Using the method of induced radioactivity we have measured the (n,2n) cross sections of C 12 , Cu 63 , and Mo 92 as a function of energy from threshold to 27 Mev. The positron activities of 20, 10, and 15.5 min, respectively, were employed. K-capture corrections were applied to the copper and molybdenum data. We have compared the copper and molybdenum data with the statistical theory of Weisskopf and collaborators and find satisfactory agreement from threshold to the onset of tertiary reactions.The reaction a-\-nucleus->/3+nucleons is considered where a and /3 are particles of quantized fields and the coupling to nucleons is assumed linear in the fields. By an extension of the recent work of Chew and Goldberger on potential scattering, the collision matrix has been manipulated into a term representing the impulse approximation plus three correction terms which are, respectively: The error in the impulse assumption arising from nuclear potentials; the "multiple scattering" which is at least fourth order in the coupling; and finally, a small term representing the effect of lowest order absorption. It is significant that there is no large term present, of the order of the impulse approximation term, which represents processes, second order in the coupling, where one nucleon absorbs a and another nucleon emits 0.