Table of contents

A survey is given of some of the more outstanding developments in magnetism during the post-war years, including the great extension of our knowledge of the ferromagnetic domain through the theoretical work of Néel and others and by the systematic use of the Bitter powder figure techniques, the main ferromagnetic, paramagnetic and nuclear resonance phenomena and experiments on the diffraction of neutrons by anti-ferromagnetic crystals are also described

Paramagnetic resonance absorption in single crystals of the copper Tutton salts has been investigated at 3.10 cm wavelength between 90° k and room temperature. In salts diluted with zinc Tutton salt the hyperfine lines are clearly resolved at 90° k but broaden and merge into one another as the temperature is raised. From these measurements the spin-lattice relaxation time τ can be estimated and is found to be about 2×10^-9 sec It is found that 1/τ varies linearly with temperature between 150° k and 290° k An investigation of the crystalline field in the Tutton salts shows that for the concentrated salts the field has a strong rhombic component above 150° k, whilst in the salts diluted with zinc to less than 1% copper the crystalline field has tetragonal symmetry at 290° k, but the tetragonal axis lies in a plane perpendicular to that containing the tetragonal axis at 90° k.

One obtains two different theories of semiconductors, depending on whether the free energy due to the spin of electrons in donor (or acceptor) centres is taken into account (approach A) or not (approach B) it is shown (i) that the former approach can be carried through for a degenerate semiconductor, (ii) that it applies formally to the case when the interaction between impurity centres and conduction electrons is taken into account, provided one of the symbols is reinterpreted, and (iii) that it is to be preferred. Approach B has, however, been used almost universally It is shown that it corresponds to the unlikely situation in which fifty per cent of the electrons from each impurity level come from paired states, and the remaining fifty per cent from unpaired states having the same activation energy.

The quantization of the complete gravitational field is carried out by extending the work of an earlier paper. The main obstacles in the quantization of Einstein's field are overcome by expressing the field quantities in the Riemannian space as expansions in the flat space, and then splitting the gravitational field into the linear and the non-linear parts. The linear part of the gravitational field is regarded as the free gravitational field, while the non-linearity is treated as a direct interaction between the gravitons. This treatment is quite general, but it suffers from the usual limitations of the perturbation method.

The gravitational self-energies of the photon and the electron are also investigated by using the linear approximation of the gravitational field It is found that the photon self-energy vanishes unambiguously, while the electron self-energy is quadratically divergent.

A model of liquid helium is described, which incorporates the main features of the Landau (1941) and London (1939) theories, making use also of Mayer's concept of clusters. The ordinary process of liquefaction is associated with the formation of clusteis in coordinate space, while the λ-transition is probably associated with the formation of small clusters in momentum space The latter process is probably prevented, or greatly modified, in helium 3

Possible reasons for the appearance of superfluid effects are given, and it is suggested that the unique properties of helium II may be traced to the fact that the interactions between two atoms is of about the strength needed to form a molecule.

Gaseous FaF and InF are formed when the metals are heated with aluminium trifluoride, and sufficient pressures of the monofluorides for observation of their absorption spectra are obtained at about 1000° c for GaF and 1400° c for InF. The three ultra-violet band-systems, ¹Π, ³Π₁, ³Π₀₊ ← ¹Σ⁺ expected by analogy with GaCl and InCl have been photographed on spectrographs of moderate dispersion, and vibrational analyses have been carried out. The ¹Π states converge rapidly to give rather sharp dissociation limits: the products are probably Ga or In, (²P_3/2), + F(²P_½). The bearing of the present results upon the interpretation of the electronic levels of TlF and on the dissociation energy of AlF is briefly considered

The determination of the product pv for particles leaving tracks in photographic emulsions is discussed on the basis of measurements of the scattering in the tracks made by the `coordinate' method (Fowler 1950), allowing for errors of measurement due to grain size The use of Fowler's method with the comparatively large interval lengths used in practice is shown to be justifiable in spite of its neglect of much information contained in the track (This neglect increases the root mean square error in the estimation of the scattering over the minimum attainable error by a factor less than approximately 2.6.) A possible method of estimating pv, which approximates to the `best possible', based on the use of mean squares, is considered, and the root mean square relative error in the estimation found, the necessity for cutting off the scattering distribution at large angles being taken into account A possible method of estimation based on the use of mean moduli is also discussed.

Examination of the relatively long-lived isotopes produced by pile irradiation of samarium using the proportional tube spectrometer leads to the elucidation of the decay schemes of ¹⁵¹Sm and ¹⁵⁵Eu. The conclusions are supported by experiments involving β-e^- and β-γ coincidences, utilizing Geiger-Muller and scintillation counters.

The energies of the lowest states of ⁶Li have been calculated from the standpoint of the shell model, using recent data for the proton-neutron forces. It is concluded that it is necessary to take into account the tensor force in discussing the lowest states of this and other odd-odd nuclei.

The photo-ionization cross section of methane is calculated using self-consistent wave functions. The value at the spectral head (105 850 cm^-1) is found to be 9.4×10^-17 cm². An approximate general formula is employed to obtain results in a wave number range up to 200 000 cm^-1 and the associated oscillator strength is computed. It is pointed out that exchange effects probably reduce the cross section. With the aid of the general formula an estimate is also made of the photo-ionization cross section of argon.