Re: Puzzler

["Dale E. Boyce" <dale@radpro.uchicago.edu>]

A little information, but more speculation....

Daughter products of beta- decay are positively charged and can
be attracted (repelled) by positve (negative) charges.  The daughters
may actually electroplate to metalic objects.  I've heard stories of
radon daughters plating out on fan blades due to static charge.

Looking at the chart, I don't see any suspects leaping out at me
that would give you the kind of count rate you see at environmental
concentrations unless there is a reason that the activity is systematically
attracted to the metal.  I would think even chance reaction with zinc
in the galvanizing would not give count rates that high unless there
was either a high radioactive gas background or some concentration
mechanism.

It could be that a relatively positive static charge exists on
the grounded fence post relative to the atmosphere.  Radon daughters
may be plating (and selectively attracted to the posts).  Static
could be from wind or some other source.  Are there high tension lines
nearby?  I haven't tried very hard to work any numbers, but let's
do a back of the envelope.  Let's say that the wind is blowing 5 m/sec
= 50 decimeters/sec.  Every second 50 liters of air passes a section
of post 10 cm long and within 5 cm of the surface horozontally.
In one minute 3000 liters passes by.  If radon daughters are in
equilibrium with radon in the air.  3000 pCi/(pCi/liter) pass by
the post.  

If the fence post is 5 cm wide about half of the air in this volume
must divert around it.  Perhaps only a fraction has time to plate out.
Without measured outdoor air concentrations it is hard to guess
what collection efficiency would be needed.  If it is 0.1 pCi/liter
then about a third of the daughters would need to plate from the above
volume to explain ~200 dpm.

Lot of words for pure speculation ;)