On the day of the accident, Slotin’s screwdriver slipped outward a fraction of an inch while he was lowering the top reflector, allowing the reflector to fall into place around the core.
Slotin received a lethal dose of 1,000 rad (10 Gy) neutron and 114 rad (1.14 Gy) gamma radiation in less than a second, while the position of Slotin’s body over the apparatus shielded the others from much of the neutron radiation. Slotin died nine days later from acute radiation poisoning.
After reading the wiki, this is sort of macabre:
But why he no wear lead suit?
Given that it had already tasted human blood.