Be-7/I-131 decay corrections in air samples

["Frame, Paul" <FRAMEP@ORAU.GOV>]

> Our continuous air samples taken around the Millstone Nuclear 
> Power Station
> routinely see Be-7 in the range of 0.05 to 0.1 pCi/cubic 
> meter (2 sigma error
> around 0.03).  The source is cosmic.

When we've done the measurements in Oak Ridge, we usually measure a little
less than 0.1 to 0.2 pCi/cubic meter - highest in the spring when there is
more mixing between the stratosphere and troposphere.  

Power plant Be-7 concentrations are usually based on a quarterly composite,
this is equivalent to a 90 day sampling period. Since Be-7 has a 53 day half
life, there can be significant decay during the sampling period. If the
decay is unaccounted for, it would lead to a 70% or so underestimate of the
Be-7 concentration. When the concentration is simply calculated by taking
the activity on the filter and dividing it by the volume of air sampled,
decay has not been accounted for. 

I'm not suggesting that Millstone's numbers are low, concentrations can vary
with latitude as Franz noted - I'm just pointing out a necessary correction
that is easy to overlook.

I'm sure the same sort of thing often happens when labs calculate I-131
concentrations in air based on weekly charcoal samples - the decay of the
I-131 during the sampling period is not taken into account.

best wishes

Paul Frame
Professional Training Programs
ORAU
framep@orau.gov
http://www.orau.gov/ptp/ptp.htm
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