Alpha particle monitor

Apparatus for monitoring gas-borne radioactive matter emitting weak ionizing radiation substantially less energetic than alpha particles, comprising:

• a flow-through linear ion chamber having an outer electrode enclosing an inner collector electrode between which an electrostatic potential is maintained of a magnitude insufficient to produce charge amplification avalanching within said chamber, said chamber including input and output gas ports to facilitate flow therethrough of a gas to be monitored bearing radioactive matter substantially less energetic than alpha particles,
• an operational amplifier circuit connected to said collector electrode for providing at its output a steady state signal related to the average level of said weak radiation,
• said operational amplifier circuit having (a) a feedback resistance of at least approximately 10.sup.11 ohms, (b) feedback capacitance, including stray feedback capacitance, below approximately 10.sup.-13 pico farads, to substantially eliminate electrical integration of the input to said amplifier from said ion chamber collector electrode, and (c) a time constant for providing, in response to collection by said collector electrode of an electrical charge of approximately 10.sup.-14 coulombs associated with an alpha decay event, an output pulse having a width sufficient to be discriminated in the presence of noise, and
• signal separation means connected to the output of said operational amplifier for separating said steady state signal associated with said weak radiation and said pulses associated withs said alpha emission, to enable measurement of said weak emission either alone or simultaneously with measurement of said alpha particle emission.