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Radiation Monitoring

I've always been curious about geiger counters, and last year I got a
computer-interfaceable Geiger Counter from Aware Electronics,
model RM-60 (more details).
The
calibration on this particular model indicates that counts per minute
(CPM) is nearly equal to microrem per hour: (CPM * 0.95) = uR/Hr.

I connected the counter to an old '386 PC that a friend gave
me
(thanks
Richard!) and left it running in a corner of my apartment. When I made
the
plot below, it had been running continuously for about four months. If
you
look at the radiation count for a short time period (minutes to hours),
the
random variation masks any trends, since the counter only counts about
11
times per minute and, in radiation counting, the standard deviation of
your
data is the square root of the count, so the more counts, the
better. Averaging over four-hour periods, or as in this graph, days,
shows
definite peaks.

These peaks (mostly) correspond to cold days when the gas room
heater went
on. Natural gas contains radon. The greatest increase is only 5% above
background, so it's not a hazard, but it's interesting that you can
measure
it. In the last 20 days of the chart, the heater has been turned off,
so
peaks there must have some other explanation (eg. random noise).

The plot below shows 200 days of radiation monitoring,
starting
with
the same data in the 120 day plot above, and adding a 7-day running
average as the red line. The position of the red line on each day
corresponds to the average of the previous seven days' radiation
count. Although the variation is small as a percentage, the data does
show trends well beyond expected counting-statistics noise. For
instance, on day 180 the running 7-day average reaches 8 sigma below
the mean. I do not know if these deviations are correlated with local
wind patterns or solar activity.

Here is a PDF plot showing a
diurnal variation in radiation, based on
47 days in July-Aug 2007.

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