Table of contents

The proton spectrum of the reaction ³⁹K(γ, p) has been obtained at 17.6 MeV. Transitions to known levels in ³⁸A at 2.16 and 3.75 MeV were identified and proton groups of energies as low as 2 MeV were resolved, corresponding to levels in ³⁸A at 4.3, 5.0, 5.4, 6.3, 6.6, 7.3 and 7.85 MeV.

The total (γ, p) cross section for the reaction at 17.6 MeV is found to be in excess of 11 mbn.

Cross sections for excitation of atomic levels by electrons often vary sharply with energy near threshold, as if resonating with a state of the compound ion. Some consequences of this `compound ion model' of excitation are explored. In particular it is shown that information bearing on the validity of the model can be obtained from measurements of the angular distribution and polarization of the light emitted following excitation to radiating states. Possible applications to mercury and to helium are discussed.

Using a nitrogen filled proportional counter the resonant absorption of γ-rays to the 8.06 MeV level in ¹⁴N has been observed by means of the ¹⁴N(γ,p)¹³C reaction. The inverse reaction ¹³C(p,γ)¹⁴N at the 554keV resonance was used as the source of γ-rays. The γ-ray flux was accurately measured with a scintillation counter and from the results the width of the 8.06 MeV level for ground state γ-ray transitions was found to be 10.5 ± 0.6 eV. This is in good agreement with the value obtained from the results of Seagrave on the inverse reaction ¹³C(p,γ)¹⁴N. Since the γ-ray energy is a function of angle due to a Doppler shift the resonant character of the absorption has been shown by varying the angle of the proportional counter with respect to the proton beam direction and noting the change in proton yield.

The paramagnetic susceptibilities of 90, 71, 60, 42, 20, 10 and 5 atomic per cent cerium in thorium alloys have been measured over the temperature range 90°K to 700°K. The temperature dependence of the resistivity of 90, 60, 40, 20 and 5 atomic per cent cerium-thorium alloys has been investigated over the range 90°K to 293°K. The thermal expansion of the 60% alloy has also been measured over this temperature range. All the alloys exhibit a sharp decrease in susceptibility at low temperatures. This has been attributed to the excitation of some of the 4f electrons of the cerium atoms into the conduction band at these temperatures, and the resistivity and expansion results support this hypothesis. Above room temperature the susceptibilities of all the alloys obey a Curie-Weiss law, χ=C/(T - θ), over at least part of the temperature range. The effective magnetic moments per cerium atom calculated from the Curie constants C increased with decreasing cerium content. A model has been proposed in which coupling between the cerium f electron and the conduction electrons causes polarization of the latter in the neighbourhood of the cerium atoms. The apparent increase in p_eff is attributed to this induced moment.

Thermal diffusion in mixed gases depends on a `thermal diffusion factor ' α, which is less than unity in all known neutral gas mixtures (in which the particles have no electric charge).

In simple ionized gases α exceeds unity, being of order Z+1, where Ze is the charge on the ions. However, in the steady state of such a gas, at non-uniform temperature, thermal diffusion does not produce a concentration gradient of the electrons relative to the ions; instead it sets up an electric field, which keeps the gas electrically neutral almost everywhere. In an ionized atmosphere subject to gravity, in which the temperature increases upwards, this electric field opposes and reduces the electric field that keeps the electrons and ions together, despite the great difference between their masses. The thermal diffusion factor seems to be greatest for the ions of different kinds in a mixed ionized gas, composed mainly of electrons and light ions of small charge Ze, with a small admixture of heavy ions of greater charge Z'e. In this case α is of order 2.5 (Z'/Z)², and can amount to several hundred. In such a gas, at non-uniform temperature, thermal diffusion can much increase the (still small) proportion of the heavy multiple ions in the hotter regions. The electric field that keeps the electrons with the ions, so that the gas is neutral, reinforces this tendency of the multiple ions, in the cases here considered.

This tendency may have some importance in the solar corona, if turbulence is not too great and temperature inequalities persist long enough.

It seems likely also that thermal diffusion will influence the distribution of the multiple ions present in the zeta nuclear fusion apparatus.

An assessment of the method used to measure values of the Townsend primary coefficient of ionization at low pressures is given. This shows that the secondary processes of ionization must, in general, be taken into consideration in the analysis of the data, especially at pressures of the order of a few millimetres of mercury, in order to obtain accurate values of α/p₀.

General boundary conditions associated with diffusion across a solid - vapour interface are considered in detail and the diffusion equation is solved subject to these boundary conditions. Analytic solutions are derived for: (a) diffusion outwards of an impurity into a finite volume with or without simultaneous pumping; (b) diffusion inwards from a limited source of impurity, initially in the vapour phase, in a closed system. These results are discussed with reference to the diffusion of Group III and V elements across germanium and silicon surfaces.

