Levels of 19F in the excitation region 4<Ex<11 MeV were studied with the 12C(11B, α) reaction, using a recoil-coincidence technique to measure values of γγ/γ. The information obtained has been combined with known values of ωγ to determine alpha-particle widths in the range ~1−100 eV for various levels. The highest lying level observed to have a significant gamma-decay probability was a new level at Ex = 10.927 ±0.008 MeV with γγ/γ = 0.051±0.004. The 1124−, 9.873 MeV level was observed to have a surprisingly large value for γ /γ of 0.43 ±0.04, which gives γα = 1.4±0.3 eV, γp = 0.10 ± 0.03 eV, and a total width γ = 2.6±0.4 eV. The present measurement of γinα for the 1121− and 1124− levels, together with previous information for the 1122 among these 112− levels and clarify a long outstanding problem concerning the 112− member of the Kπ = 12− alpha-cluster band. We conclude that the 1122−, 8.953 MeV level is the best candidate for this alpha-cluster state, and there is very little configuration mixing among the 112− levels.

Abstract Gamma-decaying high-spin states of 22Na populated in the 12C(14N, α) reaction were studied, using a recoil-coincidence technique to measure values of Γ γ Γ and an α-γ coincidence arrangement to measure lifetimes and γ-decay branches. Good candidates have been found for the yrast 8+ and 9+ levels at excitation energies of 8572±3 and 9813±3 keV, respectively. Their properties are compared with those predicted by the shell model and test these calculations in the region where band termination is expected.

Measurements of Γγ/Γ have been carried out for about 40 states of 24Mg in the excitation region 10 to 12 MeV using a recoil-coincidence technique. The information obtained has been combined with known values of ωγ to determine Γγ, and Γα. Widths of the order of 0.01 to 100 eV can be determined accurately using this method. Some previous data on levels in this excitation region are discussed. Excitation energies and electromagnetic transition rates are compared with theoretical predictions. Two notable conclusions are the observation of a considerable excess in E2 strength in the 10 to 12 MeV region compared with that predicted by the shell model, and the identification of the 4+ 11.219 MeV level as possibly being a member of a collective band based on the 0+, 6.432 MeV level.

The reaction 19F(6Li, d)23Na has been studied at a beam energy of 34 MeV using a high resolution magnetic spectrometer. A total of 36 states in 23Na have been observed in the range Ex = 4–13MeV and deuteron angular distributions obtained for 26 states. Compound processes accounted for less than 10% of the forward angle cross section in most instances. Finite-range distorted-wave Born approximation calculations have been performed and compared with the data. The calculations reproduced the observed angular distributions well, allowing relative α-particle spectroscopic factors to be extracted and several new or improved spin assignments to be made. The 6Li-19F optical potential was derived from elastic scattering data obtained in the present work. The results are discussed with reference to the rotational structure of 23Na.

A search has been made for high-lying states in 20Ne which decay by γ-ray emission. The states were populated via the 12C(12C, α)20Ne reaction at a beam energy of 37.1 MeV. A technique which allowed the measurement of γ-ray emission probabilities down to less than 1% was employed. In addition to previously known γ-ray emitting states, states at 11.89, 12.01, 12.10, 12.17 and 12.38 MeV were found to have observable γ-ray decay modes.

The alpha decay width and total width of the first 0+, T = 2 state in 40Ca have been measured by the elastic scattering of α-particles from 36Ar. The results are Γα0 = 80 ± 10 eV and Γ = 81 ± 10 eV.

A new high resolution magnetic spectrometer — the Oxford MDM-2 spectrometer — has been designed, installed and tested. The layout of the magnetic elements is in the order: entrance sextupole and multipole, gradient-field dipole and exit multipole. The device has a “normal” focal plane, and the 1.6 m radius dipole magnet has a maximum mass-energy product of 315 MeV amu. At the maximum solid angle of 8 msr, the ratio of energies that can be accepted by the spectrometer (Emax/Emin) is 1.31.

