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Stargazer 40
USA
431 Posts |
 Posted - 10/29/2018 : 05:40:07
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As I mentioned earlier in intro, I have a friend with a Mazur 9000. This is the meter that uses same GM tube as the GMC-600+. One of the hallmarks of the Mazur 9000 is that the design is certified and First Responders send their meters in for calibration. Where the vast majority of both Mazur and GMC users are unlikely to send their specific meters in to be calibrated, knowing that the design is calibratable and that out of the box it is using best numbers to provide credible results is a requirement for any semi-professional/professional First Responder use. So I wanted to see first hand this meter's response to higher activity sources and just how it's features compared. Cost of the 9000 is $600.
Here is the meter with the GMC-600 next to it.
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About the same size, but the GMC-600 is a little thicker (The 600+ uses the same case so valid comparison). The LCD screen is only two lines of information. No graphing capability or Large Font display with this little screen. The four buttons on the Mazur have pretty much same functions as those on the GQ meters. The menu provides only display and audio adjustments, and setting time interval recording and quick averaging response adjustment. There is no access to any user adjustment of response. Only a calibration process that requires known calibrated source and distance to get to specific output into the meter. The Mazur feels heavier so wanted to see why. Here's a picture with the front of case removed. The key pad is part of front case cover and a clear window is present to cover the LCD screen. The power is a 9V battery compared to the Lithium 3+V battery in the 500/600 GMC meter. The LND 7317 GM tube is visible in the case. There is also metal shielding all around the electrical components. This really isn't mentioned anywhere in Mazur marketing materials that I've read through, and don't think it adds to structural integrity as not a screwed together cage, but only seems to be folded shielding. Perhaps an attempt to eliminate stray energy, or even EMP protection. A question for Mazur.
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I was hoping to get some idea of similarity of response with the meter, so brought out my sources.
Here is the potassium salt with Mazur - 401CPM and 1.16uSv/h
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With the GMC-600 I got 163CPM and .466uSv/h
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My friend has owned the meter for a couple years and has not had it calibrated. The Mazur meter shows about 2.5 times the dose rate of the 600 (conversion set to same 350CPM/uSv/h in both meters with my setting based on higher activity source. GQ out of box uses 300CPM/uSv/h so would increase 600 to .54uSv/h or half of Mazur) If I went back to the first setting I used in the 600 Review to match background field measures across M4011 and LND-12 GM tubes in 500+, it would be 150CPM/uSv/h and the dose rate reading would be 1.087uSv/h. Pretty close to same.
Here is the Mazur on my stack of Thorium gas mantles - 35,195CPM and 100.5uSv/h. I can tell you that adjusting the display units is a pain as requires entering menu and selecting. Once selected you get average, min, max, current, and other data by cycing through with up and down arrows, but the two line display is simply very limited if you want more than a dose rate reading.
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For the 600 I get 9,798CPM and ~28uSv/h
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For the 500+, M4011 only, I get 4,781CPM and ~31uSv/h
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So 3-3.5 times the dose rate in the Mazur 9000. Changing to the matching background conversion only improves the 600 to 65.32uSv/h.
I attempted to use both the 5uCi and 10uCi sources with the Mazur meter unshielded, but got the following
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The meter also showed >1250uSvh. So not useable at high levels. It simply stops as out of range. The conversion is 350CPM/uSv/h, so the dead time correction is dealt with transparently and actual counts and dose rate not clear. I needed to drop back to shielded sources and gammas only.
Here is 5uCi source shielded with Mazur - 14,056CPM and 37.10uSv/h
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With the 600 I get 8,180CPM and 23.371uSv/h. The result with the background matching factor is 54.53uSv/h. Now more than the 7317 by close to 50%.
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And with the 500+. M4011 only, I get 3,289CPM and 21.38uSv/h
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For the shielded 10uCi source the Mazur gives 24,079CPM and 70USv/h
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The 600 gives 13,452CPM and 38.434uSv/h. Again with the lower factor the 600 reads 89.68uSv/h. Over Mazur 9000 by 28%.
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The 500+, M4011 tube only, gives 5,817CPM and 37.81uSv/h
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So still looking at data, but the SBT-11A in the 600 may not be as sensitive to gammas as I thought (the counts are a little more than half the LND7317 in the Mazur instead of the same). Converting using factor that matches background measures for other GM tubes, the SBT-11A is giving higher dose rates than the Mazur. As well, the M4011 may also be less sensitive to gammas than the .0065 conversion indicates for perhaps a much broader range of energies and radiation types. Will look at this all a lot closer. Most important though is the subject of calibration remains important. I don't have calibrated sources so +/-20%. Calibrated sources are +/-10%. Still a lot of room for differences, but I expect to narrow it down once I better understand the calibration/certification process. Dead time correction not a significant issue at these count rates.
We didn't run alphas. More to come.
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Stargazer 40 |
Edited by - Stargazer 40 on 10/29/2018 15:00:24
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ikerrg
United Kingdom
334 Posts |
Posted - 10/29/2018 : 07:04:24
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Thanks Stargazer_40. Interesting set of measurements!
Your measurements demonstrate again that different GMC tubes (or pancakes) are not well suited to compare the absolute radiation of a source, but they are very sensitive to know its relative intensity versus a defined source. I have indicated before that the dose rate in uSv/h is a SI unit to know the biological effect of the radiation in Grays (J/kg). A GM counter cannot know the energy of the radiation, only its intensity (density), because it only detects events above a defined energy threshold. That is why the ratio between uSv/h and CPM for a GM tube is unique for a single energy source and radiation type (e.g. photons at 3 MeV), and therefore the “calibration” is only valid for that type of radiation. The only way to guess the radiation energy with a GM counter is by adding shielding to the GMC in stages and measuring the relative count rate, doing the appropriate calculations to know what radiation type/energy can go through the shielding.
I am really interested to know about the calibration process used in certification labs for GMC based detectors. They will probably calculate the uSv/h vs. CPM ratio for different calibrated sources with different types of decay (Cs, Co, etc.) and then the user should use the average of those for an unknown radiation or use only one specific number if the source type is known.
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Stargazer 40
USA
431 Posts |
Posted - 10/29/2018 : 07:12:21
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Thanks ikerrg,
What I know at the moment is that Cs137 is the primary source. I am sure that distance is involved to get the output necessary for various levels with a single source (similar to ullix's measures with radmon+ and mantles. I haven't seen any reference to shielding, which speaks mixed energy source. Even the Mazur calibration process describes a two point measure with Cs137 specified, but no reference to any shielding. I found that strange. Like many things out there the devil is in the details. I will find out as I expect to send something off to be calibrated/certified when I am more knowledgeable.
Oh, and sorry for confusion. The GMC is model designation for GQ meters, likely Geiger Mueller Counter. GM is Geiger Mueller tube.
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Stargazer 40 |
Edited by - Stargazer 40 on 10/29/2018 07:48:06 |
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Comparison of 500/600 meters with Mazur 9000
