Gamma decay of virtual states in ²⁰Ne and ¹⁶O excited in radiative alpha-particle capture
A cryogenically pumped, windowless gas target has been used to study the radiative alpha-capture reaction on 16O and 12C targets, giving information on the electromagnetic decay properties of various unbound states in 20Ne and 16O. Several states in 20Ne have been observed in this reaction for the first time. The first five unit-isospin, natural parity states in 20Ne have been observed. The isospin assignment of the 11.27 MeV (1-)) state has been made definite by the observation of a strong Ml decay to the 8.85 MeV (1-)state, and the analogue of the 1.97 MeV state in 20F has been identified with the 20Ne state at 12.25 MeV. Angular distribution measurements have shown the spin of this state to be 3-), and the total width has been shown to be less than 1 keV, contradicting previous reports. The 12.22 MeV (2+)state has been resolved clearly as an individual resonance for the first time. The electromagnetic transition rates for the lower T=l states in 20Ne are compared with shell model calculations. Five zero-isospin capture resonances in 20Ne have been studied, the gamma-decay of the 8.70 MeV (1-), 9.11 MeV (3-) and 11.92 MeV (4+) states being observed for the first time. A remeasurement has been made of the enhanced E2 decay strength of the 8+ member of the ground state rotational band, giving ω=l31 ±18 meV, in agreement with a previous less accurate value. This result is compared with shell model calculations. A study of the capture reaction on 12C has been performed in the beam energy range 5.22 MeV < Eα < 9.53 MeV. No new capture resonances have been found but the known inelastic resonances have been seen. The 11.10 MeV (4+) state has been studied and the strength for the decay to the 6.13 MeV (3-) state has been shown to be only half of a previous result. The 12.05 MeV (0+) state has not been observed, but a limit has been placed on its strength.