Proton Threshold States in OXYGEN-14 and Silicon -27

The observations of anomalous ('15)N/('14)N and ('26)Mg/('24)Mg isotope ratios in meteorites and the observations of gamma rays from the decay of ('26)Al in the interstellar medium suggest that explosive hydrogen burning may play a significant role in nucleosynthesis in our galaxy. In order to determine the sites for explosive hydrogen burning and to understand its role more quantitatively, it is necessary to determine the rates of the key nuclear reactions in this process. We have established upper limits for the ('13)N(p,(gamma))('14)O and the ('26)Al(p,(gamma))('27)Si reaction rates at temperatures of astrophysical interest by measuring the properties of the 5.17-MeV 1('-) state in ('14)O and the properties of the proton threshold states in ('27)Si, respectively. The 5.17-MeV 1('-) state in ('14)O serves as an s-wave resonance in the ('13)N + p system and dominates the ('13)N(p,(gamma))('14)O reaction rate at temperatures corresponding to the Hot-CNO cycle. By measuring the total width ((GAMMA)), the gamma-ray width ((GAMMA)(,(gamma))), and the excitation energy (E(,x)) of this state, we are able to determine the ('13)N(p,(gamma))('14)O reaction rate. Using a high-precision QDDD (Quadrupole-dipole-dipole -dipole) magnetic spectrograph, we have measured the excitation energy of this state and its total width. On the basis of ('14)N(('3)He,t)('14)O*(('14)O)(gamma) recoil-coincidence measurements, we have been able to set the current best limit on the gamma-ray branching ratio for this state ((GAMMA)(,(gamma))/(GAMMA) < 4.5 x 10('-4)) which implies (GAMMA)(,(gamma)) < 17 eV. The production of ('26)Al via the ('25)Mg(p,(gamma))('26)Al reactions has been studied in detail down to very low temperature. However, the possible destruction of ('26)Al via the ('26)Al(p,(gamma))('27)Si reaction has not yet been studied at center-of-mass energies corresponding to those temperatures (below 2 x 10('8) K). Using the ('26)Al(('3)He,t)('27)Si reaction we have located seven new states between the proton threshold and the lowest directly observed ('26)Al(p,(gamma))('27)Si resonance (E(,c.m.) = 287 keV). We have eliminated all but one of these states as possible s-wave resonances for the ('26)Al(p,(gamma))('27)Si reaction, the one remaining possibility in this energy range is the state at E(,x) = 7589 (+OR-) 3 keV (E(,c.m.) = 126 (+OR-) 3 keV). Our results determine the upper limit for the rate of the ('26)Al(p,(gamma))('27)Si reaction at temperatures below 2 x 10('8) K which is consistent with the ('26)Al/('27)Al ratios observed in the Allende meteorite and in the interstellar medium. (Abstract shortened with permission of author.).