The ¹⁸F(α,p)²¹Ne Reaction and Its Astrophysical Implications

Recent observations of the abundance distribution of heavy elements in metal-poor old stars suggest an existence of more than one r-process site, proposing a different site for the synthesis of light r-process nuclei. One model is the r-process nucleosynthesis in the supernova shock traveling through the He-rich shell of the pre-supernova star. In this scenario, the ¹⁴N from preceding CNO burning is converted to ¹⁸F, via the reaction ¹⁴N(Ì_å±,γ)¹⁸F and subsequent Ì_å_⁺-decay forms ¹⁸O, which then produces neutrons via ¹⁸O(Ì_å±,n) or ¹⁸O(Ì_å±,γ)²²Ne(Ì_å±,n) reactions. <p>The statistical model predicted that the ¹⁸F(Ì_å±,p)²¹Ne reaction can compete with the Ì_å_⁺-decay at high helium density and temperatures. Since ²¹Ne(Ì_å±,n) has a positive Q-value, it has been identified as a stronger neutron source than ²²Ne(Ì_å±,n). Because no published data on the ¹⁸F(Ì_å±,p)²¹Ne reaction are available, we have measured the total cross sections of the ¹⁸F(Ì_å±,p)²¹Ne reaction in inverse kinematics and the time-reverse reaction, ²¹Ne(p,Ì_å±)¹⁸F. The results were compared with Hauser-Feshbach statistical model calculations. The astrophysical implications of the new experimental reaction rate are discussed.</p>