Search for the 1.2 MeV γ-ray line of Walraven and Haymes

Abstract

WALRAVEN AND HAYMES¹ have reported the possible detection of a ∼1.16 MeV γ-ray line at a flux level of (3.4±1.5)×10⁻³ photons cm⁻² s⁻¹ from a point in the galactic plane (l = 345°, b = 5°). If correct this could be an enormously important result constituting the first direct confirmation of theories of explosive nucleosynthesis (ref. 2 and refs therein). Clayton et al.^3–4 have predicted that the most intense γ-ray line to be emitted from the radioactive debris of a typical supernova after ∼3 yr will be the 1.156 MeV line from the decay of ⁴⁴Ti (τ_1/2 = 48 yr). However, the reported detection was at a marginal level of statistical significance (2.6σ). It was mainly the “remarkable double coincidence that just the theoretically-predicted spectral anomaly occurred” when the telescope was pointed at a region of the galactic plane believed to contain young supernova remnants, that encouraged these workers to report this result. Clearly an independent attempt to confirm this observation with a telescope of finer energy resolution is called for. Here we report the result of such an attempt. Our conclusion is that the reported positive detection was probably a statistical fluctuation.