X/γ-rays detection during streamer development

X- or gamma-rays have been produced in the developing discharge of a 2 MV lightning surge generator. The generator provides a voltage waveform with 1.2/50 mus rise/fall time. The waveform is truncated by a spark gap: two sharply pointed electrodes placed horizontally at 0.7 m distance with one electrode grounded. Gap breakdown occurs at about 850 kV. High energies has been recorded with BaF₂ and LaBr₃(Ce) gamma detectors facing the surge. Aluminum windows limit the detection to gamma's with energy above 17 keV. The scintillator- photomultiplier combination has been calibrated by the 662 keV line of a ¹³⁷Cs source; the pulse output rise/fall time is 11/23 ns. With positive surges applied to the floating spark gap electrode, hard radiation is observed in 100% to 60% of the surges depending on the electrode's distance to the measurement equipments. The radiation occurred in the leading edge, during the streamer development before a conducting channel was formed between the electrodes. The gamma signal consists of several pulses, well resolved in time. Gamma pulses equivalent to 2-3 MeV were detected. From absorption measurements we conclude that these high energy pulses are pile-up of multiple gamma-quanta, each over 100 keV. The pile-up occurs within the rise time of the detector response, 11 ns. The consistent timing with the HV-surge excluded background radiation as a source for these high intensity pulses. For negative discharges three things change: more surges resulted in gamma-pulses, more gamma-pulses per surge, and more gamma energy. Gamma-pulses up to the equivalent of 30 MeV have been recorded. The high equivalent energies are caused by streamers from the measurement cabinet, right in front of the detector. X-ray was also measured from streamer-corona plasma in a wire-plate reactor. No breakdown occurred during the experiment. A high repetition rate 15/25 ns high HV-pulse with 100 ns width was applied to the wire. The voltage pulse's amplitude was 65 kV with an additional DC offset of +20 kV. Energies equivalent to 45 keV have been recorded.