On Wednesday afternoon, 12/2/42, Compton took me over to West Stands to see the crucial experiment on Pile #1. When we got there, the control rod had been pulled out to within 3 inch of the point where K would be 1.0. The rod had been pulled out about 12 inch to reach this point. The resultant effects were being observed 1) by counting the neutrons as recorded on an indium strip inside the pile (see previous notes) and 2) on a recorder connected to an ionization chamber placed about 24 inch from the pile wall. The pile itself was encased in a balloon cloth envelope.
From Fermi's Own Story by Enrico Fermi
The year was 1939. A world war was about to start. The new possibilities appeared likely to be important, not only for peace, but also for war. A group of physicists in the United States—including Leo Szilard, Walter Zinn, now director of Argonne National Laboratory, Herbert Anderson, and myself—agreed privately to delay further publications of findings in this field.