Interference effects among J = 3/2⁺ resonances in ¹⁹Ne system & Searching for resonances in the unbound ⁶Be nucleus

The ¹⁸F(p,α)¹⁵O reaction plays a crucial role in understanding γ-ray emission from novae. Because of the importance of understanding the ¹⁸F + p reactions, a number of studies of the A=19 isobars have been made using stable and exotic beams. The interference effects among J ^π = 3/2⁺ resonances in the ¹⁸F + p system, however, have never been measured, but they can change the S-factor by a factor of 20 at nova energies. R-matrix calculations indicate that the cross sections above the Ec.m. = 665 keV resonance are sensitive to the interference between the E_c.m. = 8, 38, and 665 keV resonances. In order to study the interference effects, an excitation function for the ¹H(¹⁸F,α)¹⁵O reaction has been measured in the energy range of E_c.m. = 663-877 keV using radioactive ¹⁸F beams at the Holifield Radioactive Ion Beam Facility (HRIBF). By measuring the ¹⁸F(p,α)¹⁵O cross section off resonance and comparing the cross section with theoretical calculations, we provide the first experimental constraints on the interference of 3/2⁺ resonances.

The ³He(³He,2p)⁴He reaction is responsible for destruction of ³He in stars, and strongly affects the calculated neutrino luminosity from the sun. Previous measure- ments demonstrated a rise in the ³He(³He,2p)⁴He S-factor at low energies, which could be due to a low energy resonance in the ³He + ³He (⁶Be) system, or be due to an electron screening effect. In the ⁶Be nucleus, however, no excited states have been observed above the first 2⁺ state at E_x = 1.67 MeV up to 23 MeV. But there is considerable evidence to support the presence of unknown excited states in ⁶Be. First of all, in the mirror nucleus ⁶He two excited states have been established below the t + t threshold at E_x = 12.3 MeV. Secondly, a recent measurement at Notre Dame University found tentative evidence for a ⁶Be level at 9.6 MeV. A search for the missing ⁶Be levels was performed by studying the d(⁷Be,t)⁶Be reaction with the radioactive ⁷Be beam at Oak Ridge National Laboratory HRIBF. No excited states in ⁶Be were found; however, we could set the upper limits on the reaction cross section.