Search for α-particle condensation in nuclei from the Hoyle state deexcitation

Abstract

Heavy ion reactions induced by few tens MeV/nucleon beams constitute an ideal framework for producing and studying nuclear excited states close to the decay threshold. The fragmentation of quasi-projectiles from the nuclear reaction 40Ca+12C at 25 MeV/nucleon performed at LNS-Catania was employed in order to populate the Hoyle state of 12C and investigate the theoretically predicted molecular structures. Complete kinematic characterization of individual decay events, made possible by the CHIMERA high-granularity 4π charged particle multi-detector, reveals that 7.5±4.0% of the particle decays of the Hoyle state correspond to direct decays in three equal-energy α-particles and thus fulfill the decay criteria of an α-particle condensate. Moreover, events with increased kinetic energy dispersion in the 12C center of mass, which amount to 9.5±4.0%, point toward the occurrence of a second competing molecular configuration, a linear α-chain type. Both α-particle condensate and linear chain structures have been theoretically predicted but, to our knowledge, not experimentally confirmed so far.