Re: Efficiency Measurement

[Jeff Sawyer <74652.2361@CompuServe.COM>]

In response to Judd Sills enquiry and for the general information of all...

Concerning the efficiencies quoted by instrument manufacturers: I first would
like to say that this is always an interesting topic since almost everyone's
data sheet states an efficiency but no one ever seems to state how that
efficiency was arrived at. For example, measuring the efficiency with a PGM
probe seems a simple enough task but what size source did you use? What type of
source was it  and did it use a protective mylar window or was it electroplated?
Did it have a metal backing or plastic? Was the decay corrected 2 pi particle
emission rate used or the activity? What diameter source was used - it does make
a difference especially as it relates to the solid angle. Is it the efficiency
for that ENERGY or is it the efficiency for the NUCLIDE. In other words, does
the efficiency stated, especially if it's in a graph of efficiency vs. energy
(often with the nuclide stated near the data point), correct for the yield. A
two inch diameter source measured with a GM detector gives a different
efficiency than a 3/4 inch source. The activity of the source plays an important
part in all of this as some efficiencies are measured without the benefit of
deadtime calculations (and that's not a dig at any manufacturer but is an
observation based on conversations with people who had to use the only sources
they had available). A related question: is the system efficiency linear over a
wide range of activities? It can be an important question but it's not one
usually addressed in data sheets.

I am sorry to have to answer a question with several more questions but the
issue of the stated efficiencies is not as simple as it would first appear.
Without knowing what type of source was used it's hard to come up with a reason
for the differences. there are only a few manufacturers of Geiger Mueller
detectors which the various manufacturers incorporate into the probes they sell.
The specifications which you noted were different are almost certainly for the
same detector. For example, Ludlum states 1.7 mg/cm^2 whereas Eberline states
1.4 to 2 mg/cm^2. I would say that you could probably rewrite the Ludlum
specification to read 1.7 +/- .3 mg/cm^2. If there's anyone from Ludlum lurking,
let me know if that's correct...

I'm sure there are many (perhaps just a few) standards or guidelines that
specify how efficiencies should be measure but you never see any references to
these on most data sheets. Off hand I can't think of a single manufacturer that
specifies a standard when stating efficiencies. Again, if there's anyone out
there who does, please correct me on this. 

As for the apparent discrepancy between the Sr/Y-90 and P-32 efficiencies that
started this, since we (Eberline) haven't measured a P-32 source yet (and thanks
to Karin gordon's posting, we'll certainly look into that neat Si-32 source) I
can't really comment. Sorry but I'm as much in the dark on that as everyone
else. Maybe a call to Ludlum might be in order to enquire as to the sources used
and the calibration procedures.

Hope that helps...

Jeff Sawyer
Eberline Instruments
505-471-3232 ext. 237
Fax: 505-473-9221
E-mail: 74652.2361@compuserve.com
or via a link on www.eberlineinst.com