Measuring Cosmic Elements with Gamma-Ray Telescopes

Gamma-ray telescopes are capable of measuring radioactive trace isotopes from cosmic nucleosynthesis events. Such measurements address new isotope production rather directly for a few key isotopes such as Ti, Al, Fe, and Ni, as well as positrons from the β-decay variety. Experiments of the past decades have now established an astronomy with γ-ray lines, which is an important part of the study of nucleosynthesis environments in cosmic sources. For massive stars and supernovae, important constraints have been set: Co isotope decays in SN1987A directly demonstrated the synthesis of new isotopes in core-collapse supernovae, Ti from the 340-year old Cas A supernova supports the concept of α-rich freeze-out, but results in interesting puzzles pursued by theoretical studies and future experiments. Al and Fe has been measured from superimposed nucleosynthesis within our Galaxy, and sets constraints on massive-star interior structure through its intensity ratio of ∼15%. The Al γ-ray line is now seen to trace current star formation and even the kinematics of interstellar medium throughout the Galaxy. Positron annihilation emission from nucleosynthesis throughout the plane of our Galaxy appears to be mainly from Al and other supernova radioactivity, but the striking brightness of the Galaxy’s bulge region in positron annihilation gamma-rays presents a puzzle involving several astrophysics issues beyond nuclear astrophysics. This paper focuses mainly on a discussion of Al and Fe from massive star nucleosynthesis.