Direct measurement of the 21Na(ρ,γ)22Mg resonant reaction rate in nova nucleosynthesis

Sodium 21


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 22Na can ensue. With a half life of 2.6 years, 22Na β-decays leading to the emission of a characteristic γ-ray of energy 1.28 MeV. This combination of long half life and characteristic gamma signature makes 22Na a possible “viewing port” into the nuclear physics of these cataclysmic events, for, γ-rays of this energy are readily detectable with past and current orbiting satellite observatories. To date, no 1.28 MeV γ-signal has been observed from any nova, and this remains an outstanding problem in astrophysics.

Within these environments, production of 22Na can proceed via isolated, narrow reso- nances in the reaction path: 20Ne(p,γ)21Na(p,γ)22Mg(β+νe)22Na. As many as three res- onant states in the 22Mg nucleus can contribute to the total nova 21Na(p,γ)22Mg reaction rate. The strengths of these resonances and, therefore, the 21Na(p,γ)22Mg nuclear reaction rate, were hitherto unknown, creating significant uncertainty in the expected yield of 22Na from an oxygen-neon nova event.

Thick target yield measurements, using a high intensity (> 108 s1) radioactive beam of 21Na 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 21Na(p,γ)22Mg. 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 22Mg nucleus has been derived of 403.5 ± 2.4 keV, rather than the literature value of -396.8 keV. The implications the results of the present work have on nova 22Na production are consistent with no observed 1.28 MeV γ-signal.

Author: Bishop, S.