Re: Gas Proportional Counters

[LIPTONW@detroitedison.com]

2 cautions: 
 
1)  The increase in efficiency due to backscatter must be accounted for if the 
sample being counted does not have the same backing as the calibration 
standard.  For example, if the counter is calibrated with a metal backed 
standard, but is used to count filter paper, then a "backscatter factor" must 
be determined and applied to the count.  Depending on the degree of accuracy 
you need, you may want to factor in that different metals would have different 
backscatter factors. 
 
2)  If a windowless proportional counter is used to assay a nonconducting 
sample, such as filter paper, the results could be inaccurate, due to a 
"dielectric effect," i.e., a static charge between the filter paper and the 
chamber which will perturb the electric field in the chamber and alter the 
efficiency.  I believe that conducting filter paper is available. 
 
The opinions expressed are stricly mine. 
Here's to a risk free world, and other fantasies. 
 
Bill Lipton 
liptonw@detroitedison.com

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• To: Multiple,recipients,of,list,radsafe@romulus.ehs.uiuc.edu
• Subject: Re: Gas Proportional Counters
• From: "radsafe@romulus.ehs.uiuc.edu" <radsafe@romulus.ehs.uiuc.edu>
• Date: 18 Apr 97 07:46:42
• Reply-to: radsafe@romulus.ehs.uiuc.edu

We have an old Nuclear Chicago 2 inch windowless proportional counter. 
For energetic betas, such as from P-32, our efficiency is 0.68. For 
infinite backing of aluminum, the theoretical efficiency is 0.669 for 2 
pi geometry. Yes, you can get efficiencies greater than 50% for 2 pi 
geometry because of backscatter.


=======================================================================
John Harvey                            | McMaster University
Senior Health Physicist                | NRB-110
(905) 525-9140 ext 24226               | 1280 Main St. W.
harveyj@mcmail.cis.mcmaster.ca         | Hamilton, Ont Canada L8S 4K1

On Fri, 18 Apr 1997 radgovwa@sage.wt.com.au wrote:

> Hi Radsafers
> 
> We were having a discussion at morning tea/coffee about the efficiency of gas
proportional 
> detectors in 2 pi geometry. I mantain that the max efficiency in 2 pi 
> should be between 40% to 45% and can't go above 50%  for 
> alpha detection. For beta detection the figure should be between 30% 
> to 40%. The sources we use for calibration are Am-214 and Sr-90.
> 
> What are your thoughts??
> 
> Rex Breheny
> radgovwa@saga.wt.com.au
> 

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