Re: Finer Points of Setting Up Scint/PM Tube Systems?

[Bernard L Cohen <blc+@pitt.edu>]

	I would advise you to measure the pulse height distribution with
an MCA and make decisions based on that. It is difficult to see how any
outsider can give useful advise without seeing the pulse height spectrum
and knowing precisely what you want to accomplish. The ratio of
efficiencies for Cs-137 and Co-60 depends only on the size of the
scintillator -- a larger scintillator improves the efficiency for Co-60
more. 

Bernard L. Cohen
Physics Dept.
University of Pittsburgh
Pittsburgh, PA 15260
Tel: (412)624-9245
Fax: (412)624-9163
e-mail: blc+@pitt.edu


On Fri, 18 Feb 2000 Glen.Vickers@ucm.com wrote:

> 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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