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 2.GQ Geiger Muller Counter
 CPM not returning to background value quick enough		  New Topic  Reply to Topic
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seb

10 Posts
Posted - 05/17/2013 :  00:29:47  Show Profile  Reply with Quote
Hi,

I am the new owner of a GMC-300. The unit work nicely. But when I test samples with high activity the unit take a very long time (a dozen of minutes) to go back to normal values. Is this related to the Geiger tube or hardware, or to the leveraging algorithm for CPM ? What is the integration time for CPM ? The last minute ? The last x minutes ?

I tested this :

- Background is 20 CPM
- GMC is exposed to fiestaware peaking at 4000 CPM for 1 minute. Then Fiestaware is removed.
- After 6 minutes the GMC still display 140 CPM, and not the real background value which oscillate between 15 and 30 CPM.

I don't use data recording only realtime display. Is there anything that can be done ?

Also Geiger tubes have some dead time, what is the CPM value where CPM display start to be not linear ?

Thank you very much :)
Reply #1

ZLM

1271 Posts
Posted - 05/17/2013 :  00:44:08  Show Profile  Reply with Quote
It is related to the tube. You can turn it off and then on after a high count measurement. That will help to back to normal faster.
The CPM is a realtime and real count, so it dies not have linear issue. Only when CPM converting to dose unit has linear issue. But I do not have such information now.
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Reply #2

harlan

USA
23 Posts
Posted - 06/05/2013 :  11:02:21  Show Profile  Reply with Quote
I had the same problem with my GMC-300 when I went radiation antiquing in my mall that sell old things. I thought the whole shop was radioactive because the GMC-300 got readings over 5000 CPM. I bought the fiestaware cup and saucer and then left the store. The next day, I went back and the readings were normal. So now I know to leave the area and turn off the unit, then come back and check it again.
Harlan
NETC.com
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