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 2.GQ Geiger Muller Counter
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David W Harris

USA
1 Posts

Posted - 09/18/2020 :  16:02:49  Show Profile  Reply with Quote
When I download data from my GMC-300E Plus, I get errors.
In the CSV file, every THIRD minute line is missing.
instead there is a "8,,,,"
I am receiving radiation treatment on a VARIAN TrueBeam LINAC.
I set up the GMC-300E to one side to get a polar scan as the LINAC rotates around me.
I import the CSV files into MathCAD to make nice plots.
Even running the download speed at 1% doesn't help.
The Realtime Monitoring appears to be working.


David W. Harris
98-310 Kamehameha Hwy. #224
Aiea, Hawaii 96701
DavidWHarris@hawaii.rr.com
2123'15.45"N 15757'13.32"W
Reply #1

ullix

Germany
489 Posts

Posted - 09/20/2020 :  01:40:20  Show Profile  Reply with Quote
@David, interesting scenario.My guess: the counter ist overpowered with too much radiation. Linear accelerators Generally Produce particles Like protons, Electrons. Ist that what you get and are trying to measure? If it is Gamma, do you know energy (MeV)?

Geiger pulses last some 100 mikrosec, some tubes more, some less. I think i measured M4011 at 125. So, theoretical max countrate is cps<=10000. In reality, and considering Poisson distribution of counts, max will be closer to cps=3000, which happens to be what you have seen.

At higher counts the counter only sees one blurr of stuff, and its electronic may be unable to find any counts, hence your empty records.

There is a second effect, which also paralyses a counter: a high count rate results in a high current being drawn thru thetube, which results in the anodevoltage collapsing. Then also nocounts is the outcome.

The remedy is the same for both situations: reduce countrate!

First, if you can, increase distancebetween source and counter to take advantage of the 1 over distance-squared law.

If not possible, reduce counts by wrapping counter in absorbing material. High atomic weight, like lead, is best but perhaps not available. Lead > (is better than) Copper > Iron > Aluminum > Wood. But Lightweight needs bigger wall thickness. For aluminum, like kitchen foil, und be prepared for 5...10mm! Stone is also possible, think of ceramic bathroom tiles.

Some experimentation required! Let us hear the results.

Edited by - ullix on 09/20/2020 01:42:01
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