Abstract
Background: The (spin and isospin zero) -particle is an efficient projectile for the excitation of the isoscalar, natural-parity states of C. Among those states that have pronounced -cluster structure, the Hoyle state ( state at 7.65 MeV) has been observed in many () experiments while the second state of C, predicted at MeV as an excitation of the Hoyle state, has not been observed until a recent high-precision experiment of the C scattering at MeV. A plausible reason is a strong population of the narrow state at 9.64 MeV and broad 0 resonance at 10.3 MeV that hinders the weak peak in the () spectrum.
Purpose: The accurate determination of the electric transition strengths of the isoscalar states of C, including an component at MeV that can be assigned to the state, based on a detailed folding-model + coupled-channels analysis of the () data measured at and 386 MeV.
Method: The complex optical potential and inelastic form factor given by the folding model for the C scattering are used to calculate the () cross sections for the known isoscalar states of C in an elaborate coupled-channels approach. The strengths of the form factors for these states are then fine tuned against the () data to deduce the corresponding transition strengths.
Results: A significant transition strength has been obtained for the state from the present analysis of the () data measured at and 386 MeV. The transition strengths of the and states were also carefully deduced, and some difference from the results of earlier analyses has been found and qualitatively understood.
Conclusion: Despite a strong hindrance by the and excitations, the presence of the state in the () spectra measured at and 386 MeV has been consistently confirmed by the present folding-model + coupled-channels analysis.
6 More- Received 16 October 2013
DOI:https://doi.org/10.1103/PhysRevC.88.064317
©2013 American Physical Society