Size distribution of photo-pulses in a DC positive corona discharge by wire-to-plate electrodes is recorded as a function of the applied voltage. Measurements are made in regions of intermittent corona and steady burst pulse corona, and the effect of external radiation on the corona characteristics is examined. The characteristic of intermittent burst pulse corona is observed to change sensitively into what resembles the characteristic of steady corona when the corona gap is irradiated with β-rays or ultra-violet rays or even with α-rays that come from a P0 source to be counted by the corona instrument used as a spark counter.