Weak Interference Between the 1 $$^-$$ States in the Vicinity of $$\alpha $$ -Particle Threshold of  $$^{16}$$ O

Abstract

The subthreshold 1 − 1 state at an excitation energy Ex = 7.12 MeV in 16 O has been believed to enhance the S-factor of 12 C(α,γ) 16 O. The enhancement seems to originate from strong interference between 1 − 1 and 1 − 2 (Ex ≈ 9.6 MeV) in the vicinity of the α-particle threshold. However, weak interference between them and a resulting small E1 Sfactor are exemplified with R-matrix theory. Including a higher-order correction of the resonance parameters, the present example appears to reproduce the experimental data consistently. It would therefore be possible that the E1 S-factor is reduced at low energies.