Table of contents

A method for studying extensive air showers is described, in which four detectors are used to record the Cerenkov light produced by the shower particles in tanks of water 20 cm deep. The size spectrum has been measured for shower sizes between 7 × 10⁵ and 2 × 10⁷ particles, and the results are in good agreement with those of other workers The lateral distribution function appropriate to the Cerenkov detectors has been determined and found to be consistent, in the range 10 m < i < 200 m, with the predictions of electron-photon cascade theory

The present investigation draws attention to the potentialities of the high-current toroidal discharge assembly, SCEPTRE, as an intense source of the higher spectra. By way of illustration, several new classified lines of Ne IV, V and VI which have been observed in the range 2200 Å to 2300 Å are reported, thus permitting some revision of, and extension to, the analyses of these spectra. The complete F VI triplet 3s³S-3p³P⁰ has also been observed for the first time. Two new experimental methods of differentiating between the stages of ionization are given. With SCEPTRE, in contrast to conventional sources of spark spectra, gaseous samples are easily introduced into the deuterium plasma. Thus, such a discharge apparatus will be of value in exciting the spectra of the rare gases.

Experimental evidence is presented to show that the contribution of the Stark effect to the large observed widths of O V and F VI lines is very small. Consideration is also given to the detection of the coronal lines with SCEPTRE and to their value in studying ion motion within the plasma.

Measurements are made of the signal picked up by a microphone in the ultrasonic beam launched across a fluid in a region containing the wake of a cylinder. Because of the disposition of vortices in the wake there is a difference of phase in sound rays reaching different parts of the microphone and the signal fluctuates periodically. These modulations of the ultrasonic signal are related to fluctuations in the velocity in the wake. The experiments are carried out (a) in water using a towed cylinder, (b) in air behind a cylinder fixed in a wind tunnel.

This paper describes a method of evaluating perturbations in classical mechanics which is similar to quantum mechanics time dependent perturbation theory.

Previous interpretations of ion mobility measurements in liquefied argon and helium in terms of gas kinetic theory have shown that these liquids do not behave as compressed gases. A more detailed analysis of the total ion energy shows that in some instances the results have been incorrectly interpreted, while in others it would appear that impurities caused the apparent failure of theory The re-interpretation indicates that kinetic theory is applicable to these liquids, and that the collision cross sections for ions in argon and helium I are very nearly the same in both the gaseous and liquid phases. Polarization attraction is shown to be of great importance In liquid helium II it is necessary to consider quantum effects, but the experimental data can still be explained adequately by gas kinetic equations.

Using carefully selected oscillator strengths the logarithm of the average excitation energy, measured in rybdergs, appropriate to the Lamb shift is computed to be 4.37 ± 0.03. Taken together with recent calculations by Pekeris, the theoretical value of the ionization potential of helium is 198310 674 ± 0.025 cm^-1 compared with 1983 10.8₂ ± 0.15 cm^-1, the experimental value.

The time-of-flight technique has been used to study the neutron spectra from the reactions ²⁸Si(d, n)²⁹P, ³²S(d, n)³³Cl and ⁴⁰Ca(d, n)⁴¹Sc. Weak neutron groups have now been resolved which correspond to levels in ²⁹P at 2.42 ± 0.04 and 3.13 ± 0.03 MeV, and in ³³Cl at 2.11 ± 0.06 and 2.53 ± 0.06 MeV. Only one group could be definitely attributed to the ⁴⁰Ca(d, n)⁴¹Sc reaction, and this denotes a level Q-value of - 2.86 ± 0.02 MeV.

A fast expansion cloud chamber, triggered by three proton telescopes was used to investigate photonuclear reactions in helium, nitrogen and neon. Various reactions yielding from one up to seven charged fragments involving the emission of a 100 MeV proton were observed in nitrogen and neon. The results basically agree with the model of photon absorption by p-n pairs in the complex nucleus, but an unexpectedly large number of events was found in which the residual nucleus disintegrated. The assumption that in (50±15)% of the cases in which the photon is absorbed by a (p, n) pair, the residual nucleus is sufficiently excited to disintegrate, brings the results into line with previous scintillation counter experiments in which p-n coincidences were studied.

A small number of (γ, p) events was found at energies substantially above the giant resonance. It was also found that 5-10% of the events occurring involved the emission of two fast protons.

