The Making of the Atomic Bomb: 25th Anniversary Edition

by Richard Rhodes

On October 18 they started a systematic investigation, a series of measurements made inside and outside a lead housing. By October 22 they were prepared to measure what might happen when only a lead wedge separated the neutron source from its target. But the experimenters had to give student examinations that morning and Fermi decided to go ahead on his own. He described the historic moment late in life to a colleague curious about the process of discovery in physics: I will tell you how I came to make the discovery which I suppose is the most important one I have made. We were working very hard on the neutron-induced radioactivity and the results we were obtaining made no sense. One day, as I came into the laboratory, it occurred to me that I should examine the effect of placing a piece of lead before the incident neutrons. Instead of my usual custom, I took great pains to have the piece of lead precisely machined. I was clearly dissatisfied with something: I tried every excuse to postpone putting the piece of lead in its place. When finally, with some reluctance, I was going to put it in its place, I said to myself: “No, I do not want this piece of lead here; what I want is a piece of paraffin.” It was just like that with no advance warning, no conscious prior reasoning. I immediately took some odd piece of paraffin and placed it where the piece of lead was to have been.818 The extraordinary result of substituting paraffin wax for a heavy element like lead was a dramatic...