Abstract
Gamma-ray lines provide a versatile probe for sites of nuclear reactions in astrophysical objects. Two sets of nuclear reactions can be explored: (1) build-up reactions of atomic nuclei from lighter constituents (nucleosynthesis), and (2) break-up of heavier nuclei in nuclear spallation reactions, including nuclear excitation reactions. Nucleosynthesis occurs in pre-supernova stars, in supernova events (e.g., explosive carbon and oxygen burning and silicon burning) and in novae (explosive hydrogen burning). Spallation reactions occur when cosmic rays interact in interstellar clouds, in the Galactic gas towards the region of the Galactic center, and with matter near their acceleration sites. Spallation and nuclear excitation are also induced by solar-flare particles. Recent experimental observations include solar lines of several elements observed at the time of the large 27 April 1981 flare and from several other flares, the galactic 26A1 line, the e+ annihilation line from an extended region about the Galactic center, and the 56Co decay lines from Supernova 1987A in the Large Magellanic Cloud.
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