Nuclear Physics Seminar with Lee Sobotka

Update on Neutron "Upscattering" on the Hoyle: The Hoyle state in 12C (O+2) is the gateway to the periodic table. The resonant capture of 3 alpha particles, (with 8Be as an intermediate) into this state with subsequent electromagnetic decay is the accepted mechanism for generating 12Cgs. This nuclide then serves as the seed for all (or almost all) heavier elements. However another mechanism was proposed 55 years ago that could provide a parallel path to bound 12C. This path, inelastic upscattering, remained a footnote – forgotten by most – as the relevant cross-section excitation function could not be measured. We have now determined this excitation function (by employing detailed balance and a first-of-its-kind measurement) and the window of relevance for this alternate path is narrow.

Action Decomposition: Barrier penetration is one of the hallmarks of QM. George Gamow grounded QM in the mid-1920’s by showing that QMical barrier penetration could explain alpha-decay systematics, the Geiger-Nuttall relation. Introductory QM presentations always solve the transmission through a square barrier and progresses to arbitrary barrier shapes with the help of the WKB approximation. With the latter, one appreciates that the penetrability is largely a function of the (abbreviated) action, i.e. the momentum-distance area (made unit less by dividing by hbar) in the forbidden barrier region. From this is appears that no observable can report on the shape of the barrier but only on the magnitude of the accumulated action. Appearances can be deceiving.