Multiprobe study of excited states in 12 C : Disentangling the sources of monopole strength between the energy of the Hoyle state and E x = 13 MeV

Abstract

Background: The Hoyle state is the archetypal α-cluster state which mediates the 3α reaction to produce 12C and is of great interest for both nuclear structure and astrophysics. Recent theoretical calculations predict a breathing-mode excitation of the Hoyle state at Ex≈9 MeV. Its observation is hindered by the presence of multiple broad states and potential interference effects. An analysis with Gaussian line shapes of measurements at the Research Center for Nuclear Physics (Osaka University) with the Grand Raiden spectrometer suggested that additional strength was needed at Ex≈9 MeV to reproduce the data; this analysis did not account for the well-known threshold effects observed in 12C. Nevertheless, various theoretical studies have since concluded that this additional strength corresponds to the predicted breathing-mode excitation of the Hoyle state. To meaningfully identify a new source of monopole strength in this astrophysically significant region, a more appropriate phenomenological analysis which accounts for penetrability and interference effects must be used to determine whether the data can be explained with previously established states.

Purpose: We aim to investigate the monopole strength in the astrophysically important excitation-energy region of 12C between Ex=7 and 13 MeV to determine whether the previously established sources of monopole strength are able to reproduce the data.

Method: The 12C(α,α′)12C and 14C(p,t)12C reactions, which are expected to exhibit contrasting selectivity towards different monopole excitations, were employed at various detection angles and beam energies to populate states in 12C. The inclusive excitation-energy spectra were simultaneously analyzed with multilevel, multichannel line shapes. Various scenarios with different sources of monopole strength and interference effects were considered to determine whether the ghost of the Hoyle state and the previously established broad 0+3 state at Ex≈10 MeV are able to reproduce the observed monopole strength.

Results: Clear evidence was found for excess monopole strength at Ex≈9 MeV, particularly in the 12C(α,α′)12C reaction at 0∘. This additional strength cannot be reproduced by the previously established monopole states between Ex=7 and 13 MeV. Coincident charged-particle decay data suggest that the strength at Ex≈9 MeV is dominantly monopole, with no evidence of a J>0 contribution.

Conclusions: The data support a new source of monopole strength at Ex≈9 MeV, which cannot be described with a phenomenological parametrization of all previously established states. An additional 0+ state at Ex≈9 MeV yielded a significantly improved fit of the data and is a clear candidate for the predicted breathing-mode excitation of the Hoyle state. Alternatively, the results may suggest that a more sophisticated, physically motivated parametrization of the astrophysically important monopole strengths in 12C is required.