Direct Measurement of the 21Na(p,y)22Mg Resonant Reaction Rate in Nova Nucleosynthesis

摘要

An oxygen-neon nova is presently understood to be the result of a thermonuclear runaway on the surface of an oxygen-neon white dwarf. During this event production, and subsequent ejection into the interstellar medium, of the radioisotope 2 2 ~ a can ensue. With a half life of 2.6 years, 2 2 ~ a P-decays leading to the emission of a characteristic y-ray of energy 1.28 MeV. This combination of long half life and characteristic gamma signature makes 2 2 ~ a a possible "viewing port" into the nuclear physics of these cataclysmic events, for, y-rays of this energy are readily detectable with past and current orbiting satellite observatories. To date, no 1.28 MeV y-signal has been observed from any nova, and this remains an outstanding problem in astrophysics. Within these environments, production of 2 2 ~ a can proceed via isolated, narrow resonances in the reaction path: 2 0 ~ e ( p , Y ) ~ u27 N ~ ( ~ , y)22~g(P+v,)22~a. As many as three resonant states in the 2 2 ~ g nucleus can contribute to the total nova 2 1 ~ a ( p , y ) 2 2 ~ g reaction rate. The strengths of these resonances and, therefore, the 2 1 ~ a ( p , y ) 2 2 ~ g nuclear reaction rate, were hitherto unknown, creating significant uncertainty in the expected yield of 2 2 ~ a from an oxygen-neon nova event. Thick target yield measurements, using a high intensity ( lo8 s-u27) radioactive beam of 2 u27 ~ a with the DRAGON facility at ISAC, have been performed in inverse kinematics resulting in a direct measurement and limit on two astrophysically important resonance strengths 2 1 ~ a ( p , y ) 2 2 ~ g . Uncertainty in this reaction rate has been reduced by more than 10-fold for nominal peak nova temperatures 0.3 GK. A narrow resonance, thick target yield curve has been mapped out for the first time using a radioactive heavy ion beam. From this curve, a new mass excess for the 2 2 ~ g nucleus has been derived of -403.5 f 2.4 keV, rather than the literature value of -396.8 keV. The implications the results of the present work have on nova 2 2 ~ a production are consistent with no observed 1.28 MeV y-signal.