Re: Day/Night background change?

[Judd Sills <sillsj@GAT.COM>]

At 11:52 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?
>	I'm sure you can suggest other reasons why the observation of
>the alarm tripping could be being misinterpreted by me.  I'm not sure
>why I don't have more spontaneous trips with the alarm set at 3 times
>background.  This must have to do with the time constant setting on the
>Ludlum 177 (slow in our case) and the "run length" of background
>radiation events.  I'm checking unattended operations in the vicinity
>of the monitor in question.  For now we'll just set the alarm level up 
>another background increment.
>	Thanks.  Tom.
You are most likely experiencing fluctuations in the background
due to radon-thoron increases.  Radon daughter products  will
have a higher concentration during atmospheric inversions which
occur most frequently during the late night and early morning
(pre-dawn) hours.  You are in an area that has somewhat elevated
radon levels anyway, so this could be more dramatic for you.  I have
seen similar problems in locations in both Colorado and Arizona
that are more dramatic during the spring and late winter months.

A good way to check this out is to run a filter type air sampler
that has a GM detector for live time monitoring and a strip chart
and see if you are getting upward trends in short-lived activity
on the air sample strip charts at about the same time that the
background on your friskers increase.

Best regards

Judd M. Sills, CHP           |   Office: (619)455-2049
General Atomics, Room 01-166C|      Fax: (619)455-3181
3550 General Atomics Court   |   E-Mail:  sillsj@gat.com
San Diego, CA  92121         |