On 21 May 1946, with seven colleagues watching, Slotin performed an experiment that involved the creation of one of the first steps of a fission reaction by placing two half-spheres of beryllium (a neutron reflector) around a 3.5-inch-diameter (89 mm) plutonium core. The experiment used the same 6.2-kilogram (13.7 lb) plutonium core that had irradiated Daghlian, later called the " demon core " for its role in the two accidents. Slotin grasped the upper 228.6 mm (9-inch) beryllium hemisphere with his left hand through a thumb hole at the top while he maintained the separation of the half-spheres using the blade of a screwdriver with his right hand, having removed the shims normally used. Using a screwdriver was not a normal part of the experimental protocol.