Can solar ⁶Li abundance really be explained by Galactic cosmic rays?

Cosmic-ray interactions are the only known source of the rare isotope 6Li. The standard picture is that the observed solar ⁶Li is produced by galactic cosmic rays accelerated in supernova remnants. Thus lithium-6 is a unique probe of the local Galactic (hadronic) cosmic-ray history. On the other hand, extragalactic gamma-ray background is a measure of cosmic-ray fluence but for the average star-forming galaxy. Using the connection between production of lithium and hadronic gamma-rays in cosmic-ray interactions we tested this assumption and came to a surprising andalarming result: extragalactic gammaray background allows for only ∼ 50% of solar lithium-6 abundance to be produced by Galactic Cosmic Rays. Although extreme assumptions yield a consistent picture, more realistic ones indicate that solar ⁶Li cannot be produced by standard GCRs alone without overproducing the hadronic gamma rays.