A.E. Smith

The construction and operation of a hybrid gas-silicon detector for studies of cluster breakup reactions

A hybrid detector system has been constructed consisting of a gas filled ionization chamber and a large area silicon semiconductor strip detector within the same gas volume. The construction and operation of these detectors is described, together with a brief description of the electronics and data acquisition system used in cluster breakup studies. The techniques for software correction and gain matching of the detector energy signals are described and results are presented for the resolutions in energy, position and time. In a typical application, elements up to Ca are easily resolved and the positions of individual ions are measured with a resolution of <0.2 mm in the reaction plane, whilst energy resolutions approaching 200 keV can be obtained from the Si strip detector over a small area. Some spectra are presented for measurements of the breakup of Mg-24 into C-12 + C-12.

Kinematic coincidence technique to identify γ-decaying highly excited states in light nuclei

Abstract 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 12 C( 16 O, α) 24 Mg and 12 C( 6 Li, d) 16 O 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, Γ γ /Γ, of individual excited states. The general properties of reactions suitable for use with the coincidence technique are discussed.

Symmetric fission of 24Mg following inelastic scattering

Abstract Symmetric fission into two 12 C nuclei has been observed from states at high excitation energy in 24 Mg excited in inelastic scattering. Fission to the 10 B+ 4 N channel has also been observed and indirect evidence for the 16 O+ 8 Be channel noted. The same experiment also reveals fission into 16 O+ 12 C from states in 28 Si populated by alpha-particle pickup onto the 24 Mg projectile. The correlation of the states in 24 Mg observed in this experiment with those previously observed in 12 C+ 12 C scattering is discussed.

A study of 19F by inelastic scattering on a 12C target at 82 MeV and 144 MeV using the techniques of resonant particle spectroscopy

Abstract The α-decay of states in 19F has been studied in the sequential breakup reaction 12 C ( 19 F , 19 F ∗ → 15 N α) at laboratory energies of 82 and 144 MeV. Double differential cross sections have been obtained for 20 excited states in 19F. The correlations show a systematic dependence on the reaction angle θ c.m. ∗ which allows the transferred angular momentum to be deduced. Using this result we present evidence for two new states (J π = 5 2 − or 7 2 − ) at excitation energies of 7740 keV and 8277 keV and two parity assignments for states at 7245 keV ( 3 2 + ) and 9287 keV ( 7 2 + , 9 2 + ) . Comparisons with shell model calculations suggest the two new states probably have J π = 5 2 − and 7 2 − respectively. The double differential cross sections have been analysed with a strong absorption model which allows extraction of qualitative information on the reaction mechanism.

10+ states in 20Ne

Abstract The α-transfer reaction 12C(16O, 20Ne∗)8Be has been studied at a bombarding energy of 150 MeV in a search for high-spin α-cluster resonances in 20Ne. Angular correlations were measured for states in 20Ne which α-decay to the ground state of 16O. A broad 10+ resonance was located at E x ( 20 Ne )∼27.5 MeV , but no definite evidence was found for any 10+ strength at lower excitation energy.

Study of neutron transfer reactions at sub-coulomb energies using a recoil separator

Abstract The results of a study of neutron transfer reactions at energies below the Coulomb barrier using a recoil separator are reported. Excitation functions for single-neutron pick-up at an angle of 180° in the center-of-mass have been measured for the systems 58 Ni+ 116,118,120,122,124 Sn over an energy range of 40 MeV from near to well below the Coulomb barrier. The data are well described in terms of both DWBA and semi-classical models of the transfer process. The relevance of the direct nucleon transfer cross sections to the sub-barrier fusion of these systems is discussed.

Symmetric and near-symmetric fission of 24Mg, 28Si and 32S

Abstract In several recent measurements the symmetric fission of 24 Mg has been reported. In this paper the results of a search for similar processes in 28 Si and 32 S are presented. In contrast to the results for 24 Mg, no evidence is seen for the fission of 28 Si to 16 O+ 12 C or 32 S to 16 O+ 16 O following inelastic excitation. However, these processes are observed to occur following pickup onto 24 Mg. The results are consistent with a simple model of the fission process developed by Harvey and suggest the existence of large scale cluster structure in all three nuclei.

The gross structure resonance near Ecm=25 MeV in 12C+12C scattering

Abstract We have observed a gross structure resonance in the reaction 12 C( 12 C, 8 Be) 16 O leading to the 11.09 MeV 4 + state in 16 O at E cm =25 MeV. Using a triple coincidence technique we suggest J π =16 + for this resonance in agreement with other work. We interpret these results in terms of recent theoretical calculations for 24 Mg.

Comparison of resonances observed in the 12C(24Mg, 12C12C)12C and 12C(20Ne, 12C12C)8Be reactions

Abstract A study of the 12 C( 24 Mg, 12 C 12 C) 12 C reaction has revealed discrete states at excitation energies between 20 and 30 MeV in 24 Mg. Angular correlation analyses of some of the lower energy states confirms the report by earlier authors of monotonically increasing spin values consistent with a rotational sequence. Comparison with breakup observed in the 12 C( 20 Ne, 12 C 12 C) 8 Be reaction suggests that the same states in 24 Mg are populated, while the difference in laboratory cross sections in the two reactions is interpreted as evidence for the association of the breakup states with a prolate hyper-deformed configuration appearing in theoretical calculations for 24 Mg.

A compact high-efficiency 8Be detector

Abstract We report the operation and performance of a compact, high-efficiency 8Be detector designed for use in breakup reaction studies. It offers a number of advantages over previous detectors in allowing accurate kinematical identification of 8Be excited states and scattering angle as well as allowing large solid angle coverage.