Measurement of the 12C Hoyle state radius via double-excitation inelastic scattering

The detailed properties of 12C second 0+ excited state, known as the Hoyle state, are both a challenge for nuclear structure theory and have a key role in the synthesis of elements. We propose to measure the mean matter radius of this state by analysing the diffraction structure of single- and double-excitation in 12C+12C inelastic scattering at 105 MeV beam energy. The experimental setup consists in the coupling of FAZIA and INDRA multi detectors. Few nuclear excited states have attracted the interest of the scientific community as much as the 12C Hoyle state (second 0+ state at 7.65 MeV excitation energy), which is remarkable in many respects: aside from its key role in the synthesis of 12C, its description remains a challenge for nuclear structure theory.
In nuclear structure, the Hoyle state is crucial to understand clustering in nuclei. Theoretical calculations show different hypotheses regarding its spatial configuration but most of them predict a strongly developed 3-alphas structure and a matter radius significantly larger than 12C ground state. In recent years, many experimental efforts have been made to characterize the Hoyle state decay properties and the rotational band built on top of it. However, only a few attempts to access experimentally the Hoyle state radius have been made, mostly using inelastic scattering of electrons, protons and light nuclei (up to 12C) on 12C target. We propose here to measure the Hoyle state radius by comparing single- and double-excitation in 12C+12C inelastic scattering.