Table of contents

Electrical contacts with faces of silicon carbide and of many other semiconductors do not obey Ohm's law. This property of silicon carbide has been attributed to an amorphous film known to occur on faces of the crystal. Theoretical work relating to such contacts is hampered by lack of knowledge of the essential physical features of the model. In this paper a study is made of the manner in which the surface texture of the crystal affects the electrical properties of the contact. The electrical characteristics of a contact between a rounded metal point and the demonstrably smooth face of a crystal were shown to vary only slowly across the surface. A series of observations was made of the electrical properties and of the electron diffraction patterns of a basal plane of a crystal as the overlying film of amorphous material was removed.

The results show that the presence of the amorphous film is not essential to the exhibition of the special electrical properties of the contact.

Electron diffraction shows that PbCl₂ is formed instead of PbO. PbSO₄ when mosaic single-crystal PbS layers a few hundred A. thick are heated on their NaCl substrates in air at 200-300° C. The structure and orientation of these PbCl₂ layers are described. This example of epitaxial crystal growth is particularly interesting because the PbCl₂ atoms adjacent to the, at least initially, atomically smooth PbS surface do not lie exactly in a single close-packed plane atomic sheet, but the atomic arrangement closely matches that of the PbS substrate in a series of parallel bands. When the composite PbCl₂-PbS layers were isolated from their NaCl substrate the transmission patterns showed that they must have partly broken up, leading to curvature about remarkably well-defined axes parallel to the film plane and PbS cube-face diagonals.

Attempts at the measurement of the absorption spectrum of cosmic rays by means of counter telescopes have been made by many authors. It is pointed out here that none of the usual methods, i.e. the determination of the number of coincidences as a function of the absorber thickness, can be satisfactory unless the influence of the geometry through scattering is known. Only then can this (integral) absorption curve be used for the determination of the momentum spectrum.

A method of determining directly the differential absorption curve is described which avoids largely the influence of scattering. This method has been used for the investigation of some specific effects which have been observed by some and denied by other authors. In particular, the region of 12 cm. Pb has been investigated, and it is found that the change of absorption observed here is due solely to the overlapping of the high momenta end of the electron spectrum and of the meson spectrum. Although the present investigation cannot confirm it, it does not exclude the possibility of a specific change of the absorption coefficient in the 22 cm. Pb region.

An account is given of experiments on the momentum spectrum of the particles in extensive air showers at sea-level. The results are consistent with an integral spectrum of the form N(>E) α (E+E_c)^-(1.1±0.3), where E_c is the critical energy in air. Penetrating particles are found in some of the denser showers, the ratio of penetrating particles to electrons being (0.8±0.4)%.

A simple multivibrator for eliminating multiple discharges in argon-alcohol counters is described. This circuit has been used successfully in cosmic-ray intensity recording equipment requiring a high degree of stability. It is shown that the increase in counter life which is observed when a multivibrator of this kind is used is probably due to quenching of the discharge before it has spread the full length of the counter wire.

The absorption of nucleons passing through any number of nuclei is calculated with due consideration of all fluctuations of energy loss. For the probability ω (, E) d for an energy loss by a nucleon with energy E hitting a nucleon at rest, a law of the form ω d = ω(/E) d/E is assumed, a law suggested by meson theory and Bremsstrahlung. It is shown that, with a primary spectrum, following a power law, the absorption is strictly exponential for any thickness of the absorber and for all energies of the nucleons, apart from small deviations at low energies due to the latitude cut-off. This agrees well with recent experimental results. The experiments also permit some conclusions about the shape of the function ω, and it is shown that ω must be a fairly broad distribution extending up to /E = 1, possibly favouring, but not to a very large extent, low values of /E. The average energy loss of a fast nucleon hitting a nucleon at rest is determined by the experiments within very narrow limits. The conclusions that can be drawn about ω, the total cross-section for meson production and the average energy loss are all compatible with the theory of Hamilton, Heitler and Peng, in its later version

Some Lorentz-invariant cut-off procedures for the electron self-energy are investigated. Expressions for the self-energy are obtained as expansions in inverse powers of the cut-off constant K. The first term, logarithmic in K, has the correct transformation properties for all the cut-offs. The second term, independent of K, is covariant only for one of the procedures. The remaining terms have not the correct properties in any of the procedures, but contain negative powers of K and, therefore, vanish for K → ∞. The one cut-off procedure which gives a covariant constant term can therefore be used for an unambiguous subtraction of the electron self-energy provided that in the final results one proceeds to the limit K → ∞.

Corrections to 1949 Proc. Phys. Soc. A 62 253