Table of contents

The (γ, p), (γ, pn), (γ, α), (γ, αα) and (γ, αp) reactions in ¹⁴N have been studied by passing the X-ray beam from a 23 MeV synchrotron through a Wilson cloud chamber. The range distribution of protons of energy up to 3 MeV shows sharp well-defined peaks at energies of 0.51, 1.63 and 2.92 MeV. These energies correspond closely to the energies of the resonances reported for the ¹³C(p, γ)¹⁴N (ground state) reaction. For these resonances the ratios of the cross sections for the (γ, p) and (p, γ) reactions are in good agreement with the predictions of the principle of detailed balancing. At higher energies the following measurements were made: (i) the (γ, p) process was investigated by measuring the range distribution of the ¹³C nuclei, (ii) both (γ, p) and (γ, pn) processes were studied by measuring the numbers of events at three different synchrotron peak energies. These measurements showed that both the (γ, p) and (γ, pn) cross sections are small at energies up to 15 MeV and large in neighbourhood of 20 MeV. The estimated values of the integrated cross sections up to 23 MeV for the (γ, p) and the (γ, pn) reactions are both approximately 0.02 MeV barn. The (γ, α), (γ, αα) and (γ, αp) cross sections are each about 0.1 millibarn. The implications of these results are discussed in some detail.

The M_IV and M_V absorption spectra of Er₂O₃ have been studied in detail and a careful comparison made with the Mα, β emission lines. The M_V absorption spectrum possesses a number of well-defined absorption maxima. It is shown that the minima between these maxima coincide closely with components of the Mα multiplet and it is suggested that the main components of the emission line previously observed by Van der Tuuk and by Lindberg are not real but are produced by self-absorption in the target material The line is probably accompanied by one or more real satellites however. The M_IV absorption spectrum is much weaker, but probably possesses a number of maxima also. The Mβ emission line probably possesses a satellite, but its form appears to be influenced by self-absorption.

Extensive air showers with energies of about 10¹⁶ eV and 10¹⁷ eV have been recorded at geographic latitude 53° N by means of counter sets separated by about 150 m and 300 m. Harmonic analysis of the shower frequencies has revealed no evidence for a sidereal daily variation with amplitude exceeding the statistical uncertainties, which are 2.5% and 6% respectively. No evidence has been found for a solar daily variation with amplitude exceeding 2.5% for 10¹⁶ eV showers. A solar daily variation has been found for 10¹⁷ eV showers with an amplitude of 19% and a time of maximum at about 1300 h. The interpretation of this result is briefly discussed.

The γ-ray beam was incident normally on Ilford G5 nuclear research emulsions, and events occurring in the emulsion were studied. The events have been analysed, and particular attention paid to interactions in which charged mesons leave tracks of length greater than 1.5 mm at right angles to the γ-ray beam. About 50% of the observed photo-mesons are not accompanied by visible nuclear disintegration `stars'. The energy spectrum of the emulsion photo-mesons has been measured. The ratio of stars to mesons is found to be 11.4:1. These results are all consistent with a theoretical model proposed by Butler, in which meson production is considered to take place preferentially on the surface of atomic nuclei. Several high-energy electrons and electron-pairs moving at right angles to the γ-ray beam have been observed, and are accounted for.

The observed cross section for the production of stars with three or more prongs is 2.9 × 10^-27 cm² nucleus^-1Q^-1 in agreement with the result of Miller and disagreement with that of Kikuchi. The relation of these results to observations of the electromagnetic excitation of nuclei by fast charged particles is considered.

Measurements of the polarization of some of the high energy γ-rays from the reactions ²⁷ Al(p, γ)²⁸Si, ²⁶Mg(p, γ)²⁷ Al and ²³Na(p, γ)²⁴Mg have been made using nuclear emulsions impregnated with heavy water. The results obtained enable assignments of even parity to be made to the ground and first excited states of ²⁷Al and to the ground state of ²³Na. A range-energy relation for protons up to 5 MeV in wet emulsions is given in an appendix.

Calculations have been carried out of the energy and angular distribution of the electrons produced in the process of pair production by incident quanta using Born's approximation and assuming different forms for the distribution of electric charge on the nucleus.

The effect of the finite charge distribution is large for large angle pairs produced by quanta of energy above 20 MeV, but there are only small differences between the effects to be expected for different distributions.

The scattering of fast electrons with a mean energy of 13.5 MeV has been investigated. Approximately 3000 stereoscopic pairs of photographs were obtained with a Wilson cloud chamber filled with xenon and helium using a 20 MeV betatron as the source of high energy electrons. A total length of corrected track of 1270 metres yielded 318 scatters greater than 5°. Comparison of these results with the Rutherford scattering gives a ratio which varies from unity at 0° to a maximum of 1.6 between 20° and 30°. An inverse square law dependence of scattering with energy is found. General agreement, within the statistical limits of the experiment, is obtained with the theoretical cross sections of Curr, McKinley and Feshbach, and Bartlett and Watson.

The stability of small oscillations of protons circulating in a synchrotron is investigated on the assumption of a uniform space charge in addition to the magnetic field of constant radial gradient.

If the space charge is that of the protons themselves, the greatest number that can be held in a uniform beam of cross section A is VB_ZA/2e, where V is the velocity of the protons, B_Z is the magnetic field on the equilibrium orbit and the units are electrostatic. One third of this number might be held in a bunched beam of 120° azimuthal length, but existing synchrotrons fall far short of this.

Consideration is given to the effects of ionization in the residual gas, and to the possibility that the space charge could be neutralized by the continuous injection of electrons.

Departures from the Landé interval rule in the ground multiplets of certain rare-earth ions are examined. It is found that magnetic spin-spin interactions between the 4f electrons are much too small to account for the apparent distortions, but that by allowing for the partial breakdown of Russell-Saunders coupling it is possible to obtain satisfactory agreement between experiment and theory. Values of the spin-orbit constant ζ are given for several rare-earth ions, and the positions of a number of excited multiplets are tabulated.

The methods described in a previous paper have been applied to the measurement of the mean lifetimes of the first excited states of the mirror nuclei ⁷Li and ⁷Be. The experimental results are, respectively, 7.7 ± 0.8 × 10^-4 sec and 2.7 ± 1.0 × 10^-13 sec. The accuracy of these results is limited, particularly in the latter case, by inadequate knowledge of the range-velocity relationship for these nuclei in their passage through solid materials.

The mean lives of the first 2⁺ states of ¹²C, ¹⁶O, ²⁰Ne, and ²⁸Si have been measured using recoil methods with following results: ¹²C τ = 2.6 ± 0.9 × 10^-14sec. ¹⁶O τ less, similar 2.5 × 10^-14sec. ²⁰Ne τ = 7.6 ± 3.3 × 10^-13sec. ²⁸Si 9 × 10^-13 less, similar τ (or τ less, similar 3 × 10^-14).