The resistivity components, Hall coefficients and low field magnetoresistance coefficients have been measured on a number of single crystal specimens of p-type bismuth telluride at 77°k. The results are shown to be consistent with a many-valley form of the valence band for which the energy extrema are on the reflection planes in momentum space. The parameters associated with the model are evaluated and are used to obtain the relations between the Hall coefficients and the density of carriers. The results are used to discuss the variation of the resistivities and the Hall coefficients with temperature over the range 77°k to 290°k and, in particular, to obtain the conductivity mobility for holes for current flow parallel to the cleavage planes. This mobility varies as T^{-1 98} and, for one specimen, has a value of 515 cm² volt^-1 sec^-1 at 290°k.

Heating the surface of an insulator by a rapidly fluctuating source results in a temperature distribution in the solid, which is usually described as an attenuated wave. For high frequency fluctuations, when this description may not be valid, the temperature is found as a solution of the Boltzmann equation. Expressions are derived for various related quantities in terms of the phonon source, its frequency and the coefficient of reflection of the phonons at the boundary.

The magnetothermal resistance and magnetothermoelectric effects have been measured for samples of p-type and n-type Bi₂ Te₃ at 77°k. Scattering laws for the charge carriers have been derived from the magnetothermoelectric effect and also from the magnetothermal resistance effect for p-type material. It has, however, not been possible to fit a simple scattering law to the results from the magnetothermal resistance effect in n-type material.

The integral expressing the frequency response of an optical system in different focal planes in the presence of primary and secondary spherical aberration has been evaluated for a large number of cases employing a Deuce computer, programmed for the numerical integration of double integrals by a method described by Hopkins.

Detailed results are given for values of the secondary spherical (wave front) aberration w₆₀= -4λ, -6λ, -9λ, -12λ, each with three different amounts of primary aberration and each in five different focal planes; and curves are given showing the frequency response that can be obtained when the performance is optimized for low frequencies according to Hopkins' criterion, where a low frequency is one for which the response is greater than or equal to 0.8. For these low frequencies it is confirmed that an under-corrected marginal ray gives the best image quality. For frequencies greater than that for which the response is approximately 0.6, an over-corrected margin is to be preferred.

It is found that, in agreement with the prediction of geometrical optics, the values of the response, D(s, ψ), plotted as functions of the product sw₆₀ (or sw₄₀, for the case w₆₀ = 0), where s is the reduced spatial frequency, lie closely on a single curve, when sw₆₀ ⩽ 1.2 (or sw₄₀ ⩽ 0.3, for w₆₀ = 0).

Electrical resistivity and magnetic susceptibility properties of two alloys based on γ-uranium are described and interpreted in terms of a theoretical structure derived from X-ray work by P. C. L. Pfeil. The implication of the earlier work, that uranium in the γ form is characterized by only four electrons per atom (as opposed to six for the α and β forms) is supported by the present results.

If parity is conserved, neutrons produced in the interaction of unpolarized protons with an unpolarized target should have zero longitudinal component of polarization. An experiment is described in which the longitudinal polarization of the 350 MeV neutron beam emerging from an internal beryllium target in a synchrocyclotron is measured. The result is that, if the reaction matrix describing the neutron production process contains a term F(σ.P/P), where σ, P are the neutron spin and momentum, then |F|² ⩽ 3.6 × 10^-6 with 95% confidence.

A discussion is given of the effects of the nuclear potential, and of multiple scattering, on the polarization of neutrons produced when nuclei are bombarded by high energy protons. The results indicate that, in order to explain the observed polarization at large angles, in particular its sign, multiple scattering must play an important part in the process.

A direct measurement has been made of the relative intensities of the L and K X-radiations emitted in the electron capture decay of ¹²⁶I. The source was distributed throughout the central portion of the NaI crystal in which the X-radiations were measured, and coincidences were taken with the gamma rays from the excited states.

The L/K-capture ratio was observed to be 0.142^{+0 005}_{-0 018}, which is close to the theoretical value of 0.126.

The dielectric constants of aqueous solutions of the chlorides of Li, Na, K, Rb and Cs have been measured over the concentration range 10^-4 to 10^-2 normal at 25°c, using a novel form of electrometer. An accuracy to ±½% is claimed, and the results indicate that the fall in dielectric constant at concentrations below 10^-3 normal is proportional to the square root of the concentration.

Measurements have been made of the amplitudes of the maximum tensile stress pulses which could propagate along rods of glass and of two glasslike plastics (polystyrene and polymethylmethacrylate) without producing fracture. Small quantities of explosive were detonated at one end of the rod so that a compression pulse travelled down the rod and on reflection at the opposite end returned as a tension pulse. The displacement-time curve of the free end was recorded with a condenser-microphone arrangement and the stress was obtained by differentiating this curve numerically. The maximum amplitudes were compared with the tensile strength of the specimen as measured in a standard tensile testing machine. Since the loading time with the stress pulses was only a few microseconds, the ratio of loading rates between the `static' and dynamic tests was of the order of 10⁶. For all three materials the dynamic value was about twice the static. In the two plastics, it was found possible to produce purely internal cracks which resulted from the slightly higher tensile stress in the interior of the rod. With glass no such internal cracks were observed. These results indicate that the strength of the plastics will not depend markedly on the surface condition of the specimen.