Observations were made of the effect of sudden ionospheric disturbances on radio waves reflected from, and transmitted through, the ionosphere. The results of simultaneous measurements of the phase changes on waves of frequency 2-6 Mc/s are shown to indicate that they are due to an increase in the electron density which extends to above the maximum of the E layer, and possibly up to 150 km. Simultaneous measurements of the increases in absorption of waves in the frequency range 2 to 24 Mc/s (which include measurements of cosmic radio noise) show that there is usually an important increase in absorption in the lowest regions of the ionosphere where the electronic collision frequency is 10⁷ or even 10⁸ sec^-1. These conclusions are consistent with the observations of other workers. The above mentioned phenomena were also compared with phase changes observed on very long waves, but since there was found to be a large variation in the individual aspects of different sudden ionospheric disturbances it was not possible to draw any detailed conclusions.

The conductivity of a number of barium titanate single crystals was measured as a function of temperature, applied field, electrode material, thickness and time. Typical results are given, and tentative theories put forward to account for the observed activation energies, the striking non-ohmic nature of the conductivity, and the influence of the electrode material. Results for some doped crystals are also briefly reported.

Measurements have been made of the shift in the applied magnetic field for resonance, caused by the presence of a conducting wall. Small spheres of nickel ferrite and magnesium manganese ferrite were used at wavelengths of 0.87, 1.25 and 3.12 cm and of yttrium iron garnet at a wavelength of 3.12 cm only. The shift was always to lower fields and was independent of frequency within experimental error. Its magnitude at room temperature was 135 and 100 oersteds in nickel and magnesium manganese ferrites respectively. In the garnet, the shift was found to be dependent upon the orientation of the steady field with respect to the conducting plane. A simple explanation is presented which gives agreement with the experimental data in order of magnitude.

The coefficients of viscosity and thermal conductivity of binary mixtures of xenon with krypton, argon, neon and helium have been determined experimentally 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. Values of the dimensionless quantity f = λ/ηc_v have been obtained for each mixture. The experimental values are compared with theoretical values based on the Lennard-Jones (6-12) potential of interaction between molecules, using force constants derived from viscosity data. Agreement between the experimental and theoretical results is not complete. It is suggested that the discrepancies may be due principally to deficiencies in the empirical rule recommended in the literature for determining the force constant, ₁₂/k, for interactions between unlike molecules.

Measurements have been made of the thermal conductivity and thermoelectric power of a range of single crystal samples of p- and n-type B1₂Te₃ between 6 and 200 K. The observed lattice conductivity could be interpreted in terms of scattering by the tellurium isotopes. Measurements on the iodine doped samples showed the existence of appreciable impurity scattering. Interpretation of the thermoelectric power results suggests a multiband structure for Bi₂Te₃.

A coaxial cylinder viscometer is described which was designed to measure the elastic and viscous properties of fluids which exhibit thixotropic behaviour, in particular of natural `wax-containing' oils. A constant torque is applied to the inner cylinder which is free to rotate, while the outer cylinder is kept stationary. The change in the rate of shear with time for which the stress is applied can be measured. The shape of the resulting `rate of shear, time' graph can be interpreted so as to distinguish and measure the elastic and viscous properties. An example is given of the behaviour of a typical `wax-containing' oil together with a suggested quantitative interpretation of the results.

A method is described for measuring cross sections for ion charge exchange at mean ion energies corresponding to gas temperatures of the order of 300 K.

The measurements are made in afterglows following pulsed radio-frequency gas discharges, using a mass spectrometer to identify the ions present, and to observe their relative rates of change of population.

The reaction cross section for the process O⁺+O₂->O₂⁺+O has been deduced from the measured population decay rate of the O⁺ ion, at gas temperatures from 200°K to 300°K.

The apparent variation of the cross section with gas temperature is not considered to indicate a systematic temperature dependence.

The cross section was found to be 3.2 ± 0.5 × 10^-16 cm², corresponding to a rate coefficient of 2.5 ± 0.4 × 10^-11 cm³ sec^-1, for the process.

The relevance of this result to upper ionospheric studies is briefly discussed.

Absolute differential cross sections are presented for the inelastic scattering of 19.6 MeV deuterons by ²⁴Mg leading to the first excited state at 1 37 MeV, in the angular range 15°-120° (c.m.). The angular distribution shows a sharp rise at forward angles (θ_cm < 40°) and a rather weak oscillation at more backward angles. It is concluded from an analysis of the results that the electric contribution to the scattering is smaller than has been previously assumed, and that a distorted wave treatment is essential if a satisfactory fit is to be achieved. Absolute differential cross sections are also presented for the elastic scattering of 19.6 MeV deuterons by Mg.

Correction to Proc. Phys. Soc.75, 329