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

The excitation functions have been measured for the ¹²C(¹²C,α)²⁰Ne and ¹²C(¹²C,⁸Be)¹⁶O reactions leading to the excited states up to 25 MeV in ²⁰Ne and 20 MeV in ¹⁶O, respectively, in the beam energy range E(¹²C)=48-72 MeV. The region of excitation investigated was E_x=38-50 MeV in ²⁴Mg which is well above the threshold for breakup into six alpha particles. In the ⁸Be channel leading to the 6-7 MeV region of ¹⁶O, excitation functions were also measured for ⁸Be-γ coincidences in ¹²C(¹²C,⁸Be)¹⁶O^*-->γ+¹⁶O_g.s. particularly to distinguish between the otherwise unresolved 6.05 MeV, 0⁺ and 6.13 MeV, 3^- states in ¹⁶O. It is observed that a cluster of resonances in the excitation region 39-43 MeV in ²⁴Mg decays via α and ⁸Be channels predominantly to the particle-hole states in ²⁰Ne and ¹⁶O 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 ²⁰Ne (which is above the 5α breakup threshold) and to a possible 4α linear chain band in ¹⁶O around 18 MeV, indicating their highly deformed nature. This latter cluster structure coincides in energy with the possible 6α linear chain resonance identified in the literature at E _c.m.=32.5 MeV in the inelastic scattering channels ¹²C_7.65,0⁺+¹²C_7.65,0⁺ and ¹²C_7.65,0⁺+¹²C_9.64,3^-. In this excitation energy region intermediate structures in the ⁸Be 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 ¹⁶O. Similar resonances in the same energy region in inelastic scattering channels ¹²C_g.s+¹²C_9.64,3^- and ¹²C_g.s.+¹²C_7.65,0⁺ have also been reported in the literature. It is conjectured from the present measurements that in this region of excitation in ²⁴Mg (44-49 MeV) there are at least two types of states excited in the collision of ¹²C+¹²C. The first type consists of 6α 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.