A reader Inquires: Origin of ‘Critical Mass’

Essayist William Safire, in a recent magazine column, ¹ attributed the coining of the term “critical mass” to British science historian Margaret Gowing in 1940. However, that seems unlikely: In 1940, Gowing was an undergraduate studying economic history at the London School of Economics. ² The Oxford English Dictionary (2nd edition, Clarendon Press, 1989) gives several examples of the use of the term, the earliest tracing through Gowing to the famous Frisch–Peierls memorandum of March 1940. ³ Otto Frisch and Rudolf Peierls used the terms critical size, critical amount, and critical radius but not critical mass.

It may be impossible to determine the term’s origin in the rush of events surrounding the discovery of fission. Determining when it first appeared in the scientific literature would be an easier task. Three papers from May and June of 1939 explored the conditions necessary to sustain a chain reaction. ⁴ The first, by Francis Perrin, was published in the 1 May edition of Comptes Rendus B. Near the bottom of the second page of that paper, in the context of achieving a moderated-neutron chain reaction in U₃O₈, he wrote, “d’ou une mass critique M_C de 40 tonnes.” The only slightly later papers by Siegfried Flügge and Peierls did not mention critical mass, but Peierls did use the term critical thickness.

Do any readers know of a precedent for “critical mass” earlier than Perrin’s paper?