Table of contents

The cross sections for the production of ¹⁸F from ¹⁹F, ²³Na, ²⁴Mg, ²⁷Al, ²⁸Si, ³¹P and ³²S by bremsstrahlung with a maximum energy of 240 MeV have been measured. The cross sections for the production of ¹⁸F from ¹⁹F, ²³Na and ²⁷Al have also been obtained for bremsstrahlung with maximum energies of 180 MeV and 120 MeV. The results are compared with other spallation work.

Relations between the single-crystal and polycrystalline saturation magnetostriction constants are derived for cubic materials of positive and negative anisotropies. Terms which are independent of the direction in which the magnetostrictive strain is measured are not neglected and the formulae obtained are thus directly applicable to the corresponding magnetoresistance constants. The method used is perfectly general and is extended to deal with materials of hexagonal (and cylindrical) symmetry for which the hexagonal axis is a direction of easy magnetization or for which the basal plane is a plane of easy magnetization. The relations obtained are compared with experimental data.

The small heat changes accompanying the magnetization of pure nickel ferrite have been measured for a rod specimen at - 10° and 18°C. By making use of the correctness of current theory for thermal changes accompanying the magnetization of ferromagnetic substances in high fields, the results have been analysed, and some slight support found for the Lilley disperse field theory.

Quasi-elastic interactions of 930 MeV protons with bound protons in carbon have been investigated, using a propane bubble chamber. A method has been devised to relate the measured scattering angles to the momenta of the bound nucleons. A Gaussian curve falling to 1/e at a momentum corresponding to a kinetic energy of 13 MeV has been fitted to the experimentally determined momentum distribution. The cross section for quasi-elastic reactions in carbon is found to be 46 ± 10 mbn, considerably higher than values which might be inferred from results of previous experiments.

Experiments are described in which small particles of cubic cobalt have been obtained by precipitation from a 2% Co-Cu alloy for the purpose of measuring the total internal magneto-crystalline anisotropy field as a function of particle radius. Two methods have been used, namely static torque measurements of the principal anisotropy constants and dynamic measurements based on spin resonance in the anisotropy field. The two methods yield fair agreement in the values calculated for the anisotropy field in a range of particle sizes.

The field is found to vary little with particle radius in a single crystal specimen and this is interpreted as evidence that internal resonance is independent of shape anisotropy.

The time dependence of the viscosities of certain thixotropic fluids has been measured at constant rate of shear, using a Couette type viscometer. A time dependent yield value has been introduced which relates the instantaneous value of the observed viscosity, for a given rate of shear, to a time independent residual viscosity. The transient behaviour of the yield value has also been examined

Using the double resonance technique a study has been made of the hyperfine structure of the 3²P₃₂ state of sodium in a magnetic field sufficiently strong to decouple completely J and I. The values of the hyperfine coupling constants are a=18.5±0.6 M_c/s and b=2 25 ± 0 4 M_c/s. The Landé factor for the state is g_J=1.3341 ± 0 0003

Are-investigation of the combined angular correlation existing between the 296, 308 and 316 kev γ-rays of ¹⁹²Pt has been found to be in agreement with spin 3 for the 4th excited state and M1/E2 mixtures of about 3% and 5% for the 308 and 296 kev transitions respectively.

The Born-Oppenheimer approximation has been used to evaluate the cross section for the detachment of electrons from H^- by electron impact over an energy range from threshold (0.75 eV) to 75 eV. An empirical correction was then applied to account approximately for the long range Coulomb repulsion between the incident electron and the negative ion. The resulting cross section has a broad maximum of 700πa₀² centred at about 45 eV.

A procedure is discussed by which the spatial structure of classical simple non-spherical condensed systems (i.e. systems composed of non-spherical particles interacting in pairs) can be determined using the well established formulae of statistical mechanics without the need to appeal explicitly to vectorial integro-differential equations. In the mathematical method, the effects of particle asymmetry are introduced gradually starting from an appropriate symmetry already known from alternative calculations. The superposition approximation is invoked but its effect may be controlled if necessary. The case of slight molecular asymmetry is the one most conveniently treated although the procedure has a wider usefulness.

In order to answer the question left open by a previous publication ultrasonic measurements have been made in liquid methylene chloride at - 22°C. These confirm that there is a relaxation of the vibrational specific heat less the contribution of the lowest frequency mode of vibration. The paper ends with a short discussion of some hypersonic measurements recently reported by Pesin and Fabelinskii.

