Re: Efficiency Determination

["Frank E. Gallagher, III" <fegallag@uci.edu>]

OK, so we get an efficiency determination that is fairly accurate in the
calibration lab.  Now we have to make real world measurements and have even
more problems than have already been discussed.

1.   In the research lab, we may have a variety of radionuclides in use in a
given area.  Do we use P-32 or C-14 efficiencies for some otherwise
unidentified contamination?  S-35 or Cr-51 efficiencies?  Do we need to wait
until we have wipe sample counts with radionuclide identification before we
report the survey meter results?

2.   In making an actual measurement, it makes a great deal of difference
for a GM pancake probe whether the contamination is in a small spot (all of
the radioactivity is under the detector) or spread out over 100 cm^2 or more
(only about 20% of the radioactivity is under the detector).  The tiny drop
of spilled activity is quite common in the research environment.

The accuracy of the attempt to measure dpm goes down fast when using a
portable survey meter for operational work.  I am, therefore, an advocate of
reporting survey meter results in net cpm.  The more hazardous nuclides
(such as P-32) will be counted with a higher efficiency than most of the
less hazardous beta emitters (such as C-14, S-35, etc.).  Thus, 250 net cpm
of P-32 will represent a lower activity than 250 net cpm of S-35, provided
that the physical characteristics of the spills are similar.  We have used
cpm values for our survey meter contamination action levels for many years,
and this policy has survived a number of state inspections.

Frank E. Gallagher, III, CHP    
Manager, Radiation Protection,
  and Radiation Safety Officer
University of California, Irvine
Environmental Health and Safety     |    Phone:  (714) 824-6904
300 University Tower                |    Fax:    (714) 824-8539
Irvine, CA   92697-2725             |    E-mail: fegallag@uci.edu