You're not entirely wrong, though I don't know anything about a barrel. One of the physicists, Alvin Graves, was right behind Slotin, and thus blocked by Slotin's body from most of the exposure. The rest were significantly further away, with the next closest person, photographer Dwight Smith Young, almost two meters back. Which was, apparently, far enough to keep him safe and allow him to survive into his 80s.