 A pre-test explosion was conducted on 7 May 1945 to calibrate the instruments.A wooden test platform was erected from Ground Zero and piled with of TNT spiked with nuclear fission products in the form of an irradiated uranium slug from the Hanford Site, which was dissolved and poured into tubing inside the explosive.This explosion was observed by Oppenheimer and Groves's new deputy commander, Brigadier General Thomas Farrell.