Other possibilities: if it is high on sunny days, you may have an older model that does not have UV coating over the electrode ends. This can be rectified and diagnosed by applying electricians tape over the electrode ends on both sides of the detectors. Is your cooldown period sufficiently long to eliminate the possibility of heat retention in the detector housing/screen causing efficiency changes due to gas density? It sounds like you and John have come to a more probable answer for your symptoms, but I have experienced these others.
One other thought. Is your flow to the detector remaining constant even in the extreme heat. Effects of heat on the gas bottle, regulator, and needle valves could cause increased flow to the detector. The increased flow could generate an increased signal due to vibrations in the cathode.
>From: AndrewsJP@AOL.COM
>Reply-To: AndrewsJP@AOL.COM
>To: Cehn@AOL.COM, radsafe@list.vanderbilt.edu
>Subject: Re: Floor monitor - background increasing
>Date: Tue, 21 Aug 2001 18:50:52 EDT
>
>In a message dated 8/21/2001 1:37:48 PM Eastern Daylight Time, Cehn@AOL.COM
>writes:
>
>
> >
> > 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
> >
> >
>
>Makes sense to me. I have seen similar phenomena with monitoring soil, but
>without the heat and with a different detector. We routinely monitored the
>background during the measurement runs and used a moving background count
>rate to analyze the data. If you are using digital logging equipment you
>could monitor the decay of the background at the end of the day and see if it
>followed the "about 30" minutes half-life you would expect from the decaying
>radon daughters. That would prove your theory.
>
>John Andrews
>Knoxville, Tennessee