Gamma-Ray Line Studies of Nuclei in the Cosmos
  Gamma-ray line studies are capable of identifying radioactive tracer isotopes
generated in cosmic nucleosynthesis events. Pioneering measurements were made
30 years ago with HEAO-C1, detecting the first interstellar gamma-ray line from
26Al, then with SMM and numerous balloon experiments, among their results the
detection of radioactivity from supernova SN1987A, and with the Compton
Observatory and its OSSE and COMPTEL instruments in 1991-2000, which performed
sky surveys in 26Al and 511 keV annihilation emission and the detection of the
Cas A supernova remnant in 44Ti radioactivity. The SPI high-resolution Ge
spectrometer on INTEGRAL was launched in 2002 and continues to collect data on
astrophysically-important gamma-ray lines from decays of 44Ti, 26Al, 60Fe, and
positron annihilation. 44Ti decay lines from Cas A have been observed with both
INTEGRAL telescopes, and constrain the expansion dynamics of the ejecta. The
lack of other 44Ti remnants is a mystery. The 26Al gamma-ray line is now
measured throughout the Galaxy, tracing the kinematics of interstellar gas near
massive stars, and highlighting special regions of interest therein, such as
groups of massive stars in Cygnus and even more nearby regions. The detection
of 60Fe radioactivity lines at the level of 15% of the 26Al flux presents a
challenge both for observers and models. Positron annihilation emission from
the nucleosynthesis regions within the Galactic plane appears to be mainly from
26Al and other supernova radioactivity, while the bulge's positron annihilation
brightness remains puzzling.
