Detectors 020
Detectors 020 : Ionization: Applications: (1) Gas Phototubes: Avalanche multiplication during Townsend discharge is naturally used in gas phototubes, to amplify the photoelectric charge generated by incident radiation (visible light or not) on the cathode: achievable current is typically 10~20 times greater respect to that generated by vacuum phototubes. (a) The starting of Townsend discharge sets the upper limit to the blocking voltage a glow discharge gas filled tube can withstand: this limit is the Townsend discharge breakdown voltage also called ignition voltage of the tube. Neon lamp/cold-cathode gas diode relaxation oscillator: The presence of Townsend discharge and glow discharge breakdown voltages shapes the characteristic of any gas diode or neon lamp in a way such that it has a negative differential resistance region of the S-type. This occurrence is typically used to generate electrical oscillations andwaveforms, as in the relaxation oscillator whose schematic is shown in the picture on the right. The sawtooth shaped oscillation generated has frequency: Where: is the glow discharge breakdown voltage, is the Townsend discharge breakdown voltage, and are respectively the capacitance, the resistance and the supply voltage of the circuit. Since temperature and time stability of the characteristics of gas diodes and neon lamps is low, and also the statistical dispersion of breakdown voltages is high, the above formula can only give a qualitative indication of what the real frequency of oscillation is
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