The absorption spectra of the alkaline earth metal vapours have been photographed in the quartz ultra-violet and Schumann regions, by means of a 3 m vacuum spectrograph and large King furnace. Considerable extensions of the are spectra of these elements have resulted, many of the new lines being two-electron transitions. Extreme cases of autoionization and many examples of series perturbations occur. The results are presented for the case of Ba, including assignments for 64 out of about 68 new lines. The presence of a doubly excited level practically coincident with the lowest ionization limit has striking effects on the photoionization cross section and on the intensities of the high members of the singlet principal series of Ba, which is extended to the m=37 member.

The energy spectra of the protons emitted from a vanadium target when bombarded by 8.9 MeV deuterons have been measured by magnetic analysis at angles of observation between 5° and 70°. The angular distributions of sixteen proton groups have been measured and compared with theoretical stripping curves to obtain information on parities, spins and reduced widths. Most of the transitions required an l=1 assignment. The results are compared with results on the ⁵¹V(n, γ)⁵²V reaction.

Various possible mechanisms, which could produce potentials in a pure liquid through which an ultrasonic wave is propagating, are discussed. Alignment of molecular dipoles, due to density gradients, appears to be the most effective mechanism, and could account for the potentials observed in polar liquids. It is suggested that the non-polar heptane (and the similar paraffins of this series) really contain weak quasi-free dipoles, whose moment is of order 0· x 10⁻¹⁸ e.s.u. cm. This could account for the small potentials which have been observed in heptane by Rutgers and Rigole.

Kirkwood's expression for the friction constant is compared with the expression implicit in Brownian movement theory. Relations between the friction constant β and the fluctuating force F(t) are obtained in the case of a white spectrum and of a general spectrum. The former relation resembles Kirkwood's expression and depends on the correlation function of F(t) being sufficiently sharply peaked to permit the integral to reach a plateau value in a time τ₁ such that βτ₁ ≪ 1.

Since Kirkwood's expression involves the total force X(t) = F(t) - βu, the correlation integral of this force is calculated in the macroscopic theory of Brownian movement. The infinite integral

∫^∞₀ <X(t)X(t + τ)>_tdτ

is found to be zero, but the incomplete integral

∫τ10 <X(t)X(t + τ)>tdτ

is found to be equal to the incomplete correlation integral of the fluctuating force

∫τ10 <F(t)F(t + τ)>tdτ

provided that the upper limit of integration τ₁ is large enough for the correlation function <F(t)F(t+τ)>_t to have decayed to zero, but is sufficiently small to ensure that βτ₁ ≪ 1.

It follows that there is correspondence between the microscopic and macroscopic theories provided this condition holds.

Using pure single crystals of rhombic sulphur, of volume of few mm³, conduction counting occurs for α-particle bombardment but only in a region close to the negative electrode, indicating a barrier effect. If the applied voltage is increased there is an increase in the counting rate proportional to Vⁿ where n = 0.7, but no increase in the maximum pulse height. Creation of a space charge barrier is essential before counting can begin, after which the counting rate decays with time but the crystal can be reactivated by light or other ionizing radiations. Examination was made of the primary photoconductivity between 4400 Å and 9000 Å, down to 130°K and the photoconductive efficiency was found to be about 1.5 μAW^-1.

Comparing these results with von Hippel's interpretation for the alkali halides, a mean value of N = 10¹³ cm^-3 was obtained for the donor density. Secondary currents were also observed and breakdown pulses were attributed to field emission at the electrodes. The free path w per unit field was estimated as gt-or-equal, slanted 5 × 10^-8 cm, and the lower limit of the mobility μ_e of the electrons, which were the charge carriers, as about 10^-1 cm² v^-1 sec^-1. The energy per ion pair was found experimentally to be about 32 ev which agrees with the value deduced from independent data on the electronic band structure of sulphur. The thickness of the barrier layer was approximately 10^-2 cm with barrier field strength of the order of 10⁵ v cm^-1. The conductivity pulse magnitude depends upon the position of the α-particle track inside the barrier and the excitation mechanism for conductivity pulses in a molecular crystal like sulphur differs considerably from that of a covalent crystal like diamond.