Table of contents

The motion of a charged particle through a low density electron plasma in an external magnetic field has been investigated. The interaction of the particle through transverse waves has only been considered, and the consequent energy losses are calculated. It is found that the coupling between the longitudinal plasma wave and the transverse electromagnetic wave modifies the nature of radiation emitted by the particle. The results show that for a velocity greater than that needed for exciting plasma waves in the medium, the particle emits non-Cerenkov type of radiation, in addition to Cerenkov radiation obtained previously by others.

The conditions under which oscillations have been observed are shown to be such that a system of transient circulating currents can arise. These currents will tend to increase the carrier concentration at one surface of the specimen, and thus modify the surface recombination. Oscillations of a relaxation type could plausibly result. The model suggests variations of frequency and amplitude with magnetic field intensity and direction, and with electric field and temperature similar to that reported, and could account for the occurrence of incoherent oscillations with an accurately aligned magnetic field.

For moderately large organic scintillators, a linear relation is found between the position of the γ-ray ` Compton peak ' and the maximum energy transfer in a first Compton collision. Extrapolation yields a positive intercept on the energy axis as a result of the dependence of the specific light output of scintillators on the linear energy transfer of charged particles. The broadening of the Compton peak on the high-energy side has three components: (a) statistical, (b) intrinsic line broadening arising from multiple interactions of the γ-ray, and (c) geometrical, resulting from the variation of light collection efficiency with the position of the event in the scintillator. The dependence of these three components on γ-ray energy is considered and their approximate magnitudes for two plastic scintillator units are calculated from experimental γ-ray spectra.

During an investigation of the spectra of metallic hydrides of group II of the periodic table, five new bands of MgH at λλ 2819, 2702, 2172, 2100 and 2088, a band system of ZnH at 2426 Å and a band system of CdH at 2483 Å, have been observed and analysed. Also the corresponding deuteride bands, where possible, have been observed. Only the MgH and the corresponding MgD bands at 2819 Å and 2702 Å are dealt with in this paper. They are classified as E²Σ->X²Σ and F²Σ->X²Σ transitions respectively. The rotational constants are calculated.

The hyperfine structure of the level 5²P_1/2 of potassium 39 has been measured by the techniques of optical-radio-frequency double resonance. The magnetic interaction constant a_1/2 was found to be 8.99±0.15 Mc/s, which leads to the value 4.56±0.3 for the ratio a_1/2/a_3/2. This is significantly lower than the value 5.08 which the Goudsmit-Fermi-Segrè theory would predict. The measured value of the Landé g-factor is 0.665±0.003.

Direct measurements of static and optical dielectric constants have been made on GaAs with a resistivity of 10⁹ ohm cm at room temperature. Values obtained were ₀ = 12.5(3) and _∞ = 10.90. An effective ionic charge e_s^* of 0.46e can be deduced from these values.

An experiment is described in which a large area plastic scintillation counter is used to measure the energy loss of relativistic μ-mesons. The results, which are not dependent on the particular method used to fit them to the Landau distribution, show that the increase in the most probable energy loss is less than 1% for mesons of energies between 500 and at least 10 000 MeV. This conclusion is consistent with the Sternheimer density correction to the ionization loss theory of Bethe-Bloch.

Current-voltage characteristics of evaporated specimens of vitreous selenium have been studied. They show an ohmic part at low applied voltages and, with increasing hole injection, a V² region which precedes the trap-filled limit as predicted by Lampert's theory. Ohmic contacts of tellurium or platinum were formed on newly prepared specimens by the prolonged application of a moderately high electric field, but this was not found possible with a low work function electrode such as magnesium. The detailed analysis of the results from formed specimens between 2 and 31 μ thick indicates the existence of a volume distribution of hole traps of density 9×10¹³ cm^-3, 0.79 ev above the valence band. These values agree with independent results from measurements of the transient photoconductivity after saturation of the centres with strong illumination. At room temperature the average trapping time is 50 seconds leading to a capture cross section of 8×10^-16 cm². The presence of the 0.79 ev level offers some explanation for the pronounced difference between the absorption and photoconductive edges in vitreous selenium. It is concluded that the level lies near the upper edge of a comparatively dense distribution of states extending towards the valence band.

The coefficients of viscosity and thermal conductivity of binary mixtures of krypton with argon, neon and helium have been determined over the full range of composition of each mixture at a pressure of 70 cm of mercury and at a temperature in the range 18.0 to 18.3°C. The experimental values are compared with theoretical values based on the Lennard-Jones (6-12) potential of interaction between molecules. Agreement between the experimental and calculated values is generally satisfactory, but there are discrepancies between some of the measurements given here and those of other workers.

