  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 of flare-accelerated ions. This delayed
X- and gamma-ray 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, 3-He and alpha-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 beta+ radioisotopes. After ~2 days however, the flux of the 511 keV
line can become lower than that of the 846.8 keV line from the decay of 56-Co.
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 and the ground state of 52-Mn, 1332 and 1792 keV from 60-Cu, and 931.1
keV from 55-Co. The strongest delayed X-ray line is found to be the Co Kalpha
at 6.92 keV, which is produced from both the decay of the isomer of 58-Co by
the conversion of a K-shell electron and the decay of 57-Ni by orbital electron
capture. Prospects for observation of these lines with RHESSI or future space
instruments are discussed.
