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Gross sections for (r, n) reactions in 016 and Si28 have been measured using nearly monochromatic photons obtained from the annihilation-in-flight of fast positrons 1, 2). The neutrons were detected in a 471 paraffin-moderated neutron counter having an efficiency of approximately 20%. The neutron pulses were electronically separated according to the number obtained per beam pulse, ie, one, two, or three and these data could be used to determine (y, n),(y, 2n), or (y, 3n) cross sections. The photons were measured by means of a transmission ion chamber filled with xenon to a pressure of 1 atmosphere. The chamber was calibrated by use of a 6 by 5 inch diameter NaI (T1) gamma-ray spectrometer. The photon resolution was approximately 3%. The oxygen sample consisted of 150 grams of H20 in a thin plastic container. The silicon consisted of 8 discs, 2 inches in diameter, by 0.25 in. thick, weighing a total of …