Table of contents

Variational methods are used to calculate phases for the s- and p-wave scattering of electrons by hydrogen atoms. Full allowance is made for electron exchange and the influence of polarization is investigated. The derived elastic cross sections are compared with experimental results.

A formalism is developed for treating scattering processes in the several hundred MeV range in which the target nucleus is left in its ground state or in a low excited state. These processes will be described in terms of collisions between the incident particle and single nucleons of the nucleus, using approximations of a semi-classical nature. The inelastic processes can be regarded as single particle excitations, the effects of scattering by other nucleons being included in a distorting potential. The target nucleons are assumed stationary during the passage of the incident particle; it is demonstrated that this is a reasonable approximation.

Recent experiments by Cohen on the inelastic scattering of protons by nuclei of medium Z have shown some unexpected effects, in particular, a pronounced increase in cross section at excitations of 2-3 mev in all elements with Z between 40 and 52. A search for this `anomalous scattering' has been made in Sn, In, Cd, Ag, Pd, Mo, Nb, Zn, Cu and Ni, using 9.8 mev protons. The prominent effect at excitations of 2-3 mev was not found at 9.8 mev. In Cu, Zn and Ni an intense group corresponding to excitations of 1-1.4 mev was observed, in agreement with Cohen's results.

The wide distribution in the values of the neutron widths and spacings of slow neutron levels leads to fluctuations in the cross section averaged over several levels. A method of deriving information on the properties of these distributions from observations on the fluctuations is described.

Fluctuations in total cross section measurements are analysed and agreement to order of magnitude is obtained with the predicted width and spacing distributions, but it does not appear that exact agreement is obtained at present The ²³⁵U fission cross section also is analysed by this method, but the results do not agree with the theory of the method. It is probable that these discrepancies are due to the effect of correlations in the distributions.

Use of a large NaI(Tl) crystal has permitted determination of the yield curves of each of the high-energy gamma rays arising from the ⁹Be(p,γ)¹⁰B reaction for proton energies from 220 to 440 keV. The results indicate that all but the ground state gamma rays arise from the 6.89 MeV level in ¹⁰B, and this confirms the postulated 20% isotopic spin impurity for this level. The ground state transitions arise from the broad 7.48 MeV level and there is some evidence for part of this gamma radiation arising from direct transitions. The 6.89 MeV level can also decay by gamma-ray emission to the 5.11 MeV level.

In the approximation of the single nucleon model for the nucleus, the effect of nuclear binding on pion-nucleon scattering is estimated using a variational method and neglecting nucleon recoil in intermediate states. Results are compared with those of a simple p-wave phenomenological theory. Similar cross sections are obtained except in the region of the p-wave resonance of free nucleon-pion scattering, where the phenomenological theory leads to a cross section an order of magnitude smaller. This result is unaffected when nucleon recoil is taken into account, but may be sensitive to the choice of trial wave functions used in the variational calculation.

The effect of simple configurational mixing, within the j-j coupling shell model, on the nuclear matrix elements for allowed beta-transitions of odd A nuclei is considered. It is shown that such mixing could be responsible for the unfavoured nature of such transitions within the 2d and 1g shells.

Relative band intensities of the Second Negative system of O₂+, the Ångstrom and Third Positive systems of CO, and the γ and β systems of NO have been measured. They have been interpreted with the aid of the Franck-Condon factors q _{ν' νdouble prime} and the r-centroids r _{ν' νdouble prime} to show the variation of the electronic transition moment R_e(r) with internuclear separation r. Arrays of the relative vibrational transition probabilities p _{ν' νdouble prime} and the intensities Iinfinity at infinite temperature have thereby been derived for these systems.

The interaction of a magnetic field and a compressional acoustic wave travelling in a conducting medium is considered and expressions for the electric current densities and fields in filamentary-shaped specimens are derived on the assumption of charge neutrality. Calculations appropriate to metals and to semiconductors are carried out, the latter materials being of interest as the presence of charge carriers of both signs makes possible changes in carrier concentration without violation of the neutrality assumption. In addition, second-order acoustomagnetoelectric effects are discussed.

