Further evidence for the radiative decay of a light, penetrating particle

Photons and photon pairs from the 590-MeV proton beam dump at SIN have been observed in a counter-triggered spark chamber, placed behind 7.5 m of iron-concrete, and 2 m of free space. Nineteen 1..gamma.. events and twelve 2..gamma.. events were found at angles ..cap alpha..< or =7/sup 0/ from the beam-dump direction, whereas a background of, respectively, 4.8 and 2.3 events had been expected. The effect was unchanged by 20 cm of iron in front of the decay region, but vanished when the iron was moved to the end of this region, suggesting these surplus photons stem from a decay.