Proton-transfer Study of Unbound ¹⁹Ne States via ²H(¹⁸F,α+¹⁵O)n Reaction

The nuclear structure of ¹⁹Ne near the proton threshold is of interest for understanding the rates of proton-induced reactions on ¹⁸F in novae. Analogues for several states in the mirror nucleus ¹⁹F have not yet been identified in ¹⁹Ne, indicating the level structure of ¹⁹Ne in this region is incomplete. The proton-transfer reaction ¹⁸F(d,n)¹⁹Ne and neutron-transfer reaction ¹⁸F(d,p)¹⁹F have been measured simultaneously at E_c.m. = 14.9 MeV. The experiments were performed at the Holifield Radioactive Ion Beam Facility (HRIBF) of Oak Ridge National Laboratory (ORNL) by bombarding a 720-µg/cm² CD₂ target with a radioactive ¹⁸F beam. The ¹⁹Ne ions of interest are highly excited and promptly decay by breakup into α and ¹⁵O particles. The same holds true for states in ¹⁹F which decay into α and ¹⁵N particles. These charged particles were detected in coincidence in position-sensitive E – ΔE silicon telescopes. Particle identification, coincidence, and Q-value requirements enable us to distinguish the reaction of interest from other reactions. The reconstruction of relative energy of the detected particles reveals the excited states of ¹⁹Ne and ¹⁹F which are populated.

We have observed fifteen ¹⁹Ne levels from the ¹⁸F(d,n)¹⁹Ne measurement with five levels below the proton threshold. Eighteen ¹⁹F levels have been observed with the ¹⁸F(d,p)¹⁹F measurement. Angular distributions were extracted for all the levels observed in both nuclei and converted to differential cross sections using all necessary normalizing parameters. However, Distorted Wave Born Approximation (DWBA) calculations of the differential cross section were only done for levels with enough statistics to allow a meaningful comparison to the calculation. The spectroscopic factors determined from this analysis for these levels are reported.

The R-matrix analysis of the ¹⁸F(p,α)¹⁵O reaction, taking into the account new information from this experiment as well as other recent measurements indicate that the astrophysical S-factoris considerably less than than previously thought by about a factor of 2 - 5 within the temperature range of typical nova. The astrophysical S-factor was numerically integrated to produce an improved ¹⁸F(p,α)¹⁵O reaction rate. The uncertainty on the ¹⁸F(p,α)¹⁵O reaction rate due to interference between the 3/2⁺ resonances is found to have been reduced when compared to the most recent work of Chae et al.