RE: GM Counters in High Backgrounds

[Fred_Haywood@radian.com]

If one selected the appropriate instrument, he/she would not have this
problem. Cannot understand why someone would attempt to count swipes in a
high background area without at least some shielding. Also, the job is
compounded by using an out of date analog device. Why beat up on yourself?
Life does not have to be this tough.

Fred Haywood

> -----Original Message-----
> From:	Joseph Guido [SMTP:GUIDJS@doe-md.gov]
> Sent:	October 12, 1999 9:19 AM
> To:	Multiple recipients of list
> Subject:	GM Counters in High Backgrounds
> 
> I believe that the issue here is the error associated with the net count
> rate.  It has been a while since I looked at an E-140, but I assume it is
> being used as  a ratemeter in this application. 
> 
> According to Eberline, the E-140's response time is variable from 2 to 10
> seconds.  So, not knowing how it is set up, look at both extremes -
> keeping in mind that the response time is equal to 2.3 time constants
> (according to Eberline). 
> 
> Note that these calculations assume Poisson statistics:
> 
> error (1 sigma) = sqrt (count rate / (2 * time constant)) :Cember 3ed p403
> 
> time constant = response time / 2.3 : Eberline tech support
> 
> 
> In the high background application-----------------
> 
> for a 2 second response time - 
> 
> a 1500 cpm gross reading has an associated 1 sigma error rate of 227
> cpm.
> 
> for a 10 second response time - 
> a 1500 cpm gross reading has an associated 1 sigma error rate of 102
> cpm.
> 
> In the low background application---------------------
> 
> for a 2 second response time - 
> a 550 cpm gross reading has an associated 1 sigma error rate of 138
> cpm.
> 
> for a 10 second response time - 
> a 550 cpm gross reading has an associated 1 sigma error rate of 62
> cpm.
> 
> ----
> 
> Going one step further, you would express the net count rate as:
> 
> net = gross -bkg +/- sqrt (error_gross^2 + error_bkg^2)
> 
> In the high background application
> 
> gross count rate = 1500 cpm
> bkg count rate = 500 cpm
> 
> 500 +/- 294 cpm (2 second time constant)
> 500 +/- 131 cpm (10 second time constant)
> 
> In the low background application
> 
> gross count rate = 550 cpm
> bkg count rate = 50 cpm
> 
> 500 +/- 164 cpm (2 second time constant)
> 500 +/- 64 cpm (10 second time constant)
> 
> ---
> 
> joe guido
> guidjs@doe-md.gov
> 
> 
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