The Making of the Atomic Bomb: 25th Anniversary Edition

by Richard Rhodes

The experiment confirmed what Fermi and Szilard had already demonstrated: that ordinary hydrogen, whether in the form of water or paraffin, would not work with natural uranium to sustain a chain reaction. That understanding left the German project with two possible moderator materials: graphite and heavy water.1359 In January a misleading measurement reduced that number to one. At Heidelberg Walther Bothe, an exceptional experimentalist who would eventually share a Nobel Prize with Max Born, measured the absorption cross section of carbon using a 3.6-foot sphere of high-quality graphite submerged in a tank of water. He found a cross section of 6.4 × 10−27 cm2, more than twice Fermi’s value, and concluded that graphite, like ordinary water, would absorb too many neutrons to sustain a chain reaction in natural uranium.