New Prospect for Gamma-Ray-Line Astronomy

Abstract

THE prospects for gamma-ray-line astronomy associated with the anticipated occurrence of radioactivity in interstellar space have already been outlined^1–4. Each successive clarification of these possibilities has resulted from major clarifications in the theory of nucleosynthesis, and in this communication I wish to report another step. The supposition that the elements are coproduced in exploding stars has been greatly strengthened by quantitative success⁵ in the past few years. A new development within that general picture now suggests that detectable amounts of ⁶⁰Fe exist in nearby explosive remnants. ⁶⁰Fe has a half-life τ_1/2 = 3 × 10⁵ yr, and the subsequent decay of its ⁶⁰Co daughter is accompanied by a two-gamma-ray cascade with energies E_γ=1.17 MeV and 1.33 MeV (ref. 6). This pair of equal-intensity lines is the signature that can identify ⁶⁰Fe in interstellar space. The other important feature is, of course, its half-life, which is long enough to allow the radioactivity to persist long after (in terms of recorded history) the explosive event.