Table of contents

When an alloy develops an ordered structure, new Brillouin zone boundaries are formed corresponding to the extra Bragg reflections. It is shown that, in this process, the Fermi energy is lowered, and this tends to stabilize the superlattice. The theory is applied to the superlattices in CuPt, CuAu and Ag₃Mg, where the ordered structures do not follow from the simple principle of lattice-strain relief.

New photographs of the absorption spectrum of nitric oxide have been obtained. Three red-degraded bands between 1711 and 1676 Å resemble the β bands and plausibly represent the ν' = 14, 15, 16 members of the β system. An intensity and spacing anomaly between the ν' = 13 and ν' = 14 members is probably connected with the diffuseness and disturbance in position of the ν' = 2 member of the δ system and the failure of the ν' = 3 member of that system to appear; and can be accounted for in terms of a crossing of the B²Π and C²Σ⁺ potential energy curves. The β system suffers a further disturbance when the B²Π curve cuts the D²Σ⁺; the ν' = 17 member of the β system appears to be replaced by the ν' = 3 member of the system. There is no need to introduce a hypothetical repulsive curve to explain the perturbations of the various systems; they all appear to be explicable in terms of successive crossings of the ²Σ⁺ curves by the B²Π potential curve. All the states A, C, D and E involve extra-valency shell (Rydberg) orbitals. Plausible interpretations of these states are given.

An account is given of measurements on the thermal conductivity of superconducting tin below 1 °K, made by observing the conduction of heat along a tin rod joining two pills of paramagnetic salt. Measurements were made on specimens of widely differing purities and a comparison of the results for the several specimens enabled an estimate to be made of the separate contributions, from the electrons and from the lattice, to the conductivity of each specimen. The electronic contribution to the thermal conductivity is in agreement with calculations due to Heisenberg and Koppe. The significance of this is discussed and it is tentatively suggested that the thermodynamic properties of the superconducting state may be determined by an energy gap of order KT_c in the spectrum of energy levels available to the electrons.

The specific heats of ferrous and zinc ammonium sulphates have been measured from 2 to 30°K; the difference between these gives the anomalous contribution of the ferrous ion. Two maxima are found in this latter quantity, these occurring at 3.8 and 20.3°K. A scheme of energy levels capable of explaining the anomaly is put forward and the question of its uniqueness discussed.

An examination is made of the effect of damping on the photo-production of pseudoscalar charged and neutral π-mesons from protons, ignoring all self-energy effects. In the case of pseudoscalar coupling the damping corrections are large, but do not greatly remedy the disagreement of second order perturbation theory with experiment; while for pseudovector coupling damping effects are unimportant.

The theory of neutron-triton scattering has been worked out, using Wheeler's resonating group structure method, and applied to a neutron energy of 14 MeV for four types of nuclear interaction: an ordinary force, an exchange force of Majorana-Heisenberg type, a symmetrical exchange force, and the Serber exchange mixture. The nuclear potential is taken in the gaussian from V₀ exp ( - μr²) and the corresponding two body ground state wave function as N₀ exp ( - ½λr²). The resulting integro-differential equations for scattering are solved by variational methods.

The symmetric and Majorana-Heisenberg forces give the right general shape for the angular distribution of scattering, but the ordinary and Serber forces give a rather higher peak of back scattering than indicated by the experiments of Coon, Bockelman and Barschall at 14 MeV. The measurements appear to favour forces of a symmetrical exchange type as against an ordinary force type.

An apparatus is described which enables angular distribution measurements to be carried out within the angular region -5° to + 140° on disintegration particles of long range. The differential range spectrum is measured at each angle by the use of a triple proportional counter of novel design.

Measurements are reported for the two proton groups of longest range produced in the reaction ²⁷Al(d, p) ²⁸Al with a deuteron energy of 8 MeV and the results discussed in the light of the theories of the stripping process.