A “hybrid” focal plane counter comprising a gridded ionization chamber with position sensitive proportional counters, 300 mm long, 510 mm deep and with an active height of 60 mm, is described. The detector is designed for use with the Oxford MDM-2 magnetic spectrometer and is suitable for both light and heavy ions. Typical results obtained are 0.6 mm position resolution and 1% total energy resolution. Two energy loss, a veto and two position signals are available as well as provision for height and timing signals. Techniques are described for the reduction of capacitive noise, correction of electric field nonuniformities, and the reduction of interference from positive ions on the ionization signals.

States with excitation energy > 15 MeV populated in the reaction 12C(16O, αγ)24Mg have been studied using a total energy γ-ray spectrometer for Elab(16O) between 40 and 67 MeV. A previously unknown state has been identified at 16.904 ± 0.003 MeV with Γγ/Γ=0.13±0.02, mean lifetime <10 fs, a predominant γ-decay branch to the yrast (8+) state at 11.86 MeV and possible un-natural parity. These measurements also show that all other states produced in this excitation energy region have Γγ/Γ less than 0.02. Possible points of correspondence between the observed properties of the 16.9 MeV level and the predictions of shell model calculations are noted.

A simple kinematic coincidence technique is described for separating γ-decaying states from particle decaying states populated in two-body reactions. The method is especially suited to the discovery and further study of γ-decaying states at high excitation energies in light nuclei, but has not previously been widely exploited. Experimental applications using the reactions 12C(16O, α)24Mg and 12C(6Li, d)16O are presented and these illustrate the range of results obtainable. In particular, the method provides an excellent means of measuring the probability for γ-decay, Γγ/Γ, of individual excited states. The general properties of reactions suitable for use with the coincidence technique are discussed.

A search has been carried out for the 1+ T = 1 level at 11.264 MeV in 20Ne formed as a parity-forbidden resonance in the 16O(α,γ)20Ne reaction. The yield of 11.26 MeV γ-rays was measured as a function of α-particle energy over an interval of 8 keV centred on the expected resonance energy of the 1+ T = 1 level. An energy and intensity standard was provided by the nearby 1− T = 1 resonance at an excitation energy of 11.275 MeV in 20Ne. A significant improvement in the fit to the measured yield curve was obtained by including a sharp resonance at the expected energy of the 1+ T = 1 level. From the analysis it is concluded that the 1+ T = 1 level has been observed with 90% confidence. The resonance strength extracted from the fit implies a parity-forbidden a-particle width of 42(+-20)× 10−6 eV for the 1+ T = 1 level. The corresponding parity non-conserving matrix element linking the 1+ T = 1 level with the 1+ T = 0 state at 11.23 MeV therefore lies within the, range 0.8 eV ⩽ 〈VPNCΔT = 1〉 ⩽ 2.6 eV.

The mean life of the 1.04 MeV 0+, T = 1 state in 18F has been measured as 2.2±0.6 fs using a refinement of the Doppler-shift attenuation method. The reaction 3He(16O, p)18F at a mean beam energy of 26.6 MeV was used to populate the 1.04 MeV level with a recoil velocity vc = 4.4 %. The targets were 3He implanted in foils of magnesium, aluminium, tantalum and gold. The result is compared with shell-model calculations and its relevance to the search for parity non-conservation in the system comprising the 1.04 MeV level and the 1.08 MeV 0−, T = 0 level in 18F is discussed.

The lowest 0+; T = 2 state in 40Ca has been excited as a resonance in 36Ar(α,γ)40Ca at Ex = 11988±2 keV; гp1 has been determined and limits placed on Γ, Γp2, Γp3 and Γα1. Transitions to 1+;T = 1 levels at 10321.0 ± 1.6 keV and 9868.0 ± 1.7 keV were observed and their strengths measured. Ground-state M1 strength from 1+;T = 1 levels is inferred.

Many gamma-decaying levels up to 8.25 MeV in 21Ne have been populated via the 12C(13C, α)21Ne reaction at E13c = 19.7 McV. Gamma-rays de-exciting these levels were detected in coincidence with α-particles, making it possible to study the γ-decay of individual levels in isolation. A variant of the Doppler shift attenuation method due to Branford and Wright was used to measure the lifetimes of most of the states observed. The relevance of the information obtained to hand structure in 21Ne is discussed. Shell-model calculations are also presented for positive-parity states, and compared with the available experimental information.