Breakup reactions populating cluster states in "2"8Si and "2"4Mg

The "1"2C+"1"6O breakup of "2"8Si and the "1"2C+"1"2C breakup of "2"4Mg have been studied following the interaction of a 170 MeV "2"4Mg beam with "7Li, "9Be, "1"2C and "1"6O target nuclei. The measurements were performed at the Australian National University in Canberra, using the technique of Resonant Particle Spectroscopy. The breakup fragments from the decay of the resonant nuclei were detected in two Gas-Si-CsI telescopes positioned on opposite sides of the beam axis. The data suggest that the same states in "2"8Si are populated via the "7Li ("2"4Mg, "1"2C"1"6O)"3H, "9Be ("2"4Mg,"1"2C"1"6O)"5He and "1"2C("2"4Mg,"1"2C"1"6O)"8Be reactions. This implies that the cluster decaying states are populated by direct #alpha#-transfer. Breakup has been observed from states in "2"8Si at excitation energies (spins) of (26.15), 28.17 (13"-), 29.51, 29.95, 30.45, 30.76, (31.3), 31.65, 31.90, 32.51, 33.14, 33.41, 33.77, 34.45 (12"+,14"+) and 35.13 MeV. A consistent theoretical interpretation of the "2"8Si molecular structures has been given, taking into account the predictions of Nilsson-Strutinsky, #alpha#-cluster model and two centre shell model calculations. The present results for the "1"2C("2"4Mg,"1"2C"1"2C)"1"2 C reaction agree with previous measurements. In addition, new spin assignments have been proposed for several of the breakup states in "2"4Mg. States have been observed at excitation energies (spins) of 20.54 (2"+), 21.07 (4"+), 21.88 (4"+), 22.33 (4"+), 22.90 (6"+), 23.80 (6"+,(8"+)), 24.56 (8"+), 25-14 (6"+), 25.72, 26.41 (8"+) and 27.12 MeV. Evidence for the population of many of these states via the "1"6O("2"4Mg,"1"2C"1"2C)"1"6O reaction has also been observed. However, the data gave no evidence for either the "7Li("2"4Mg,"1"2C"1"2C)"7Li or "9Be("2"4Mg,"1"2C"1"2C)"9Be reactions. The presently available information did not allow an unambiguous determination of the reaction mechanism responsible for the population of the "2"4Mg breakup states. The performance of the Gas-Si-CsI telescopes has been investigated. For multiplicity 2 events in the silicon strip detectors, a crosstalk has been observed between the two active strips. The energy calibration of the silicon strip detectors for penetrating particles has also been found to differ to that for stopped particles. Empirical corrections for both of these effects have been deduced allowing the simultaneous detection and identification of heavy and light ions within a single telescope. These techniques have been extended to the detection of "8Be #-># #alpha#+#alpha# events over a wide range of #alpha#-particle energies. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:DXN027831 / BLDSC - British Library Document Supply CentreGBUnited Kingdo