large value resistors

[Mike I.]

Martin,
Apparently large value resistors have been used in some types of smoke 
detectors (the radioactive alpha-emitting kind with the americium 
source). These are ionization chambers. I have seen two types of 
circuit: one is a balanced detector using two ionization "cans", the 
other scheme uses a single "can" along with a resistor of about 0.5 x 
10^12 ohms. I think these were described in some 1970's General 
Instrument mosfet app notes. Wouldn't want to monkey around cutting the 
enclosed americium source open, but if the resistor is conveniently 
soldered somewhere....