The general differential equation of wave-motion for a Burgers body is used to express the attenuation and retardation time of a visco-elastic material in terms of the mechanical quality factor of a cylinder of the material set into forced vibration. It is assumed that the vibrations are induced by the action of a sinusoidal stress at one end of the cylinder.

Investigations have shown that a high skidding resistance on wet roads is associated with the presence of sharp edges in the road surface: on these sharp edges high pressures are set up which assist in breaking through the lubricating water film between tyre and road. This paper gives some details of laboratory tests now being made to investigate further the dependence of skidding resistance on such localized pressures.

Pressure distributions beneath rigid spheres and cones pressed into rubber have been calculated, on the basis of the elastic theory, from measurements of their penetration into tyre-tread rubber under load. The experimental evidence confirms that under the conditions of test employed the rubber is behaving as an elastic material.

The effect of the pressures on friction under wet conditions has been investigated by sliding different shapes over wet rubber, using a laboratory friction machine designed to simulate conditions between tyre and road when skidding takes place. These tests indicate that the coefficient of friction recorded under wet conditions is closely related to the pressure over the contact area between slider and rubber, and that, to ensure a satisfactory skidding resistance in wet weather, the shape of individual projections in the road must be such that average pressures of the order of 1000 lb in^-2 are set up on them. Ideally, what is required is that the individual projections in the surface of the road should have angles at their tips of 90° or less: the necessary pressures are unlikely to be obtained with rounded or polished projections, whatever their size or the load applied to them.

A study has been made of the friction of hard spheres and cones on a well lubricated rubber surface under conditions where relatively large deformations are produced. It is found that with spheres the sliding friction is almost the same as the rolling friction. Earlier work has shown that in the latter case the friction arises primarily from hysteresis losses in the rubber and it is concluded that, under the experimental conditions described, the main source of friction in lubricated sliding arises from the same cause. With conical sliders it is not possible to make a direct comparison with rolling experiments, but a simple calculation suggests that here again the friction is largely due to deformation losses in the rubber. For cones of small semi-angle there is a large increase in friction but this is associated with penetration of the lubricant film and tearing of the rubber.

The friction results are in close agreement with measurements described by Miss Sabey for spheres and cones sliding on wet rubber at relatively high speeds (c. 6 ft sec^-1). This suggests that in her experiments that water film provides very effective lubrication and that the friction arises largely from hysteresis losses in the rubber itself.

As a general conclusion this study of the friction of lubricated rubber suggests that where interfacial adhesion is small and where relatively large local deformations occur the friction may well be dominated by deformation losses in the rubber. Under these conditions we may therefore expect the friction of hard spherical or conical sliders on rubber to be greater for rubbers of low Young's modulus and high hysteresis losses.

Measurements of carrier lifetime have been made on both n- and p-type silicon crystals over the temperature range 0°C to 200°C. Results are given for both high and low values of injected carrier concentrations. A comparison is made with the Shockley-Read picture of carrier recombination. The results and theory may be reconciled on the assumption that the capture probabilities of the recombination centres are temperature dependent, and that the energy levels of the centres lie 0.45±0.05 eV above the valence band.

The measurements of total absorption in the ionosphere have been made at vertical incidence by pulse technique on various frequencies during the day. It has been observed that at lower frequencies a single maximum is generally obtained after mid-day due to sluggishness of the lower absorbing layers as suggested by Appleton. At frequencies higher than the critical frequency of F1 layer, however, two such maxima are observed one before and the other after the local noon. This anomaly has been attributed to the thermal expansion of the F2 region of the ionosphere, the effect of which is usually observed before mid-day.

Previous work on the coefficient of reflection R of bent crystals used in transmission type focusing γ-ray spectrometers has shown a power law dependence on wavelength λ for λ less than 0.5 Å. Theory is here presented which suggests that at longer wavelengths the power law does not hold and that R reaches a maximum. The wavelength corresponding to R_max depends on the assumptions made about the amount of secondary extinction occurring but could be about 1 Å.

An experiment on a bent mica crystal shows that R is proportional to λ^{1 9} for λ less than 0.5 Å and seems to confirm the existence of a maximum near 1 Å.