High multiplicity α-particle breakup measurements to study α-condensate states

An experiment was performed to investigate α-condensate states via high α-particle multiplicity breakup. The nucleus of interest was 28Si therefore to measure multiplicity 7 particle breakup events, a highly granular detector with a high solid angle coverage was required. For this purpose, the CHIMERA and FARCOS detectors at INFN LNS were employed. Particle identification was achieved through ΔE-E energy loss. The α-particle multiplicity was measured at three beam energies to investigate different excitation regimes in 28Si. At a beam energy where the energy is sufficient to provide the 7 α-particles with enough energy to be identified using the ΔE-E method, multiplicity 7 events can be seen. Given these high multiplicity events, the particles can be reconstructed to investigate the breakup of α-condensate states. Analysing the decay paths of these states can elucidate whether the state of interest corresponds to a non-cluster, clustered or condensed state

Experimental investigation of α condensation in light nuclei

Background: Near-threshold α-clustered states in light nuclei have been postulated to have a structure consisting of a diffuse gas of α particles which condense into the 0s orbital. Experimental evidence for such a dramatic phase change in the structure of the nucleus has not yet been observed. Purpose: To understand the role of α condensation in light nuclei experimentally. Method: To examine signatures of this α condensation, a compound nucleus reaction using 160-, 280-, and 400-MeV 16O beams impinging on a carbon target was used to investigate the 12C(16O,7α) reaction. This permits a search for near-threshold states in the α-conjugate nuclei up to 24Mg. Results: Events up to an α-particle multiplicity of seven were measured and the results were compared to both an extended Hauser-Feshbach calculation and the Fermi breakup model. The measured multiplicity distribution exceeded that predicted from a sequential decay mechanism and had a better agreement with the multiparticle Fermi breakup model. Examination of how these 7α final states could be reconstructed to form 8Be and 12C(02+) showed a quantitative difference in which decay modes were dominant compared to the Fermi breakup model. No new states were observed in 16O, 20Ne, and 24Mg due to the effect of the N−α penetrability suppressing the total α-particle dissociation decay mode. Conclusion: The reaction mechanism for a high-energy compound nucleus reaction can only be described by a hybrid of sequential decay and multiparticle breakup. Highly α-clustered states were seen which did not originate from simple binary reaction processes. Direct investigations of near-threshold states in N−α systems are inherently impeded by the Coulomb barrier prohibiting the observation of states in the N−α decay channel. No evidence of a highly clustered 15.1-MeV state in 16O was observed from [28Si★,12C(02+)]16O(06+) when reconstructing the Hoyle state from three α particles. Therefore, no experimental signatures for α condensation were observed

A new method for the determination of very small Γ γ

We present a new method for the measurement of very small Γγ partial width that is important for the synthesis of elements in astrophysics. The method is based on the simultaneous detection of scattered beam, residual nucleus and decay γ rays. This method is optimized for the use of the CHIMERA detector at LNS. Experimental details are described