Finer Points of Setting Up Scint/PM Tube Systems?

[Glen.Vickers@ucm.com]

I am looking to discuss some of the following points with those who may be
very knowledgeable with the set up of solid scintillator and PM tube
systems.  I am working with systems that have fixed lower discriminators and
I am adjusting the gain to best discriminate between the source and
background spectrums.  Mr. Knoll's book has some decent curves and
information, but I'm looking to increase my knowledge to able to apply the
concepts over a wide range of conditions.  

Our nuclear power plant has a pretty broad range of energies for source and
background photons.  The magnitude of the background spectrum can also also
be quite significant when placing large surface area detectors in the plant.

I believe I understand the fundamentals of HV scans, "S^2/B", and the
gain/discrimination curves in Mr. Knoll's book.  I have a few specific
questions concerning different energy mixes and curve shapes.  Is the
"trough" at the point of discrimination set at the point between the true
background energy and source energy, or does it really fall in the trough
between the photopeak energy and the compton photons?  Can the activity of
the source can impact the shape of the compton and background response
curves and therefore the gain setting?  Is it all insignificant because the
discrimination point would likely fall just below the tail of the photopeak
energies anyway?  Is there a good rule of thumb that the discrimination
point generally falls at the lower energy side of the full width at tenth
max (FHTM)?

Any philosophies concerning voltage compromises to achieve greater source
efficiencies at the expense of increased background?  This is primarily
related to count times, false alarm probabilities, and probabilities of
detection.

There is a question concerning effieciencies with Cs-137 and Co-60.  Photon
for photon, the photopeak/compton ratio appears to be better for Cs-137 than
Co-60.  This should in turn result in a higher efficiency for Cs-137 than
Co-60.  If you set the HV based on the Cs-137 S^2/B curve and determined
your efficiency with Co-60 energy, wouldn't you capture some of the Co-60
compton photon energies that would be otherwise be lost if the HV were set
with Co-60?

We are asking a lot out of our scintillator systems and I want to ensure
procedures can handle a wide range of conditions.  I'm sure many of the
designers never thought we'd be putting these devices in areas with such
high background levels.  These a but a few questions and a few good reading
references would be beneficial as well?

Sincerely,
Glen Vickers
Nuclear Power HP
815-458-2801 x2792
glen.vickers@ucm.com
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