6 alpha-cluster resonance structures in C-12+C-12 system and their decay in alpha and Be-8 channels

The excitation functions have been measured for the C-12(C-12,alpha)Ne-20 and C-12(C-12,Be-8)O-16 reactions leading to the excited states up to 25 MeV in Ne-20 and 20 MeV in O-16, respectively, in the beam energy range E(C-12)=48-72 MeV. The region of excitation investigated was E-x=38-50 MeV in Mg-24 which is well above the threshold for breakup into six alpha particles. In the Be-8 channel leading to the 6-7 MeV region of O-16, excitation functions were also measured for Be-8-gamma coincidences in C-12(C-12,Be-8)O-16*-->gamma+O-16(g.s.) particularly to distinguish between the otherwise unresolved 6.05 MeV, 0(+) and 6.13 MeV, 3(-) states in O-16. It is observed that a cluster of resonances in the excitation region 39-43 MeV in Mg-24 decays via alpha and Be-8 channels predominantly to the particle-hole states in Ne-20 and O-16 which are members of deformed bands. Another cluster of resonances in the region 44-49 MeV (centered at E-c.m.=32.5 MeV) decays predominantly to the 20.48 MeV state in Ne-20 (which is above the 5 alpha breakup threshold) and to a possible 4 alpha linear chain band in O-16 around 18 MeV, indicating their highly deformed nature. This latter cluster structure coincides in energy with the possible 6 alpha linear chain resonance identified in the literature at E-c.m.=32.5 MeV in the inelastic scattering channels C-12(7.65,0+) + C-12(7.65,0+) and C-12(7.65,0+) + C-12(9.64,3-). In this excitation energy region intermediate structures in the Be-8 channel are found at E-c.m.=31.5 and 33.5 MeV decaying to the 6.13 MeV, 3(-) state and at E-c.m.=32.5 MeV decaying to the 6.92/7.13 MeV states of O-16. Similar resonances in the same energy region in inelastic scattering channels C-12(g.s) + C-12(9.64,3-) and (12)Cg.s. + C-12(7.65,0+) have also been reported in the literature. It is conjectured from the present measurements that in this region of excitation in Mg-24 (44-49 MeV) there are at least two types of states excited in the collision of C-12+C-12. The first type consists of 6 alpha cluster resonances, possibly with large deformation, leading to outgoing channels above the breakup threshold into constituent alpha particles in both the products and the other type leading to outgoing channels below this threshold.