Summing Spectral Measurements

["Carbaugh, Eugene H" <gene.carbaugh@pnl.gov>]

We have been occasionally summing spectra for in vivo chest count measurements
at Hanford for maybe 15 years.  It's not a routine practice, but is used to
improve the MDA and Lc of already-long measurements.  Our standard chest count
(looking for Am241, Th234, and U235) is 50 minutes - going much beyond an hour
counting time becomes tedious and a little stressful for the worker in the same
chair.  If the count indicates a result above the Lc, we need to investigate.
So we'll take a break, let the worker walk around, get a drink, or whatever,
then sit him back down for another hour.  We'll examine both spectra
individually and then electronically combine them (channel by channel) to
determine if the initial indication was most likely statistical fluctuation or
real detection.  As long as the measurements are performed using the same
detector system, and on the same day, we are confident that not much is likely
to change in the analyses.  

I'm not sure there is any significant difference between this and stopping a
measurement in mid-count, then resuming the measurement.  That's something I
remember doing many times in my college lab days.  May not have been a good
practice, but I don't ever recall being called on the carpet for it by
instructors or professors.

Gene Carbaugh, CHP
Hanfor Internal Dosimetry Program
Pacific Northwest National Laboratory
gene.carbaugh@pnl.gov
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