Observing cosmic nuclei in gamma rays

Nucleosynthesis events in cosmic objects create new nuclei, admixtures of radioactive isotopes being part of the matter ejected into interstellar space by these events. Gamma rays are emitted in radioactive decays, and can be measured with space-based gamma-ray telescopes. Four mission years of INTEGRAL have led to discoveries of new sources and to detailed astronomical refinements of already-known gamma-ray line emission. As part of the main science objectives of INTEGRAL's spectrometer SPI, diffuse emission from annihilation of positrons has presented a new puzzle, as the Galactic distribution of their presumed sources does not agree with the gamma-ray image. Recent massive-star nucleosynthesis is traced throughout the Galaxy with radioactivites seen in ²⁶Al and now also ⁶⁰Fe gamma rays; precision line spectroscopy now reveals Doppler shifts for the ²⁶Al line, separately for different parts of the Galaxy. This provides new insights into the dynamics of hot interstellar gas. The processes generating new atomic nuclei in stars and supernovae are reflected in the abundances of the ejected radioactivities. For individual supernovae, measurements of ⁴⁴Ti with its 85 year decay time probes the symmetry of the supernova interior. Since such gamma-ray measurements probe the generation of new nuclei in cosmic sites through a radiation process which is independent of environmental parameters such as temperature or ionization, it will remain worthwhile to further develop instrumentation in this window to the nuclear-physics universe.