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 2.GQ Geiger Muller Counter
 Need help!!! GMC-300s readings

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radioactiveclass Posted - 10/25/2023 : 07:55:27
I have 5 GQ GMC-300s Geiger counters I purchased to use in my classroom. I also have 2 CDV-700 Geiger counters. When my students were comparing the measurements between the GMC-300s and the CDV-700 they noticed that the measurements are nowhere near comparable. They were measuring in milliroentgens per hour. They compared several different sources and the GMC 300s was always about one decimal place lower than the CDV-700. Is this a programing error? I even used the Depleted uranium standard that comes with the CDV-700 to check that they were reading within range and they were both very close. The CDV-700 measures around 2.5-3.5 mr/h using its test source (which from what I read online is within range of what it should) and the GMC-300s measures around 0.38 mr/h.,22%2Dyear%20half%2Dlife.

We would appreciate some help on this.
7   L A T E S T    R E P L I E S    (Newest First)
EmfDev Posted - 11/27/2023 : 10:59:20
thomasfrank is a spam account.
ullix Posted - 11/23/2023 : 01:03:46
From my point of view, the Geiger-Mueller tubes in the GMC-300s are simply less sensitive than the ones in the CDV-700s.

Apparently the opposite is true!

On my GeigerLog site I have a Wiki page with some a collection of some interesting web pages

The "Museum of Old Geiger Instruments" gives details on many old counters, among them the CDV-700 types. They show this scale:

This shows the equivalence of 0.2 MR/HR (which I take as milli-Röntgen per hour) and 200 C/M (which I take as CPM=200). Converting to uSv/h gives 0.2 MR/HR = 2 uSv/h. And finally, thus the sensitivity is 200 / 2 = 100 CPM/(uSv/h).

The sensitivity of the tubes in the GMC-300S counters is claimed by GQ to be 161.54 CPM/(uSv/h) (no data provided).

To summarize - Tube Sensitivity in units of CPM / (uSv/h):

GMC-300S     161  
CDV-700      100

So the GMC-300S is claimed to be 60% more sensitive (to Gamma radiation) than the CDV-700.
ullix Posted - 11/07/2023 : 08:40:56
Since removing the back of the GMC-300s made such a large difference, there must be something else going on.

Not necessarily. It is at least mostly due to beta radiation.
GBG12 Posted - 11/06/2023 : 19:09:33
Since removing the back of the GMC-300s made such a large difference, there must be something else going on. Distance from source to the tube, blocking metal/plastic layers, tube voltage, and tube consntruction may all play a part. The tube in the 300s is reportedly a glass J305 (detailed specs not provided by GQ), requiring 380-450 V, while the CD-V-700 uses 900 V based on Wikipedia. That alone would make it more sensitive to lower energy radiation.
radioactiveclass Posted - 10/27/2023 : 03:45:22
If I can find time I will verify the voltages as suggested. The metal shutters were closed during storage on the CDV-700 and open during use. I did remove the back from one GMC300s and the readings were very close to the CDV-700. Would be nice if they would have designed a sliding window into the plastic case.
ullix Posted - 10/26/2023 : 00:55:57
A 10fold difference is huge! And all 5 GMC-300s show approx the same value? Please, provide some numbers of both CPM and uSv/h for the GMCs.

Can you measure the anode voltage of the GMC counters? You need a DVM and a 1GOhm resistor (for details see: "The Anode Voltage" in this article Are they in the range 380... 450 Volt?

If all 6 counters are NOT defect, then the only way a significant difference could result is from detecting or not the beta radiation. What samples are you using? Is their gamma and beta profile known, and if so what is it?

I understand the cdv-700 has a metal shutter, which is supposed to keep out all betas when shut. Was it shut? The GMC-300, by its case, holds back most of the betas, but not all.

You said you tested but are not fully sure that the CDV-700 complies with the expected test-source readings. Verify again. Does the test source come with numbers for both shutter-open and shutter-closed scenarios? If it does comply, this device seems ok.

It remains to verify the GMCs.

First, what is their background count rate in CPM?

Second, for higher count rates the only option for an easily reproducible source for the GMCs is to use KCl. See details in article "GeigerLog-Potty Training for Your Geiger Counter" Do you get count rates near those given in the article?

Finally, the unit mR/h is deprecated for several decades! In particular with an education environment I think you need to use current scientific standards, which here is uSv/h (micro-Sievert per hour). Gives some additional training in converting the units for the old device ...
EmfDev Posted - 10/25/2023 : 13:20:19
Does your CDV-700 detect alpha? If you are sure that the 2.5-3.5 mR/h is correct, then you can update the calibration on the GMC-300S. And when measuring the source with the 300S, make sure it is as close as possible to the tube located behind the buttons.

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