WU's adventures with the Hoyle state
The second 0+ state in 12C was predicted by Fred Hoyle on the basis that if such a state did not exist just above the α+8Be threshold (which in turn is just above the 3α threshold), building the elements heavier than C would (essentially) not occur. The state was subsequently found and its width (Γ = 9.3 ev, i.e. τ ~ 7.1 as) determined. The synthesis of 12C occurs when this highly excited state decays to the ground state. While questioned already in the late 1960s, it has been assumed that this decay occurs via sequential-gamma emission through the intermediate 2+ state.
The Hoyle state has been the focus of intense experimental and theoretical work in the past decade and much has been learned. For example we now know that there is a rotational excitation (2+2) built on the Hoyle and that this state is likely a bent (as O3 is) α-cluster state. Our group got engaged in "Hoyle" work when we were asked by a senior nuclear astrophysicist to address the issue of whether the Hoyle state decays to any appreciable extent directly to 3 alphas, bypassing 8Be. (He was perturbed by a group saying that this branch was 7.5%.) We were able (in 2012) to place an upper limit for direct decay of 0.5% at the 3σ confidence limit. Subsequent work by other groups pushed down the upper limit further.
Our interest turned to whether the Hoyle state de-excite in stars by inelastic up-scattering, i.e., a low-energy neutron scatters off of the Hoyle and a higher-energy neutron emerges. (This issue is why the two-gamma de-excitation was questioned already 50 years ago.) Despite numerous attempts, the relevant cross section(s) could not be measured. About 2014 we figured out how - in principle - to answer this question. It took 6 years, and forming a new collaboration, to field the experiment but it was just concluded (during this past "covid" summer). While I cannot present the cross sections at this point, it is clear - they are in the bag, and the relevant cross sections will be available in 2021. I will describe the experimental logic and tools. As a byproduct, we shed more light on the direct 3-alpha process and collected data on the reaction producing n's for the s-process in most stars: 13C(α,n)16O. Once the unique logic we employed to study the Hoyle state is described, it will be clear why these byproducts fall out.
Understanding this talk will require no more than an introductory knowledge of atomic nuclei.