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 \sim 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 ^ { 56 } Co into ^ { 56 } 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 ^ { 52 } Mn ^ { m } ( T _ { 1 / 2 } =21.1 min ) and the ground state ^ { 52 } Mn ^ { g } ( T _ { 1 / 2 } =5.59 days ) , 1332 and 1792 keV from ^ { 60 } Cu ( T _ { 1 / 2 } =23.7 min ) , and 931.1 keV from ^ { 55 } Co ( T _ { 1 / 2 } =17.5 hours ) .
The strongest delayed X-ray line is found to be the Co K \alpha at 6.92 keV , which is produced from both the decay of the isomer ^ { 58 } Co ^ { m } ( T _ { 1 / 2 } =9.04 hours ) by the conversion of a K-shell electron and the decay of ^ { 57 } Ni ( T _ { 1 / 2 } =35.6 hours ) by orbital electron capture .
Prospects for observation of these lines with RHESSI or future space instruments are discussed .