Fig. 1. Schematic diagram of the “onion-shell” internal structure of a pre-SN massive star.

Zones are labeled by their most abundant elements (26). Neutron capture taking place in the outer C-rich He/C zone converts ²⁸Si and ⁴⁸Ti to neutron-rich Si and Ti isotopes, respectively, whereas α-capture in the inner Si/S zone overproduces α-nuclides, including ²⁸Si and ⁴⁸Ti. Abundant short-lived ⁴⁹V is also made in the Si/S zone.