layers of the target employed. The data obtained were used in determinations of the angular distributions of the D+D and N+D neutron fluxes, and in tests of the reliability of the photographic plate method for relative flux measurements.

During a new investigation of the NO band spectrum many bands of the β and γ system were photographed and analysed, and the rotational constants evaluated. Intensity drops and perturbations were found and two predissociation limits at 53800 and 50500 cm^-1 were established. In this way the dissociation energy of NO is fixed at 4.29 eV. The X²π ground state of NO dissociates in the N(²D)+O(³P) atomic term combination. The new dissociation scheme is strongly supported by the results of photochemical experiments.

The widths of the lines of the inversion spectrum of pure ammonia near 1 cm^-1 previously determined experimentally (Bleaney and Penrose 1947) are shown to vary from line to line in a manner consistent with the assumption of a simple dipole-dipole interaction for the collision mechanism. Measurements of the line breadth constant for the line (J,K)=(3,3) in mixtures of ammonia with six different non-polar gases are described, and from them the collision diameters are calculated. They are found to be approximately the same as the diameters obtained from the kinetic theory, whereas in pure ammonia the diameters are two to four times greater.

A survey of microwave spectra indicates that the collision diameter is only significantly greater than the kinetic theory diameter for encounters between permanent electric dipoles.

A review is made of the derivation of formulae for the efficiency of counting systems, comprising in the general case a counter and amplifier, a scaling circuit and a recorder. In the case of recorders of constant resolving time no complete formula has been found in the literature, and this has been derived. A number of existing formulae are shown to be valid only for a particular type of variable recorder. A discussion is given of the use of the formulae in practical counting experiments.

Phosphorescence and thermoluminescence emission from photoconducting impurity activated phosphors have been satisfactorily explained by the storage of electrons, freed from luminescence centres or other atoms of the solid, in metastable energy levels known as electron traps. Electrons escaping from these traps give rise to emission when they recombine with luminescence centres but there is a probability that they may be retrapped in empty electron traps before their final recombination with centres. The present theoretical and experimental studies attempt to determine the extent to which retrapping does occur and what effects it will have in modifying the phosphorescence and thermoluminescence characteristics. Theoretical treatment shows that there are marked differences in these characteristics for conditions when the retrapping process is present and for those when it is negligible. Experimental investigations of the characteristics of specimens of zinc sulphide, zinc silicate and strontium silicate phosphors indicate that, except under special conditions, retrapping of electrons is negligible. These results together with other work can be explained theoretically if it is assumed that electron traps operative in the luminescence process are spatially associated with the immediate neighbourhood of the luminescence centres formed by activating impurities. This new concept is also supported by the relations found between the luminescence characteristics and the dielectric changes in phosphors of the zinc sulphide type.

An account is given of a simplified calculus applicable to a moving-coil galvanometer in which use is made of the concept of motional impedance. The fundamental galvanometer equations are presented in a novel and very simple form. Examples of the use of the formulae are given, notably with reference to the ultimate "noise level" of a galvanometer.