Experimental Study Of The Key¹⁸Ne（α,p）²¹Na Astrophysical Reaction

A indirect measurement of the key astrophysical18Ne(a,p)21Na reaction was discussed in this thesis. Type I X-ray bursts (XRBs) are characterized by sudden dramatic increases in X-ray luminosity. Among the possible breakout reactions, the Ne(a, p)21Na reaction may provide the principal route from the HCNO cycle into the rp-process in XRBs. Over stellar temperatures achieved in XRBs, this rate has not been sufficiently determined. In this work, the resonance parameters of the compound nucleus22Mg have been investigated by measuring the resonant elastic scattering of21Na+p. The18Ne(a, p)21Na reaction rate has been recalculated based on these parameters, and the astrophysical impact of our new rate has been investigated.The experiment was performed using CRIB (CNS Radioactive Ion Beam separator) at the Center for Nuclear Study (CNS) of the University of Tokyo. An89MeV21Na radioactive ion beam was produced and bombarded an8.8mg/cm2thick polyethylene target. The recoiled light particles were detected with three Micron silicon AE-E telescopes centered at angles of θlab=0°,+14°and-14°with respect to the beam line, respectively. The Na+p elastic-scattering excitation functions were reconstructed with a thick-target method over energies Ex(222Mg)=5.5-9.2MeV. The resonance parameters have been determined through an R-matrix analysis.23states in22Mg were observed,10of which were above the a threshold. Most of these parameters were first experimentally determined in a single consistent measurement. The18Ne(a,p)21Na reaction rate has been recalculated, and the astrophysical impact of our new rate has been investigated through one-zone postprocessing X-ray burst calculations. We find that the18Ne(α,p)21Na rate significantly affects the peak nuclear energy generation rate and the onset temperature of this breakout reaction in these phenomena. Not only does the peak energy generation rate increase by a factor of1.4-1.8with the present rate, but the profiles of the curves around the maxima are also rather different. We also note a change in the 18Ne(α,p)21Na reaction flux at these early times. For example, at0.35s, this reaction flux increases by a factor of2-3with our new rate. This contributes to the depletion of15O and18Ne at early times by a factor of3-4relative to abundances calculated using previous rates. In addition, with the present rate, the18Ne(a,p)21Na reaction dominates over the β-decay of18Ne at an onset temperature of T≈0.47GK, which is noticeably lower than the previously0.6GK.