Light Sensitive GM Tube -Reply

[Keith Brown <KDB1@nrc.gov>]

>>>>>>>>>>>>>>>
     We have a Ludlum Model 44-7 thin end window G-M detector which
appears 
     to have become light sensitive.  When the window is exposed to bright 
     light you get a significant response.  If you mask half of the window 
     you find that the response varies as you rotate the mask.  The 
     detector still responds appropriately to radiation and the background 
     is not inordinately high at normal indoor lighting levels.
     
     Any thoughts as to the cause of the phenomenon.
     
     Ed Pombier
     Director 
     University of Miami Radiation Control Center
     (305) 243-6369
     epombier@mednet.med.miami.edu


<<<<<<<<<<<<<<<
UV occurred to me, too. There are UV's at energies high enough to be
ionizing, are there not? I think, though, that the tube is a standard end
window GM, i.e., it's halogen-quenched. I am curious to know just what
countrates were seen.

Chris Alston
<<<<<<<<<<<<<<<
It could be simply a question of numbers.  I suspect that when the tube is
subjected to bright light (sun light by any chance?), the number of uv
photons admitted is higher than created in an event, i.e., the quench gas is
being overwhelmed.  To determine if this is indeed what is occurring, one
could subject the face of the tube to a low intensity light source (preferably
monoenergetic uv) and observe the behavior as the intensity is increased. 
A threshold for induction of an increased count rate should be observable
(at the point at which the quench gas begins to be overwhelmed).

Keith Brown
kdb1@nrc.gov