Unambiguous identification of the second 2+ state in C12 and the structure of the hoyle state
2013
The second Jπ=2 + state of C12, predicted over 50 years ago as an excitation of the Hoyle state, has been unambiguously identified using the C12(γ,α 0 )Be8 reaction. The
alpha particlesproduced by the
photodisintegrationof C12 were detected using an optical
time projection chamber. Data were collected at beam energies between 9.1 and 10.7 MeV using the intense nearly monoenergetic gamma-ray beams at the HIγS facility. The measured angular distributions determine the cross section and the E1-E2 relative phases as a function of energy leading to an unambiguous identification of the second 2 + state in C12 at 10.03(11) MeV, with a total width of 800(130) keV and a ground state gamma-decay width of 60(10) meV; B(E2:22+→01+)=0.73(13)e2 fm4 [or 0.45(8) W.u.]. The Hoyle state and its rotational 2 + state that are more extended than the ground state of C12 presents a challenge and constraints for models attempting to reveal the nature of three
alpha-particlestates in C12. Specifically, it challenges the ab initio lattice
effective field theorycalculations that predict similar rms radii for the ground state and the Hoyle state. © 2013 American Physical Society.
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