RE: Respect

["Jim F. Herrold" <Herrold@UWYO.EDU>]

Peter Skeels said:

> 	A Ludlum Model 3 with a pancake probe is a decent instrument for 
> medium to high energy betas.  It has an average efficiency of 5% for 
> C-14 and 15 to 20% for P-32 and Si-32.  S-35 is only detectable in 
> large quantities, so I wouldn't use it for self surveys after jobs.  
> 
The efficiencies you list agree with my experience, although I have never
calibrated specifically with a NIST P-32 standard. We have a NIST C-14
standard, as well as Tc-99 and Sr/Y-90, which we use to construct a
calibration curve showing beta energy vs. detector efficiency. Ludlum claims
32% efficiency (4 pi) for P-32 and 5% for C-14 with their model 44-9 pancake
probe. The Ludlum model 3 meter, incidentally, can be outfitted for just
about any detector type, and is not always associated with the 4-9 pancake
probe.

I'm not sure I agree, however, that this detector is not good for S-35. The
efficiency for betas is dependent on beta energy, and the energy of S-35
(.167 MeV) is not that different than C-14 (.157 MeV). I personally had
microcurie levels of S-35 laboratory waste spilled on my clothing once, and
it was easily detectable for months afterward. The article of clothing (a
tennis shoe) hung in our waste building as a reminder to wear PPE.

Jim Herrold, Radiation Safety Officer
herrold@uwyo.edu

University of Wyoming
Environmental Health & Safety
303 Merica Hall
Laramie, WY 82071-3413

(307) 766-3277
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