Re: Scint Counting

[avest@magnus.acs.ohio-state.edu (Albert Lee Vest)]

>Bruce Busby
>Idaho State University
>Radiation Safety Office
  Wrote:
>Question - Anyone use liquid scint counting 
>for alpha, primarily for source swipe-leak checks?  Problem -  We 
>were going to try, so we made up some Am-241 standards, 50 and 100 
>pCi.  The problem was that our efficiency came out to 200%.  A quick 
>call to our friendly beckman rep confirmed our fears, he said that it 
>should be about 100%.  The lab swears that their standards were the 
>correct activity (Hmmmm....). We have counted the standards on two 
>machines, with the same result.  
     
I occasionally check wipes of Am-241 sources on our scintillation counter, 
but my primary method for this isotope is to look for the gamma signature 
with a NaI(Tl) tube hooked up to an MCA.  Yes, I've got a reference standard 
and use statistics to come up with a number of microcuries the wipe has, 
based on the number of events in the peak region.  Ever have a source wipe 
that had more than 0.005 micro's, only it wasn't the RAM from the source?  
We've found sources contaminated with H-3 (before my time), and more 
recently a Co-60 source contaminated with I-125.  In the latter case, the 
LSC data (the technician's instrument of first resort ...) could have been 
taken to mean we had a leaker, but a second wipe, counted on the NaI system, 
was much more conclusive. (And a relief, given the potential $$ involved. :)

Why the 200% efficiency?  My limited LSC knowledge includes the fact that 
what this kind of instrument counts is coincidences, i.e., simultaneous 
discharges in the two photomultipliers.  My Beckman literature indicates it 
watches every 20 ns time-slice for a coincidence.  My guess is, maybe the 
American alpha particles take a long *time* to slow down in the cocktail, 
and give rise to a *longer lasting* flash of photons.  This could show up as 
more than one coincidence per alpha, and hence, the 200% efficiency.

It could also explain why the events show up well below the "2 MeV" limit; 
Maybe only a fraction of the alpha *energy* is deposited in 20 ns.

Any E-mail you get from beckman.com supersedes my meager guesses, though.

Albert Lee Vest    avest@magnus.acs.ohio-state.edu
health physicist        Office of Radiation Safety
(614)292-0122            The Ohio State University
My employer did not review or approve this message.