Table of contents

In the solid state the orbital momentum of the divalent vanadium ion 3d³ is quenched by the cubic crystalline field, leaving a spin quadruplet as the ground state. Paramagnetic resonance measurements show that in V(NH₄)₂(SO₄)₂.6H₂O this is split into two doublets with a separation of 0.36 cm^-1 by a component of the crystalline field with rhombic symmetry. An extensive and isotropic hyperfine structure is observed, from which the value of 7/2 for the nuclear spin of ⁵¹V is confirmed. The contribution from the observed splittings to the tail of the magnetic specific heat curve is evaluated.

Electron diffraction patterns from films of long-chain paraffins show the effects of secondary elastic scattering of electrons in the form of extra spots, groups of spots, arcs of circles, diffuse bands with sharp edges, and forbidden reflections in single-crystal patterns. Examples of such effects are given and analysed to demonstrate their origin in the successive diffraction of electrons from two different single-crystal regions. It is pointed out that many effects recorded in the literature on electron diffraction may be explained in the same way. The modification of the intensities in single-crystal patterns and the appearance of forbidden reflections are treated quantitatively on the basis of secondary diffraction by parallel but non-coherent regions of crystal. Expressions are derived whereby observed intensities may be corrected for secondary scattering when once the scattering efficiency of the crystal has been deduced from the observed intensities of forbidden reflections. The importance of such corrections for purposes of structure analysis is demonstrated in the comparison of two Fourier projections made from a cross-grating pattern of dicetyl (C₃₂H₆₆), one using observed intensities and the other using corrected intensities. Both show clearly the general arrangement of the molecules, but that based on corrected intensities contains much less spurious detail than the other, and gives greater accuracy and a much clearer indication of the positions of the hydrogen atoms

Inelastic scattering of charged particles can take place through the electrostatic interaction between the particles and the nucleus. Expressions for the total cross section of the process have been derived using several methods and are found to be in close agreement. The angular distributions are less reliable. Questions for more practical interest are discussed and conditions given for the best observation of the effect. The total cross sections are not negligible and certain experimental results can be explained. Inelastic nuclear scattering has been extensively used hitherto to obtain the positions of nuclear energy levels. It is suggested that more information about nuclear states might be provided by experiments exploiting the electrostatic interaction.

Correlations between the nuclear shell structure and the density distribution are analysed on the assumption that the former is associated with the successive appearance of particles of higher angular momentum. General requirements for the possible form of the nuclear density distribution are derived. It is shown that a resonable density distribution conforming with the above requirements and other known facts reproduces unmistakably the `magic numbers'.

A method has been developed for calibrating neutron sources absolutely, using the Graphite Low Energy Experimental Pile. The method used is to convert the measurement of the number of neutrons emitted by a source to a determination of the absolute beta activity of an irradiated sodium or phosphorus sample.

One source calibrated by this method, which contained 1,290 mg. of radium in the form of sulphate, mixed with 12 gm. of beryllium, was found to have a neutron strength of 9.3 × 10⁶ n/second (± 4½%).

The energy levels of F- and F'-centres are calculated on a very simple model. The ground state of F-centres is found to be determined mainly by the radius of the hole that represents the missing ion in Jost's model, and to vary only slightly with the dielectric constant.

In thallium and silver halides the long-wave photoconductivity is probably due to F-centres. In PbS the energy levels of F- and F'-centres are much too deep to be responsible for photoconductivity. D-centres consisting of a pair of adjacent missing ions are also investigated: they may act as shallow traps, thereby reducing electron mobilities.