[Phys-L] Re: Survey Meters

[ajm at CSUPOMONA.EDU (John Mallinckrodt)]

Brian writes:

> I see there is a URL for a time series for a
> radon wipe at Buda, Texas  taken 2 or 3 years ago.
>
>     <http://home.austin.rr.com/cthompson15/GM10.JPG>
>
> ... I am  missing the obvious.
> The half life of radon is 3.8 days, and this half life
>   is about 40 minutes.....

My guess is that the meter is recording the rate of beta decay from
an initially stable population of Po-218 created by the alpha decay
of Rn-222 in the air.   When the replenishing Radon source is
removed, the Po-218 quickly (3 minute half life) alpha decays to
Pb-214.

The Pb-214 beta decays to Bi-214 with a 27 minute half life and the
Bi-214 beta decays to Po-214  with a 20 minute half life.  The Po-214
very quickly (0.2 ms half life) alpha decays to Pb-210 which is
relatively stable (22 year half life).

The result of the two beta decays from initially stable populations
of Pb-214 and Bi-214 produces radiation with an approximately 40
minute half life.  I constructed a simple spread sheet to model the
decay sequence and it produced a graph of beta production versus time
that very nicely matches the Buda figure.

--
John "Slo" Mallinckrodt

Professor of Physics, Cal Poly Pomona
<http://www.csupomona.edu/~ajm>

and

Lead Guitarist, Out-Laws of Physics
<http://www.csupomona.edu/~hsleff/OoPs.html>