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 2.GQ Geiger Muller Counter
 GQ300E+ modification
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8 Posts

Posted - 09/17/2018 :  14:25:16  Show Profile  Reply with Quote
Hello all.
I have the GQ300E+ and want to make it more sensitive. I've seen were people open up the slots to form a window. As I understand, this will completely mess up my calibration or am I wrong and if so, what will happen to calibration
Reply #1


United Kingdom
334 Posts

Posted - 09/18/2018 :  01:14:24  Show Profile  Reply with Quote
The slits for the tubes will only increase the sensitivity to the beta particles. I do not know what default calibration factor are you using (ullix might help with his 300E+), but it is probably mainly defined for gamma/xrays. So yes, your device will be able to detect more beta particles (more counts for the same source), but the calibration figure to uSv/h would still be not clear/exact/known. GM tubes are not well suited to quantify the absolute radiation, and even less suited to assess the effect on the human body (uSv/h).

My recommendation is that you only open the slits in the plastic case if you want to detect more absolute counts in beta emitting sources (uranium pottery, thorium gas mantles, Cs sources, etc.). But do not worry too much about the conversion factor between CPM and uSv/h.

On the other hand, be careful as some M4011 tubes are light sensitive, and opening them to the exterior could artificially increase your count rate. That is why new M4011 tubes are painted black, which is another possibility if somebody in the forum can recommend a black paint that it is safe to use on a GM tube (and thick enough).
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Reply #2


738 Posts

Posted - 09/18/2018 :  01:21:15  Show Profile  Reply with Quote
The short answer is: you cannot make the counter more sensitive!

The longer answer requires and understanding of the differences between beta and gamma radiation, and lastly what that famous "calibration" is about. You'll find answers in my script "Potty Training" ( )

and in the manual to my GeigerLog software ( ).

In short:

beta particles will be absorbed to a large degree by the case of the counter. Removing the backplate completely, or cutting a big hole into it, will allow more betas to reach the Geiger-Müller tube and be counted, if (!) the source has sufficient beta emission, and this is what you are interested id.

Gamma particles -- at least those which could potentially make a count, i.e. have a large enough energy above 100keV -- will only be absorbed by a minor degree, likely below or even well below a few percentage points.

Any "calibration" is valid only(!) for gammas. Once you count betas, the meaning of the calibration factor is undefined. Geiger-Müller tubes are generally significantly more sensitive to betas, if the betas can reach the tube at all. It is your radioactive source which determines the applicability of the calibration factor.

Whether this calibration factor is correct even for gamma is a different question. There has never been a measurement presented which demonstrated the appropriateness of this factor. More details in the GeigerLog manual.

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