The Smoluchowski equation has been used to obtain the distribution function in coordinate space of pairs of molecules in a non-uniform liquid and hence to calculate the viscosity and flow anisotropy. This equation is the first approximation in an expansion of the Kramers-Chandrasekhar equation in inverse powers of the friction constant. In this paper, the second approximation, which leads to a fourth-order differential equation, is considered. The equation is solved numerically and the contributions to the radial distortion of the pair distribution and to the flow anisotropy are calculated for chloroform. It is found that the friction constant must be at least 4×10¹² sec^-1 for the Smoluchowski equation to be a reasonable approximation.

Some results which were derived by use of finite temperature perturbation theory are used to investigate the possibility of a low-energy collective mode in a superconductor. It is found that such a mode should exist, and the ratio of its energy to its momentum is calculated. This ratio approaches infinity exponentially as the temperature goes to zero, in agreement with Anderson's result that at zero temperature the collective mode is destroyed by the Coulomb force. The effect of this collective mode on the specific heat is negligible. The relation of this mode to `second sound' is discussed. A comparison is made between the behaviour of this collective mode in a charged superfluid system (superconductor) and the behaviour of the collective modes in a superfluid system of neutral fermions.

The ground state energy of the free electron gas is calculated using the Rayleigh-Schrödinger variational method with the wave function ψ=DΠ_i<jf(x_ij) where D is a determinant of plane waves and f(x_ij) a correlation function. Consideration of the wave function in terms of the collective coordinates p_k' the Fourier components of the density, suggests an accurate approximation for the energy integral which is then evaluated over the coordinates of the particles so that the use of subsidiary conditions is avoided. Effects omitted in the random phase approximation are included and the final results extend continuously over plasma and particle modes and should be valid in the range of densities encountered in real metals. The results agree closely with those of Nozières and Pines, and of Hubbard obtained by more elaborate methods.

High resolution photographs of the near infra-red spectrum of an argon discharge at various pressures revealed that lines whose lower levels are connected by resonance transitions to the ground state suffered markedly greater collision broadening than those whose lower levels are metastable. The excessive broadening of the former group of lines is believed to be due to resonance collision broadening of their lower levels. The collision broadening of one line from each group, namely 1s₂ - 2p₂, λ8264 Å and 1s₅ - 2p₂, λ6965 Å, was measured. The half intensity collision width of λ8264 was 14.7 ± 1.0 × 10^-20 cm^-1 per atom per cm³ of argon, while that of λ6965 was 2.0 ± 0.3 × 10^-20 cm^-1 per atom per cm³ of argon. The ratio of collision width to collision shift (the latter being taken from Hindmarsh and Thomas 1959) was 3.1 ± 0.8. This is in agreement with the value 2.75 predicted by the Lindholm theory for interaction due to van der Waals forces between the atoms. The resonance broadening of λ8264 is related to the f-value of the argon resonance line 1p₀ - 1s₂, λ1048, and this is found to be f=0.13 ± 0.03, in agreement with calculations quoted by Knox in 1958.

The angular correlation of the gamma radiation from the first excited state of. ²¹Ne, in coincidence with neutrons, has been studied using an axially symmetric neutron counter at 0° to the incident beam direction. Our results for ²¹Ne together with the published lifetime measurement of the first excited state of ²¹Ne, show that the amplitude of the electric quadrupole radiation divided by the amplitude of the magnetic dipole radiation is 0.004 less-than-or-eq, slant δ less-than-or-eq, slant 0.03.

The gamma radiation from the first excited state of ²⁵Mg, in coincidence with neutrons, was also observed but no detailed study of the angular correlation was made.

A direct measurement of the L/K electron capture ratio has been made for ²⁰⁴Tl. An improved internal source scintillation counter technique was used. A crystal was grown containing ²⁰⁴Tl and this crystal was totally enclosed in a well-type crystal. The experimental value obtained for this unique first forbidden transition (ΔJ = 2, yes) is 0.42±0.05. This value may be compared with the value 0.45±0.05 calculated from the electron capture transition energy.

In addition, information has been obtained on the β^- decay branch. The low energy end of the Fermi-Kurie plot of the β^- spectrum, after applying an exact shape correction factor, deviates slightly downwards from a straight line. This deviation is in the opposite sense from that obtained by previous workers with magnetic spectrometers.

Correction to Proc. Phys. Soc.77, 261