Experimental and theoretical study of nuclear reaction rates in the <i>rp</i>-process
Proton rich nuclei 37Ca and 36K have been studied by single neutron removal from a radioactive cocktail beam produced at the National Superconducting Cyclotron Laboratory. The excited state structure of each nucleus near the proton threshold was determined by in-flight gamma spectroscopy. The results of this work have reduced dramatically the uncertainty in the 35Ar(p,γ)36K and 36K(p,γ) 37Ca reaction rates, important at early times in the rp-process in Type-I X-ray bursts, and the 36K(p,γ)37Ca rate has increased by up to 4 orders of magnitude for burst temperatures. Under burst conditions these reaction rates are dominated by resonant capture contributions from individual resonances because of the low level density just above the proton threshold in the proton-rich compound nucleus. Therefore, precise structure measurements are required to reduce the orders of magnitude rate uncertainty in these key reactions and thereby constrain X-ray burst models. The resulting level scheme for 37Ca includes six previously unmeasured states and the implications for burst observations and for A = 37, T 3/2 isobaric mass quartets are discussed. The first excited state in 36Ca was coincidentally observed, with improved precision at 3045.0 (2.4) keV.
A comprehensive study of the dependence of simulated Type I X-ray bursts' lightcurves on individual uncertain (p,γ) and (α,p) reaction rates along the rp-process path has been performed. A two-step process was employed using a specially calibrated version of the single-zone burst model of Schatz et al. to identify sensitive rates from the full set of rates involved. These were followed by multi-zone simulations in the KEPLER burst code of entire sequences of bursts to consider observable changes under steady state bursting conditions. Several rate sensitivities were identified in the single accretion model of solar metallicity material accreted at a rate of 1.75 × 10-9[special characters omitted] per year, and the rate sensitivity of the calibrated single-zone burst model shows great similarity to that of the multi-zone model.