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Re: RADSAFE digest 2042

Bob, I would agree, it's the way they're used/calibrated.  There is a widely held "generic" efficiency sometimes given of 10 % for pancakes, but of course this is just an estimate.  It works ok for field use when you're doing mixed fission products, or similar.  There are lots of factors.  As you said, some probes have thin wire mesh screens, some thicker ones, and some have an extra window.  The distance the tube is mounted from the face of the probe is another one.  Also, some folks may be using them as lab counters, mounted in a fixture where geometry is held fixed and close (and the screen may be gone altogether).

Also,  you mentioned the difference in the sources people are using.  Someone mentioned the backscatter issue when making measurements, and of course it applies to calibration.  I've got some cal sources of the same nuclide, but they're on different substrates, and I get different numbers.  When you cal the probe are you holding it a specified distance from the source?  I think the end use of the instrument has to drive your calibration methodology.  Might be interesting to take ONE pancake probe, send it around to 20 different labs and ask for an efficiency determination on it based on say, Cs137, or whatever nuclide.  I bet the differences might scare us.
  

GM probe efficiencies
If the commonly used isotopes are:

C-14, S-35, P-32, P33 and Ca-45 which are all beta emitters and if a
survey meter like the Ludlum Model 3 with a Model 44-9 pancake probe
are being used for surveys, what are the counting efficiencies that
you use to determine the activity present and at what distance from
the source are you determining your efficiency? I raise this question
because I hear different efficiencies when I ask other people this
question.

One reason could be the type of screen covering the window face. I
understand that Ludlum might have used 4 different screens over the
years. Of course, there are other reasons like the calibration source
used, its geometry, and its distance from the meter.

Any way, if you feel this comparison between meter users is worth
while, respond to me, I will summarize the data and post it.  What
prompted me to ask is the comment about the S-35 in the post Lambert 
responded to and also the post that just arrived in which Joel  
stated what he found as the efficiency for C-14.
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