Positronium Origin of 476 keV Galactic Feature

Abstract

LEVENTHAL¹ has noticed that the γ-ray spectrum due to the annihilation of positronium, which consists of two 511 keV photons from the singlet state and three photons from the triplet state, produces a spectral feature (line) with an apparent peak at an energy less than 511 keV when viewed with a γ-ray telescope having a Gaussian energy resolution. With the energy resolution of the telescope² used to detect the 1.8 × 10⁻³ cm⁻² s⁻¹ galactic feature at 476 ± 24 keV, Leventhal¹ calculates that the observed peak will lie at 490 keV. I calculate, moreover, that if the positronium spectrum sits atop a steeply falling continuum due to other sources, as is observed, then the apparent peak can easily fall near 476 keV where it was observed. Furthermore, the actual data² on the observed feature look like a positronium feature, because there are very few (statistically zero) photons (as opposed to pulses) with energies greater than about 511 keV, indicating a sudden drop in the spectrum there. I therefore take Leventhal's suggestion seriously and show here that explosive nucleosynthesis is a plausible source of the positrons.