Table of contents

New measurements of the decay of the fluorescence produced in CsI(Tl) have been made for different particles. The detailed shape of the decay is found to depend on the average ionization density ρ. The lifetimes τ of the main components are 0.425 ± 0.025 μsec for 4.8 MeV alpha particles, 0.52 ± 0.01 μsec for 2.2 MeV protons, 0.595 ± 0.02 μsec for 8.6 MeV protons and 0.70 ± 0.025 μsec for 0.66 MeV electrons. The variation of τ and ρ is very similar to that observed by others for the variation of τ with temperature for alpha particles, which suggests that the former may be explained in terms of variations in the effective local temperature. It is shown that the differences in τ are sufficient to allow a single crystal detector to be used in experiments where particle type as well as energy determination are required.

The infra-red absorption spectra of α-quartz and vitreous quartz between 4 and 15 microns are given and values of the absorption coefficients have been calculated with the help of published reflectance data. Certain results are stated which are not in agreement with the earlier classification of the bands on the basis of point-group D₃.

The Lorenz number of the semiconductor Bi₂Te₃ has been evaluated, for the range of partial degeneracy, from data provided by the measurements of electrical conductivity and thermoelectric power. The calculated electronic component of the thermal conductivity, for p-type material, has been found to agree with the experimental results, assuming the lattice thermal conductivity to be independent of the electrical conductivity. Agreement has also been obtained for non-halogen-doped n-type Bi₂Te₃, but not for halogen-doped material. It has been shown that the reason for this is the high effective scattering cross section, for phonons, of the halogen atoms.

The theory of the transport of ionization energy has been found to agree very closely with experiment.

The outstanding problem of semiconductor noise is to account for a power spectrum of which the intensity appears to increase without limit as the frequency is decreased. There is no simple explanation in terms of relaxation times for a spectrum which is proportional to a constant power of frequency over many decades and which is found in a wide range of different materials. But it can be explained if the rate at which carriers leave the conduction band is treated as a queuing problem rather than a relaxation problem. The phenomenon in question is related to the presence of current, which implies that the carriers are swept away from the neighbourhood of the centres from which they originated, and the time to recombination is governed by the time taken to find a vacant centre elsewhere, not by the probability of falling back to the originating centre.

The occurrence of strain ageing in single crystals of arsenical copper (0.36% As) has been established by simultaneous determinations of stress-strain curves and changes in electrical resistivity. During yielding the electrical resistance increased more rapidly than normally.

The change in resistance during yielding is explained in terms of Cottrell's model of an impurity atmosphere around dislocations.

Replica electron microscopy of rupture surfaces has been used to study the rupture process in solid `two-phase' systems obtained by dispersing colloidal carbon in rubber. Two parameters are introduced to characterize the surfaces; the total proportion of filler in the surface, and the proportion extracted by the gelatine first-stage replica. The latter quantity provides a measure of carbon-to-rubber adhesion in the rupture surface, whilst values obtained for the former quantity indicate that the rupture deviates strongly from a random plane through the material.

The results, together with a correlation found between adhesion and tearing energy, may be largely explained in terms of the local concentrations of stress which occur at the particles of the dispersed phase.

The effect of nucleon current contribution to nucleon magnetic moment is considered up to the fourth order of the coupling constant in the cut-off meson theory using the renormalization procedure as developed by Chew. The same parameters as employed to fit the meson-nucleon scattering data have been used here. It is found that the additional contribution due to this effect improves the previous theoretical results where only the mesonic current contribution has been taken into account.

The fluorescence quantum intensities of normal and oxygen-free solutions of para-terphenyl in toluene have been measured as a function of concentration c and of excitation wavelength λ from 2200 Å to 3300 Å. The absorption spectra of toluene and terphenyl have been observed. The energy transfer coefficient f from excited toluene molecules to terphenyl molecules has thus been obtained.

f is found to depend on λ. For a given c, f=f₁ in region 1 (λ=2500-2650 Å), f=f₂>f₁ in region 2 (λ=2200-2350 Å), and f₂>f>f₁ for intermediate λ. Regions 1 and 2 correspond to the first and second excited electronic singlet states of toluene. The solvent-solute energy transfer from the second excited state, which occurs in approximately 10^-11 sec, competes efficiently with conversion to the 1st excited state of toluene.

Measurements have been made of the Hall coefficient and resistivity of a number of specimens of compressed cadmium oxide powder both during and after various heat treatments. The results are explained by assuming that during such heat treatments there may be changes in the intercrystalline resistances and also in the carrier concentration of the microcrystals. In particular it is suggested that the very large change of resistance during sintering is mainly due to a change in intercrystalline resistances. A manner by which the carrier concentration of a specimen may be changed after heavy sintering is described.

Neutron scattering in the low energy region where the compound levels are well separated is considered in detail. It is shown that the average phase can be reproduced by that calculated from a complex potential. The imaginary part of this potential is simply related to the absorption out of the incident beam into compound states. Formulae for the average behaviour of various quantities are derived in a simple and straightforward fashion.

Ultrasonic measurements in MnSO₄ in the frequency range 1-6 Mc/s made by the optical method have been compared with those in MgSO₄. The concentration behaviour of the absorption and the relaxation frequency, which has been studied in great detail in the range 0.0025 to 1 molar, has been compared with the earlier results in MgSO₄. The effect of the dielectric constant and the temperature on the absorption, relaxation frequency and the reaction rate support the dissociation hypothesis.

The polarization of the ground state protons emitted when ¹²C is bombarded with 8.9 MeV deuterons has been measured at several angles. These results have been compared with recent calculations of Newns and Refai. The angular distribution of the polarization was found to have the predicted shape but its sign and magnitude suggest an improbable proton-nucleus interaction.

A simple rule is given for modifying the formula for the angular distribution of any process, however complicated, which involves absorption of a particle (or radiation) p in the transition stage p + A → B so as to yield the angular distribution of the process which differs only in the replacement of the absorption stage by an emission stage A → p + B. The time-reversal operation is involved, and one is led to the time-reversal of density and efficiency matrices. and of statistical and efficiency tensors.

The LCAO method of finding approximate one-electron wave functions is applied to an atom and crystal in mutual interaction. The case of a hydrogen-like atom and a crystal whose valence electrons occupy a band of states derived, in the LCAO approximation, from a single atomic state is discussed with the help of the simplifying assumptions of the tight binding approximation. Formulae are derived for the wave functions and energy levels of this system. If the interaction extends only over a group Ω of crystal atoms near the foreign atom then, depending on the values of the interaction parameters, there may be localized states associated with the foreign atom and the group Ω. The energies of these states lie outside the normal band (including any surface band) of crystal states and their existence allows for the formation of localized bonds between the atom and the crystal.

The scaling laws for the stabilized pinch are derived. The recent results obtained with such discharges are considered in the light of these relations.

The zero order partial cross section for the excitation of hydrogen from the (1s) to the (2s) states by electron collision has been calculated by a numerical method. Full allowance has been made for electron exchange and all direct coupling between the initial and final atomic states has been retained.

The cross sections obtained are compared with those that have been calculated by more approximate methods. In particular, the cross section near the threshold is found to be considerably smaller than that given by Erskine and Massey using the distorted wave approximation.