RE: Floor monitor - background increasing

["Abelquist, Eric" <AbelquiE@ORAU.GOV>]

Joel-

 

It sounds like a temperature effect on the voltage plateau for your gas proportional detector ....we've experienced the opposite phenomenon in cold weather, where the plateau shifts to the right with lower temperatures.  This means that the operating plateau at the calibration temperature (68 F) might be 1700 volts, but at 30 F the operating plateau might move up to 1775 volts (meaning that if we set the operating plateau in the cold, it would be at a voltage of 1775 volts, as opposed to 1700 volts). So, the effect of operating the detector in the cold at only 1700 v was that we were not on the plateau, so our efficiency and background were lower than normal.  We have modified our operating procedures for surveys in cold weather.  For your case, it seems that at 105 F, your off the plateau on the high side.  [I agree that the radon theory sounds more interesting.]

 

Hope this helps.

 

Eric

-----Original Message-----
From: Cehn@AOL.COM [mailto:Cehn@AOL.COM]
Sent: Tuesday, August 21, 2001 12:15 PM
To: radsafe@list.vanderbilt.edu
Subject: Floor monitor - background increasing

I have a Ludlum 43-37 floor monitor surveying a large concrete pad -
outdoors.  On very hot days (>105), the instrument background goes up.  We
take it into a cool trailer, cool it down, and background stays up.  Any
ideas?

My favorite theory so far is radon coming off the concrete in the heat, and
the daughters sticking to the detector.  That would also account for the
backgrounds returning to normal the next morning.

Joel I. Cehn, CHP
cehn@aol.com
Oakland, CA