UNIVERSITÉ PARIS SUD UFR de Physique Habilitation à diriger des recherches

We have studied the radioactive line emission expected from solar active regions after large flares, following the production of long-lived radioisotopes by nuclear interactions offlare-accelerated ions. This delayed Xand gammaray line emission can provide unique information on the accelerated particle composition and energy spectrum, as well as on mixing processes in the solar atmosphere. Total cross sections for the formation of the main radioisotopes by proton, He, and -particle reactions are evaluated from available data combined with nuclear reaction theory. Thick-target radioisotope yields are provided in tabular form, which can be used to predict fluxes of all of the major delayed lines at any time after a gamma-ray flare. The brightest delayed line for days after the flare is found to be the 511 keV positron-electron annihilation line resulting from the decay of several + radioisotopes. After 2 days however, the flux of the e–e annihilation line can become lower than that of the 846.8 keV line from the decay of Co into Fe. Our study has revealed other delayed gamma-ray lines that appear to be promising for detection, e.g., at 1434 keV from the radioactivity of both the isomer Mn (T1/2 1⁄4 21:1 minutes) and the ground state 52Mn (T1/21⁄4 5:59 days), 1332 and 1792 keV from Cu (T1/2 1⁄4 23:7 minutes), and 931.1 keV from 55Co (T1/2 1⁄417:5 hr). The strongest delayed X-ray line is found to be the Co K at 6.92 keV, which is produced from both the decay of the isomer Co (T1/2 1⁄4 9:04 hr) by the conversion of a K-shell electron and the decay of Ni (T1/2 1⁄4 35:6 hr) by orbital electron capture. Prospects for observation of these lines with RHESSI or future space instruments are discussed. Subject headinggs: nuclear reactions, nucleosynthesis, abundances — Sun: flares — Sun: X-rays, gamma rays Online material: color figures