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We have investigated the effect of pre-ionization by a radioactive -ray source on the atmospheric air breakdown conditions in a high-voltage spark gap. A standoff millimeter-wave (mmW) system was used to monitor the breakdown properties. A decrease in breakdown threshold was observed with an increase of radiation dose. We attribute this to a space charge-controlled electron diffusion process in a cloud of radiation-induced ion species of both polarities. The space charge-dependent diffusion coefficient was determined from the measurement data. In addition, we found that the breakdown process shows random spikes with Poisson-like statistical feature. These findings portend the feasibility of remote detection of nuclear radiation using high-power mmWs.