Direct observation of a new 2+ state in C12

Abstract

The photodisintegration of 12 C into three α particles was observed using intense, nearly monoenergetic γ-ray beams available at the HIγS facility. An optical time projection chamber was used to detect the recoiling α particles from the 12 C(γ,α) 8 Be reaction. A J π = 2 + state in 12 C was found at 10.03(11) MeV with a width of 800(130) keV. Recently EFT lattice calculations have been performed in order to understand the structure of the 'Hoyle state' at 7.65 MeV, and extensions of these calculations have identified a 2 + excitation of the Hoyle state. This newly observed 2 + state in 12 C provides an important test for these calculations as well as for other models of light nuclei. Furthermore, a state in 12 C near this energy increases the rate of stellar thermonuclear helium burning at the high temperatures which occur in core-collapse supernovae and other astrophysical scenarios.

Three-body bremsstrahlung and the rotational character of theC12spectrum

“…A simple rotational sequence is also observed in 8 Be, where the ground state and the two first excited states follow as well the angular momentum and parity sequence 0 + , 2 + , and 4 + (also 6 + and 8 + states with large widths, are found numerically as poles of the S matrix). All the states in the 8 Be spectrum are unbound (resonances), with the ground state only about 0.1 MeV above the threshold for emission of two alpha particles.…”

“…In fact, when increasing the angular momentum of the initial states, the transition strength was found to decrease, which is precisely the opposite to the prediction of the rotational model [20]. Nevertheless, allowing the separation between the two alphas in the 8 Be resonances to change according to the computed root mean square radius, a very nice agreement between the computed strengths and the rotational model predictions was observed.…”

“…All the states in the 8 Be spectrum are unbound (resonances), with the ground state only about 0.1 MeV above the threshold for emission of two alpha particles. Therefore, the question arises about a possible rotational (or in general collective) character for a sequence of continuum states of considerable width.…”

“…This problem was recently investigated in previous works [19][20][21]. This was done by computation of the electric quadrupole cross sections for bremsstrahlung emission after transitions between the 8 Be states, which were described as two-body systems made of two alpha particles. These calculations required a careful treatment of the continuum wave functions and clarification of the definition of the cross section for transitions between states with a none well-defined energy.…”

“…From these 8 Be cross sections it is possible to extract the transition strengths, which for the case of transitions within a rotational band must follow well-established rules based on the assumption that all the states have the same intrinsic spatial structure (rigid rotor). The main conclusion was that the computed transition strengths do not behave as expected for states in a rotational band.…”