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GMlearner
United Kingdom
2 Posts |
 Posted - 08/27/2025 : 15:02:57
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I'm a beginner, so apologies for my ignorance.
I'm looking into a more modern GM counter to use alongside my 5 year old GQ GMC-300E Plus, which I've had from new. This is mainly for checking if vintage camera lenses are radioactive (due to thorium included in the optical glass). I use counts per minute for this. I have two other counters to compare, the GQ GMC-800 and the Bosean FS-5000.
I tested all three on a 1959 camera lens I know is radioactive. It emits from its slightly-recessed front element (less than 2 inches/5 cm diameter), mounted in a metal barrel. I rested each counter on the front filter ring of the lens barrel, with the GM tube centred over the lens as much as possible, and waited several minutes, noting the highest count reached. The GM-800's Fast Estimate was set to 60 seconds, to standardise it with the others as much as possible.
I believe the GM-800 has an M4011 tube (or J321), but I haven't looked inside. The Bosean has a tube marked both J321 and M4011 (I've checked that one). My old GQ GMC-300E Plus is a V4, and has a J305 tube (I'd expected a J321/M4011). All three counters give similar background readings in my home, but the GMC-800's result for the lens is noticeably higher than the others. The maximum readings I got for the lens were:
385 cpm from the GMC-300E Plus
360 cpm from the Bosean
605 cpm from the GMC-800
What would be the likely reason(s) for the GMC-800's noticeably higher count? It does seem to collect the count faster than the other two, even with Fast Estimate off, but I don't know why it would be more sensitive, if that's the reason for the higher count.
I also tested a physically-smaller lens from the same decade, which I knew was radioactive. For that one, the results were:
GMC-300E Plus: 201 cpm
Bosean FS-5000: 192 cpm
GMC-800: 250 cpm
Happy to be educated about this. I don't mind that the GMC-800 reads higher - I prefer to be over-cautious! But I just wondered why it does so, as I'd thought the lack of slots in its back panel might have the opposite effect, from what little I know so far.
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| Reply #1
ullix
Germany
1214 Posts |
Posted - 08/28/2025 : 00:06:34
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All those tubes in all those counters should give similar readings both for background and higher count rates.
However, if you have a significant contribution of beta radiation, then the readings will depend on the geometry of your setup and any beta-radiation-absorbing material between source and tube. Such material could well be the plastic of the counter's case. A proper comparison between these counters may be tricky. Try by putting the naked tube (but keep the black cover) in direct contact with the lens.
I do also suggest that a gamma-spectrometer would suit your intentions better. My GeigerLog software does support spectrometers, and the GeigerLog Manual has a chapter "Gamma Spectra in GeigerLog" with an intro to spectrometers. https://sourceforge.net/projects/geigerlog/files/
I use a Radiacode 102 (https://www.radiacode.com/), the cheapest ($250) among probably all spectrometers and is plenty good already. Further, it has a higher sensitivity than even the pancake tube in a GMC-600 counter.
And not to forget: it can tell you what the source of the raadioactivity is. Thorium is easy to identify in a spectrum!
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| Reply #2
GMlearner
United Kingdom
2 Posts |
Posted - 08/28/2025 : 06:21:36
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Thank you for that very useful information - it's much appreciated. I will take a look at your intro to spectrometers too. Those Radiacode spectrometers do look really impressive and interesting, though they're maybe more than I need, since really I only use a detector to see if a lens produces enough radiation to register at levels noticeably above background, just so I know to be more cautious in handling it.
For the moment, as I find the GMC-800 to have the user interface and speed (and possibly sensitivity) that I prefer amongst the three models I used, I will probably check it against another GMC-800 to see if they give roughly the same readings for the two lenses I am measuring.
Thanks again for all the useful information. |
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| Reply #3
Kaninbjerget
Denmark
28 Posts |
Posted - 09/01/2025 : 11:05:34
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The tubes are for same length more or less identical - at least when it comes to gamma detection. Some of the older 300 series has the longer 107 mm tube where as the 800 only has ~90mm +/- a few mm depending on which seller you ask.
And that's the key here - gamma. You're measuring a combination of gamma and beta. The older GMC-300 series use a kind of plastic that's not as permeable to beta radiation as that used on the GMC-800 - even though the 800 doesn't have any holes. To better compare you need to shield the beta and only measure gamma - that is also a number closer to the real dose rate it gives but not spot on since it's not calibrated for thorium. Also be aware of the internal distance from casing to tube as up close a few mm difference makes a big difference in measurred value. So to better compare you need a relatively strong gamma only (shield it) source and a distance to tube 5-10 cm. |
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Why would CPM readings for the same object differ?
