Mg and the 21 Na „ p , g ... reaction rate

Outbursts of nova are thought to be caused by explosions arising from hydrogen burning of proton-rich unstable nuclei. The detection of g radiation of such radioactive nuclei offers a method to study the parameters of these nova. The recent stellar observation of the 1.275-MeV g ray of Na suggests its possible origin in such outbursts. The expected reaction sequence is considered to be Ne(p ,g)Na(p ,g)Mg(b)Na. While the sequence Ne(p ,g)Na(b)Ne(p ,g)Na is more favorable, the former is more likely to occur at high temperature and density @1#. Therefore, the emphasis in recent investigations has been the resonant structures of Mg @2–7#. Of importance are the precise resonant energies and the strengths of proton capture to the levels near threshold. Information concerning the corresponding levels in the better-known nuclei Ne and Na (T51) can aid in determining these strengths. Several reactions have been used to populate states in Mg. These include Mg(p ,t) @4,8#, Mg(He,He) @7#, C(O,He) @5#, and Na(p ,g) @6#. In the excitation energy region just above the Na1p threshold, different reactions appear to populate different states—as is, of course, partially understandable on the basis of reasonable selection rules. Various papers also disagree on some of the J assignments and on mirror correspondences between Ne and Mg. We point to these discrepancies and try to clarify them below. With a Na beam incident on a CH2 target, Ruiz et al. @6# observed three Na1p resonances between 6.3 and 6.8 MeV excitation, and gave estimates of their widths. With uncertainties of 610 keV, their energies are 9, 9, and 33 keV, respectively, higher than those of known states from other reactions that had uncertainties of 6–10 keV. Thus, the lowest two states can probably be identified with the two previously known, but such an identification is less likely for their third state. Ruiz et al. conclude these are all ,50 resonances, and the shape of the first two resonances confirms that conclusion. The third is less obviously s wave. The authors do not state whether their widths are total or elastic widths or, indeed, whether they assumed Gel5G tot in their analysis.