Re: GM probes and RF / EMF

[don@radpro.uchicago.edu]

Common GM tube configurations, the wire in the middle of a metal tube filled 
with a low pressure non-conducting gas, are essentially capacitors.  Capacitors
will transmit AC signals,  the quality of which will depend on other elements
in the circuit, such as the series resistor usually in the HV supply and the 
capacitor coupling the tube to the pulse detection circuit.  GM counters typicallly only are useful for detecting a few hundred pulses per second, but those
pulses do have frequency components in the 100 kHz to MHz range, which is in the RF  area.  A GM tube would probably transmit such frequencies to the pulse
detection circuit, but the amplitude of the Geiger pulses is so much larger 
(usually) that they are rejected as noise by the discriminator circuiitry.  

The only times that I've noticed interference to be a problem is when I've
surveyed near a HV gel electrophoresis apparatus (typically about 1 kV).  This
couples very strongly to the GM detection circuitry - in my instrument it 
sounds more like a regular humming than the usual slightly random pulses.

___________________________________________________________________

Don Jordan                          Tel. (312) 702-6299
Office of Radiation Safety          Fax        702-4008
The University of Chicago           email: don@radpro.uchicago.edu
1101 East 57th Street, Room 11
Chicago, Illinois  60637  
                -- Any opinions are the author's --