Re: GM Response (formerly Respect)

[LIPTONW@dteenergy.com]

Good Point! - I'd like to add a little to this. 
 
You should correct for "backscatter" if you use a metal backed, beta 
calibration source, especially if you will be using the instrument for a 
sample which may have a different backscatter factor, such as a smear or air 
sample.  Figure 2-17, of Glenn Knoll's book, "Radiation Detection and 
Measurement", (New York, Wiley, 1989) includes backscatter fractions for 
various electron energies and materials.  (If you haven't guessed this by
now, 
I'm a big fan of this book.  Every HP should have a copy!)  Note that, per 
Glenn, "Backscattering is most pronounced for electrons with low incident 
energy and absorbers with high atomic number."  There is also a useful graph 
in the 1970 edition of the "Radiological Health Handbook", page 127.   
 
I've found that the metal used for the source backing will have a
significant 
impact on the apparent efficiency, even for metals with similar backscatter 
factors, eg. stainless steel and nickel.  It's probably due to the way the 
radioactive material adheres to the metal, leading to different degrees of 
self shielding.  If you participate in a cross check program where the cross 
check standards are a different metal than your calibration sources, this
may 
result in an apparent systematic error.  
 
One final point and I'll leave you alone. Use a great deal of caution if you 
use a windowless counter for assaying nonconducting sources, such as smears
or 
air samples.  Static charge buildup may lead to nonreproducible results.   
 
The opinions expressed are strictly mine. 
It's not about dose, it's about trust. 
 
Bill Lipton 
liptonw@dteenergy.com   
 
You wrote: 
 
>As a caveat - One can get surprisingly good efficiency (close to 50%) with 
high energy >betas (e.g. P-32 and/or Sr-90) if you are on a hard flat
surface 
because the betas >actually bounce back from the surface into the detector 
(enhancing the response).  >This would not be the case with a check source 
which would contain the P-32 (you only >get about 35% eff.) but with a 
contaminated surface like a lab-bench top which >could/would/will interact 
with the surface you get more "bang for your buck"...
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