Experimental investigation of α condensation in light nuclei

Abstract

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. Method:To examine signatures of this α-condensation, a compound nucleus reaction using 160, 280, and 400 MeV 16 O beams impinging on a carbon target was used to investigate the 12 C( 16 O, 7α) reaction. This permits a search for near-threshold states in the α-conjugate nuclei up to 24 Mg.Results: Events up to an α-particle multiplicity of 7 were measured and the results were compared to both an Extended Hauser-Feshbach calculation and the Fermi break-up model. The measured multiplicity distribution exceeded that predicted from a sequential decay mechanism and had a better agreement with the multi-particle Fermi break-up model. Examination of how these 7α final states could be reconstructed to form 8 Be and 12 C(0 + 2 ) showed a quantitative difference in which decay modes were dominant compared to the Fermi break-up model. No new states were observed in 16 O, 20 Ne, and 24 Mg 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 multi-particle 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 16 O was observed from ( 28 Si , 12 C(0 + 2 )) 16 O(0 + 6 ) when reconstructing the Hoyle state from 3 α-particles. Therefore, no experimental signatures for α-condensation were observed. arXiv:1907.05471v2 [nucl-ex]

Resonant and scattering states in the α+α system from the nonlocalized cluster model

“…It is also interesting to extend the analysis here to heavier nuclei such as 12 C, 16 O, and 20 Ne. It is particularly interesting to study the resonant and reaction properties of the Hoyle and high-lying Hoylelike states [1][2][3][4][5][6][76][77][78][79][80] with explicit treatments of the asymptotic boundary conditions.…”

The Hoyle Family: The Search for Alpha-Condensate States in Light Nuclei

“…In 16 O, the form factor for transitions from the ground state to the 15.1 MeV 0 + 6 state has not been measured. This measurement is needed since break-up measurements through the characteristic 4α final state [57,64,65] are inconclusive. Beyond oxygen, a high-multiplicity study into the decay of high energy states in 28 Si [57] assessed the equivalency of all the α-condensate decay modes.…”

“…An ideal demonstration of the clustered nature, and in particular of the α-condensate nature would be to observe an enhanced sequential α-particle emission from one α-condensate state to another. To do this, a high-energy compound nucleus reaction 12 C( 16 O, 28 Si ) was employed at beam energies of 160, 280 and 400 MeV, to populate a wide range of states in 8 Be to 28 Si [57]. By looking at the complete decay to a 7 α-particle final state, a direct search for Nα condensate states was performed by examining their complete dissociation into an Nα-particle final state.…”

“…In the 12 C( 16 O, 28 Si ) study [57], to overcome the limitations of the 4α penetrability, populating this state in the 12 C( 16 O, 12 C(0 + 2 )) 16 O reaction was attempted, by reconstructing the 16 O from measuring the 12 C(0 + 2 ). From the compound nucleus, if one decay product ( 12 C(0 + 2 )) is produced which is heavily clustered, one would expect the other decay product to also be preferentially populated by heavily-clustered states.…”

Report on scipost_201911_00026v1

“…In the 12 C( 16 O, 28 Si ) study [36], to overcome the limitations of the 4α penetrability, populating this state in the 12 C( 16 O, 12 C(0 + 2 )) 16 O reaction was attempted, by reconstruct- ing the 16 O from measuring the 12 C(0 + 2 ). From the compound nucleus, if one decay product ( 12 C(0 + 2 )) is produced which is heavily clustered, one would expect the other decay product to also be preferentially populated by heavily-clustered states.…”