GM Counters in High Backgrounds

[Joseph Guido <GUIDJS@doe-md.gov>]

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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