He was stacking WC blocks, by hand, around a spherical piece of Pu 239 weighing about 6 kg, and measuring the neutron flux as he did so. He wanted to see how the configuration of WC blocks changed the reactivity of the system. This was about nine months before the Slotin incident, on August 21, 1945.
Keep in mind that, at that time, these guys were deep into a heavyweight tech-out in pursuit of the atomic bomb. They had essentially no practical knowledge of what they were doing, especially as related to handling large amounts of Pu, which (if you recall) had never existed... anywhere before it was first manufactured in bulk in 1945 in Hanford WA. The guys at Los Alamos, including Daghlian and Slotin, were getting it "hot out of the reactor," literally (in more ways than one...).
You know how, when you were a kid, and someone told you "be careful of that fan, it can cut your finger off," and you had this almost irresistable urge to put your hand near the blades to see what it would be like? I've always wondered if, maybe, those guys at Los Alamos weren't subject, just a little bit, to that same mentality. I know I would have been. You have all these theoretical guys saying "be very, very careful with this stuff, and whatever you do, don't put more than "X" amount together in any one place at any one time," but no one's ever done it, and you find the question of what would happen if you did put "X" amount together almost irresistable.
Anyway, I doubt very much this is what happened, but I've wondered.
(steely)
ohh i think i get the picture. the ball is encased in bricks except for the top. then he lays a brick over the ball, and zap!