Re: Day/Night background change?

[Bob Flood <bflood@SLAC.Stanford.EDU>]

At 11:53 AM 6/9/97 -0500, you wrote:
>Radsafers,
>	We have GM foot monitors at our restricted space exits set to
>alarm at about 3 times normal background.  There has been little in
>the way of spontaneous alarm events in the last year, except the last
>two months when security guards are encountering nighttime alarm-trips
>when no occupational workers are in the building.  This suggests
>the possibility of a nighttime background change which would increase
>the probability of a spontaneous alarm trip.  Is there a day/night
>difference in background radiation?

Saw a lot of this at nuclear plants - it's radon! At night, a surface-based
inversion usually forms in the atmosphere that prevents upward mixing of
the air. This inversion breaks up in the morning as the sun stirs the lower
atmosphere and starts the mixing again. Radon evolving from the ground is
trapped very near the earth's surface, and the depth of the radon layer
slowly increases during the night. The strongest effect from this that I
observed happened at Browns Ferry Nuclear Plant. Hand and foot monitors
near the large bay doorway at the back of the plant would alarm nearly
every night, depending on weather conditions and whether the door was open.

The radon effect was also observable on the environmental monitors - the
radon layer would thicken until the top of the layer reached as high as the
GM detector in the monitor, resulting in the count rate suddenly jumping,
and then dropping after sunrise as the morning mixing began.

If this is the cause of your phenomenon, it should be more likely on clear
(or nearly clear) nights with little or no wind and should not happen on
overcast or windy nights. It should not happen in daytime, either. I also
suspect the the alarming monitor is near an open door or window, but this
may not be a necessary condition if there is a lot concrete in the makeup
of your building.


Bob Flood
Stanford Linear Accelerator Center
(415) 926-3793     bflood@slac.stanford.edu
Unless otherwise noted, all opinions are mine alone.