Transition between sequential and prompt two-proton decay
Even though two-proton decay of ground and excited states is often viewed as either one of two distinct processes, sequential or prompt, reality is more nuanced. We will demonstrate some of these subtleties by examining two-proton decay in excited states of 12O, 13O, 9C, 16Ne, and 17Ne from experimental data collected at the National Superconducting Cyclotron Facility with the HiRA detector. Theoretically, sequential two-proton emission can be considered the limiting case when a one-proton intermediate state is accessible and its lifetime approaches infinity. In this case, the two decay steps should be independent. In reality, even for long-lived intermediate states (Γ∼20 keV), the second emitted proton retains a memory of the first with a small preference for it to be emitted in the same general direction as the first. This preference increases as the width (lifetime) of the intermediate state increases (decreases). Final-state Coulomb interactions between the protons have also been observed in some cases. There also exists a transition region where the correlations between the momenta of the decay products show characteristics of both sequential and prompt emission, for instance, some probability of the protons being emitted with two distinct energies corresponding to the two steps in sequential decay and some probability of the protons having very similar energies as expected for prompt emission. For 12O states, we see a smooth evolution of these decay correlations with excitation energy. The ground state decays by prompt two-proton emission, while the excited states are in this transition region with highest excited states appearing closer to sequential emission, but still maintaining some characteristics of prompt emission.