Re[2]: Ionization at 80 eV? (Yes)

[jeff.king@srs.gov]

     I'm confused.  I was under the impression that the Geiger-Muller 
     region was at a distinctly higher voltage than the Proportional 
     region.  My DOE Fundamentals of Instrumentation book places the 
     Proportional region of the gas ionization curve at between ~300 and 
     ~800 volts and the Geieger-Muller region at ~1000 to ~1400 volts.  I'm 
     guessing that these curves are gas dependant (the handbook is quiet on 
     just what gas the curve is relevant to).
     
     A GM Tube at 900 volts and a proportional detector at 4000 volts 
     doesn't make sense to me unless the ionization curve is gas dependant 
     and differing gases are used in each.  Is this the case?
     
     This is of interest to me since my FacRep oral board is tomorrow and I 
     may or may not have to talk about the mechanics of radiation 
     detectors.  (Probably not, but better safe than sorry.)
     
     Jeff King
     US DOE Facility Representative
     Savannah River Operations Office
     In-Tank Precipitation Facility
     


______________________________ Reply Separator _________________________________
Subject: Re: Ionization at 80 eV? (Yes)
Author:  dale@radpro.uchicago.edu at Mailhub
Date:    5/16/97 5:05 PM


It is not so simple.  An 80 ev photon  or electron can easily ionize 
something, but it is the electric field not so much the potential 
that is important.
     
Think about it.  Gm tubes typically have 900 volts between the anode and 
cathode.  Proportional detectors I have used needed up to 4000 volts. 
Both of these need ionizing radiation to trigger an event.  The _field_ 
is insufficient to ionize even though the potential is there.
     
Dale Boyce
dale@radpro.uchicago.edu