Robert Serber's 1943 Los Alamos Primer Analysis of Fission Bomb Efficiency

In an analysis of the expected efficiency of a fission bomb presented in his 1943 Los Alamos Primer, Robert Serber remarked that ``it is just possible for the reaction to occur to an interesting extent before it is stopped by the spreading of the active material.'' Due to the exponential nature of the time-dependence of a chain reaction, the efficiency of the explosion can be very sensitive to parameters such as the number of neutrons liberated per fission, the fission cross-section, and the number of critical masses of material used. In this paper I examine efficiency predictions for both uranium and plutonium bombs. Had some parameter values proven only modestly different from their true values, history may well have been very different.