| T O P I C R E V I E W |
| ncotso |
Posted - 01/11/2023 : 01:58:34
Hello,
I have just purchased a GQ gmc 500+ to identify possible radioactive lenses in my photographic equipment.
I first want to measure the ambient radioactivity.
The counters of the French nuclear safety authority near my home measure on average 0,07 µSv/h. The measurements I made outdoors, on 10-minute cycles, give me approximately 0,16 µSv/h, i.e. 2.2 more than the official measurements. In my apartment, I measure an average ambient radioactivity of 0.17 µSv/h with the highest measurement equal to 0.21 µSv/h in my bathroom.
I am a bit surprised by the background measurement results obtained with my counter. I read that it is an uncompensated counter, that could produce overstated results at low energies.
With this observation, can I consider that my counter overestimates the ambient radioactivity by a factor of approximately 2.2. It would then be necessary to correct my measurements of background noise by dividing the result obtained by 2.2.
For example, when I measure 0.21 µSv/h in my bathroom, it should rather be considered that this corresponds to 0.21/2.22, therefore approximately 0.09 µSv/h.
Thank you for your answers.
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| 4 L A T E S T R E P L I E S (Newest First) |
| ullix |
Posted - 01/12/2023 : 00:16:11
I redirect you back to my first answer; do some reading and you will get answers to most of your questions.
Until you grasp the issues, I would refrain from making any non-understood changes to your instrument.
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| EmfDev |
Posted - 01/11/2023 : 12:34:26
You can change the calibration values from Main Menu -> Init Setup -> Calibration. If you want to change it exactly 2.2x lower, you can increase the CPM of all calibration factors by 2.2x. e.g. 100CPM -> 220CPM. Keep the µSv/h values. |
| ncotso |
Posted - 01/11/2023 : 04:07:17
Thank you very much for your answer.
Outside, I have just checked it again, the background measured over more than an hour is on average 25-26 CPM, or an estimate of 0.15 µSv/h. This is double the background knowned and measured in my area.
Inside, I was unaware that the background noise was influenced by the beta rays emitted by the K-40 isotope derived from potassium. Is this isotope safe?
Is it also possible that the level I am measuring is due to radon?
To measure a background noise that neutralizes beta radiation and only retains gamma radiation, is it possible to put a screen around the meter (aluminum for example)?
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| ullix |
Posted - 01/11/2023 : 03:46:04
I see some misconceptions in your comments.
You can certainly measure ambient radioactivity. The GMC-500 will probably show something like CPM=15 ... 25 when you are outside.
Inside it will depend on the building material used for the house, and in particular for floor-tiles, bathroom-tiles, kitchen-tiles, kitchen-counters, sanitary-ware, and so on. It is quite possible that this even doubles the CPMs!
Convince yourself, hold the counter in contact with any tiles, walls in your home; you will likely see a higher count rate.
Almost all of these count increases at home are due to their content of Potassium; of which isotope K-40 is radioactive. For details see my "Potty Training" (under Articles in https://sourceforge.net/projects/geigerlog/files/)
These differences are real, and you can't "correct" them by normalizing to a system measuring elsewhere!
Next, if you have a significant K-40 contribution, then most of the counts are due to beta radiation. And any calibration ever applied is solely for gamma. Therefore it is meaningless to use a "uSv/h" reading to describe a beta scenario!
Next, to my knowledge a calibration for the Geiger tubes in the GMC-500 counter has never been done, as GQ over the last years has never shown data for the claimed calibration. So, the calibration factor of 154 CPM/(uSv/h) is largely an assumption.
Establish your background in CPM at the location, where you will measure your lenses, perhaps your desk. Record long enough, like overnight. Then measure your lenses, long enough to confirm or reject assumption of radioactivity.
Recording and analyzing such long count records can nicely be done with my GeigerLog software ( https://sourceforge.net/projects/geigerlog/ )
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