Newbie: dosimeter displays high

[tkalfaoglu]

Hello everyone! I recently got interested in radioactivity and bought myself a GR-12m dosimeter from ebay.
Um, here is its link: http://www.ebay.co.uk/itm/Radiation-detector-Geiger-Counter-SBM-20-R-Tracker-model-Gr-12m-/271322508743?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item3f2c14edc7

Now this machine seems to work fine, reading 0.27 uSv/h when turned on.
I also bought some test samples from ebay, a piece of red pottery made with radium and a vial of crystals that also suppose to have some radium in them. Anyway, as expected the machine goes crazy when I approach those samples, reading 2 to 5 uSv/h.

However, when I remove the samples, the readings stay elevated, they never go back to 0.27 levels unless a day or so goes by.
I turn off the machine, turn it back on, it still reads high.  Move the machine elsewhere, still too high.
For example, it's been several hours since my last reading, I turn on the machine, the samples are in a tin jar in another room, and it reads  1.5 uSv/h.
Is this machine defective, or is that how these tubes work, staying overly sensitive after exposure to some radiation?

Many thanks, -turgut

[Fermi2]

Partly saturated.

[Fermi2]

Actually YES saturation could be the issue.

[GLW]

The electronics package is a nifty one, the GM tube is Russian (nee USSR) from a lovely little place known as the Ukraine,...

They have a lot of SBM-20 tubes laying about in the Ukraine,...

They have a lot of SBM-20  tubes available for sale cheap to buyers all over the globe,...

The numbers you are putting up are background numbers,...

The fluctuations you are seeing are possible from so many natural or technically enhanced sources that it would be pure guessing on the part of folks in an internet forum to give you sound advice of any sort,...

Could it be saturation issues?

Could be,...

Could it just be the construction materials of your dwelling?

Could be,...

Could it be a slightly contaminated detector?

Could be,...

Could it be the stability of your electronics package?

Could be,...

Could it be a combination of some or all of the above?

Probably,...

Should you be concerned about any of the levels you are playing with?

Probably not, without having a calibrated meter and without knowing how stable your meters linearity is at low ranges and without yourself having any training or practical application skills for what you are observing what you have is a fun toy that clicks, beeps, and bleeps without the operator having acquired an innate ability to interpret the indications or respond appropriately,...

All that being taken as it is written, enjoy your device and keep playing, if you are accomplished at self study there are many good self study guides right here at nukeworker.com, with a solid eighty to one hundred twenty hours of study you could develop a much better appreciation for just how much background knowledge is needed to understand what your new toy is trying to tell you,...

Good luck and have fun,...GLW

[tkalfaoglu]

Thank you so much for your assistance! I truly appreciate all the help -- this is one topic that's difficult to research, as the internet seems to assume you are out to create an atomic bomb or something as soon as you start reading about radioactivity.

I'm very glad I finally found this forum, I'll be reading the resources and I started taking hourly measurements.

Regards,
 -turgut

[tkalfaoglu]

I found out that I had contaminated the geiger counter by putting a sample on the tube (it has a mesh window on the outside).
I opened it up, brushed it and closed it up. Now it's reading decent numbers ---  0.27 uSv/h..

Many thanks again, -turgut

[MrHazmat]

Is there a copy of the OLD "PINK" book on-line?? It is great for a pure beginner to learn from. That is where I started, back in 1974.

[61nomad]

The Radiological Health Handbook?  There is one on Ebay from 1960 for a "buy it now" of about $25. I had one that was pink also but I have seen different colors. This version might not be the best for a newbie but it would be interesting to look at.

[MercTech]

Hmm, Ukraine, could be the same as the problem with some of the older GM tubes (read 50s-70s) of quench gas depletion or even lack of quench gas altogether.

The only empirical experience I have with this was from the early 80s... my boat had an AN/PDR-27 that had a gm tube inside with a manufacture date of 1952.  (Korean War era.. gack!)

The unit would source check fine and pass calibration.  But, if you actually took a reading with it, it would take up to a half hour to settle back down to expected background levels.  Proved this by doing readings on three AN/PDR-27s side by side.

Final solution was to break out an emergency repair part, GM tube (this one was made in 1969).  Then broke the cal sticker (calibration void if opened, etc.), opened the unit and replaced the GM tube.  Sent to cal and it worked fine thereafter.

Not saying that is the problem but oddball things can happen.

[tkalfaoglu]

Aha -- I think I found the book you are referring to. Great! Many thanks!
http://www.orau.org/ptp/PTP%20Library/library/Subject/health%20physics%20general/RHH.pdf

I'll check the tube next time I open up the machine - and see if there is a date or any other markers on it.
Many thanks,
-turgut