Gamma decay of virtual states in ²⁰Ne and ¹⁶O excited in radiative alpha-particle capture

• Main Author: Hurst, Michael John
• Other Authors: Allen, K. W.
• Published: University of Oxford 1980
• Subjects: 530; Nuclear structure
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.258414

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.