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 2.GQ Geiger Muller Counter
 GMC-280 Max Count?		  New Topic  Reply to Topic
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jimcking

5 Posts
Posted - 04/21/2014 :  03:32:15  Show Profile  Reply with Quote
I put a LND 712 tube in the GMC-280 to measure Alpha Radiation. Used sample from a smoke detector to test. Background count is 5 to 20 CPM and seems normal. When sample from smoke detector is brought close to front of tube (1/2 inch away) count will go up to a little over 1,000 CPM then it is like the counter is saturating and count will go down to zero. Tube says it will support a count up to 600,000 CPM. Is the High Voltage circuit in the GMC-280 loading down and not able to support a count over about 1,000 CPM? Is there anyway to fix this? Would another model GQ counter go higher. My old HeathKit counter goes much much much higher. Initially set voltage to 500 Volts as recommended by tube manufacturer. Tried setting it higher and all the way down. Did not make any difference.

Jim K
Reply #1

ZLM

1261 Posts
Posted - 04/25/2014 :  23:07:41  Show Profile  Reply with Quote
The GMC-280 is able to supports higher than that. The maximum CPM is 65535.

Check the tube and see if it is related to the tube itself.
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Reply #2

jimcking

5 Posts
Posted - 04/28/2014 :  08:53:26  Show Profile  Reply with Quote
The tube spec shows that it should work to 600,000 CPM. The tube works at normal background levels. Can a tube be bad in such a way that it has problems at higher counts only? I don't know of any way to test other than what I have already done. The tube is brand new.

Could it have something to do with the resistors? Tube says should have a 10 Meg and 1 meg resistor to power supply. What are the value of resistors in the counter? Should I add external resistance?

Jim K.
Edited by - jimcking on 04/28/2014 08:56:23
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Reply #3

jimcking

5 Posts
Posted - 04/29/2014 :  08:32:30  Show Profile  Reply with Quote
Tried the same tube in a GMC-300e Plus. Get a count of 50,000 CPM vice the 1,000 CPM from the GMC-280.

Jim K.
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