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 2.GQ Geiger Muller Counter
 newbie question on GMC-300E
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JohnnyVirgil

USA
2 Posts

Posted - 02/10/2020 :  09:39:27  Show Profile  Reply with Quote
I purchased a GMC-300E because I am into vintage cameras and lenses and a lot of times they come from questionable locations or may contain radioactive lens elements -- but I see inconsistencies with it, and I'm not sure if it's normal or not. "Normal" CPM seems to sit at around 17-20, but if I place it next to one particular camera I own, (A Kodak medalist from the 40s) sometimes it'll start rising -- the highest I've seen so far is 60, but other times, it's sit at 38 or 39, or sometimes lower. Does that mean it's not working properly? I'm sure this is a dumb question so I'll apologize in advance! Thanks.
Reply #1

Damien68

France
241 Posts

Posted - 02/10/2020 :  11:05:31  Show Profile  Reply with Quote
The device measures the number of particles intercepted by the Geiger Muller tube inside the device.
the particles are emitted during disintegration of part of unstable nucleide (radioactive elements).
also the phenomenon is very random. The probability that a nucleide will disintegrate is a function of its half-life. after multiplied by the number of nucleid present, we arrive at a normal distribution law with an average value and a standard deviation value.

which implies for discriminating small levels of radiation to make a total count over a fairly long period (about 1 hour or more).
There are functions on the counters to do this and calculate average value over long time.

The variations that you observe are completely normal and caracterise the standard deviation of distribution law.

Mastery is acquired by studying, with it everything becomes simple

Edited by - Damien68 on 02/10/2020 11:44:39
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Reply #2

JohnnyVirgil

USA
2 Posts

Posted - 02/10/2020 :  11:25:56  Show Profile  Reply with Quote
Thanks. I was kind of expecting you'd say something like that, after reading other threads here. Some are very technical and over my head (at the moment, at any rate) so I wasn't sure I was understanding the technical aspects of normal, background readings.
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Reply #3

Damien68

France
241 Posts

Posted - 02/10/2020 :  11:40:57  Show Profile  Reply with Quote
the background is the radiation level due to the environement. for example the sun radiations, or granite or other materials present in the soil. these are radiations that we will capture a little anywhere and anytime and that will add to what we want to measure. that's why we talk about background.

Mastery is acquired by studying, with it everything becomes simple
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Reply #4

EmfDev

1243 Posts

Posted - 02/10/2020 :  11:42:28  Show Profile  Reply with Quote
Thank you Damien for your answer. It is normal. There are many factor affecting the CPM reading including the sensor position and because CPM is detected in 1 minute. If you want more accurate reading, you need to use timed count feature for long time.
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