Angular distribution measurements have been carried out on a number of proton groups from the bombardment with 8 MeV deuterons of targets of natural magnesium and of ²⁵MgO and ²⁶MgO. Spins and parities have been assigned to various states of the product nuclei on the basis of the theory of the stripping process. These are given herewith, the excitation energy of each state in MeV being followed by the possible spin values and the parity in brackets: ²⁵Mg: ground state (5/2, 3/2+), 0.582(1/2+), 0.976(5/2, 3/2+), 1.957(5/2, 3/2+), 3.405(3/2, 1/2-). ²⁶Mg: 1.825(3, 2+ and 5, 4...0+), 2.972(3, 2+), 3.969(3, 2+), 4.353(3, 2+), 6.147(3, 2+). ²⁷Mg: ground state (1/2+), 0.887(5/2, 3/2+), 3.50(1/2+). The following new energy levels have been observed: ²⁵Mg: 4.62 ± 0.05, 5.05 ± 0.08, 5.49 ± 0.05 and 6.40 ± 0.05 MeV. ²⁶Mg: 7.29 ± 0.06 and 8.28 ± 0.06 MeV. ²⁷Mg: 3.50 ± 0.05 MeV.

The variational method is used to investigate the elastic scattering of slow electrons by the positive helium ion. Zero order phases and wave functions found by this method are shown to agree, in general, with those found by numerical integrations of the wave equation, both when the wave function is of the `one-body' type and when it is of the correct symmetry. The inclusion in the trial wave function of a term involving the distance between the two electrons is found to lead to smaller phases. Detailed results are given for energies of the incident electron in the range 3-60 eV.

The Chapman-Enskog formulae for thermal diffusion in a mixture of gases have been utilized to interpret the data on several gas mixtures at different temperatures but having the same composition. Assuming the Lennard-Jones 12:6 interaction potential also for unlike molecules, the minimum mutual potential ₁₂ for interaction between two unlike molecules has been calculated from the temperature dependence of the thermal diffusion ratio k_T. These determinations are expected to provide the most accurate value for this interaction as thermal diffusion is far more sensitive to the laws of interaction than the other transport properties which have been hitherto used to estimate ₁₂. The distance r₁₂ at which the potential vanishes is calculated from the experimental data on the inter-diffusion coefficient D₁₂ by utilizing these values of ₁₂. Thus ₁₂ and r₁₂ are determined purely from experimental data. Equations have been developed to correlate ₁₂ and r₁₂ with ₁₁, ₂₂ and r₁₁, r₂₂, and these relations have been tested with the help of the experimental values. It is found that our theoretical relations give much better agreement than the assumed geometric mean relation for ₁₂ and the arithmetic mean relation for r₁₂. Equations have also been developed for μ₁₂ and λ₁₂ and their theoretically obtained values compared with the experimental results.

It is pointed out that the accuracy of the cross sections observed by Maier-Leibniz for the excitation of helium by slow electrons may be tested by using them to calculate the Townsend ionization coefficients α and comparing the results of such calculations with observed values. Information about the reliability of the difficult measurements of excitation cross sections is essential to check attempts made to develop a satisfactory theory. To carry out this test it is necessary to extend and to some extent refine the work of Smit on the velocity distribution of electrons in helium diffusing under the action of a uniform electric field F when inelastic collisions are allowed for. This has been done and numerical results have been obtained for F/p=10 and 20 v/cm/mm Hg. The ionization coefficient has been measured for the higher values of F/p and agrees quite well with that calculated when Maier-Leibniz's cross sections are used. The sensitivity of this test of the cross sections was examined by calculating α assuming that Maier-Leibniz's values were respectively twice and one-half the correct value. It was found that the calculated value of α is nearly proportional to the size of the assumed excitation cross section (it was assumed throughout that the ionization cross section is given correctly from Smit's observations). It is concluded that the observations of Maier-Leibniz may be used for checking theoretical determination of excitation cross sections in helium.

In order to provide information on the first excited state of ⁷Li the angular correlation between the protons and the γ-radiation from the reaction ⁶Li (d, p) ⁷Li^*γ⁷Li has been studied. An isotropic angular correlation was observed. The angular correlations, predicted theoretically, for the most likely transitions in this reaction have been determined. It is shown that an isotropic angular correlation indicates that, very probably, J = ½ for this state.