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 How to measure true background
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ullix

Germany
47 Posts

Posted - 01/02/2017 :  07:21:14  Show Profile  Reply with Quote
I got a shiny new GMC-300Eplus from Santa and began experimenting with it.

I measured background (18.1 CPM here in Germany, measured over 1 day) and then tried to reduce background by shielding the device. Thick layers of cardboard, a tin box, a lead bag (from the times when you had to get real photographic films through the airport scanners). Not much success; the background barely changed.

Which made me wonder: is there something like electronic noise, which generates clicks even without any external radiation? Can the inside, i.e circuit boards, housing etc, generate anything?

And if so, how can I distinguish such "electronic" background from real radiation background?

Reply #1

the_mike

Switzerland
12 Posts

Posted - 01/03/2017 :  06:14:46  Show Profile  Reply with Quote
Hi ullix

To block out gamma-radiation, you'd defenetly need some thick reinforced concrete (as used in bunkers) or lead shilding... It is though interesting, that the lead-bag didn't make a difference... (did you use the logging-function taking the readouts, or did you only cover the sensor-area of the GeigerCounter?

On the other hand, the built-in chinese-tube is not known to be "one of the best"; therefore, so many owners are changing the tubes to SBM-20, or SBT-11a or SBT-9 (later two capable of also measuring alpha- and beta-radiation), as they are more sensitive and more reliable...

(actually, it would be great to buy a GMC-Counter equiped with SBT-9 (or 11a) instead the china-tube, THAT would be worth some 50-80 bugs more :-)
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Reply #2

GBG12

Canada
23 Posts

Posted - 01/03/2017 :  18:48:44  Show Profile  Reply with Quote
I wanted to shield my SBM-20 on 5 sides to make it directional, so I kept adding lead sheet layers until I reached 20 mm thick. It weighs 25 lb / 11 kg. The background counts have dropped by only 20%, indicating that it it takes much more than this to stop Gamma radiation.
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Reply #3

ullix

Germany
47 Posts

Posted - 01/04/2017 :  10:45:44  Show Profile  Reply with Quote
Wow, some real salesman at work ;-)

No, for the time being I am actually satisfied with the counter. And furthermore, my question would also relate to the best Geiger counter available, namely: how do you distinguish between a real radiation event, which makes a count, and something which simply results from some electronic effect inside the circuit board and/or the tube? Those should happen even in a perfect shielding.

I am living in a home which has concrete floor and ceiling (which may result in some gamma from those stone materials, like k-40 gamma, Rn is unlikely to play a role). And somehow I believe that the low background, which I get is not from cosmic or concrete origin. But how to distinguish? Someone must have attempted that in order to characterize the tube and circuitry?

For example in the attached picture I have the Phase 1 with the Geiger counter laying in its casing on my wooden desk. And phase 2 with it in a tin box, in a lead bag, in a steel container with ~2mm iron walls, all on my wooden desk. This amount is a reduction from 19.3 to 17.1 cpm, or ~11%. (logged an plotted with my python script shown here http://www.gqelectronicsllc.com/forum/topic.asp?TOPIC_ID=4420)

The question remains what part is (high-energy) gamma quants, and what is "electronic and/or tube" generated? I don't consider this to be a defect; just something that characterizes the device I use. (And if that were not satisfactory for a certain application, I would have to by a new one, but then I would again like to know this very property of the new device).


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Reply #4

phgphd

USA
11 Posts

Posted - 01/04/2017 :  17:39:45  Show Profile  Reply with Quote
Electronic circuits cannot produce radiation of the energies equal to gamma radiation. Even circuits running gigahertz frequencies don't come close to gamma radiation. To be statistically significant you should run your tests with various shielding for at least 10 days each test since you can get variations of average radiation of 5 counts/minute from day to day. If you place your geiger counter too close to a circuit, then you have the possibility of the electronic radiation inducing noise into the geiger counter's own circuitry, thereby confusing the issue.
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Reply #5

ullix

Germany
47 Posts

Posted - 01/05/2017 :  02:29:51  Show Profile  Reply with Quote
I am afraid I can't make myself being understood :-/

But to be sure: I am very familiar with all things Alpha, Beta, Gamma, cosmic rays, electronvolts as energy unit, and the like, as my scientific background is nuclear physics. My thesis was about high resolution Gamma measurement in nuclear reactions.

Key to good measurements is the knowledge of the properties of your detectors, like Geiger Müller counters in the present case.

All things electronic do have their own background, like that hissing in the radio or stereo; good devices have a high signal-to-noise ratio, but even the most expensive ones do have some level of noise.

The Geiger counter should also have a similar effect, where due to some imperfection in the circuitry and/or in the Geiger tube itself, a count may be generated even if no Gamma quant has hit.

That spurious background count is what I am interested in.

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Reply #6

ZLM

1046 Posts

Posted - 01/05/2017 :  12:45:25  Show Profile  Reply with Quote
All those models installed M4011 Geiger Tubes, some users changed tube to SBM-20. Those tubes measure low level radiation and are considered as high sensitivity tubes. To let tube be high sensitivity, the Geiger Tube need to set at the marginal status between stable and active, as long as the ionized electron pass through, the tube should become conductive at that moment. However, since it is on a marginal status, it is easier to become active due to the electronics noise or other noise factors. So, the conclusion: each Geiger Tube has its own noise counting. That is very hard to be eliminate. Assume you can block all radiation by lead shield, regardless the zero radiation environment, there will still be a reading from itself. So, do not expect to read zero from unit. The higher sensitivity, the higher self noise.
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Reply #7

phgphd

USA
11 Posts

Posted - 01/05/2017 :  18:25:10  Show Profile  Reply with Quote
Ullix, you are asking for the near impossible. If a geiger tube is the only means to detect a gamma photon, how would it ever be possible to distinguish between a false count and a real count? I suppose you could provide so much shielding as to lower the probability of gamma penetration to 0.0001%. Then you could determine on a statistical basis the false count susceptibility of the geiger tube. I believe you need to go underground into a tunnel that has minimal radioactive natural minerals. I suspect that even then extra shielding would still be necessary.
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Reply #8

Distelzombie

Germany
163 Posts

Posted - 01/07/2017 :  18:35:10  Show Profile  Reply with Quote
I once used a Faraday cage made from kitchen supplies to shield my device from high amounts of electrical interference/emf. It has worked perfectly and was just a piece of cardboard with aluminum foil and a sieve. I could measure nuclear radiation in the very vicinity of a tesla coil - which was impossible before.
This will at least shield your device from external interference which could very well be always a factor if you're inside a building.

You may know, judging by your background, that gamma spectroscopy is made using a ton of lead shielding around the sample. If you have access to one of these devices maybe the counter fits inside and you can thus measure the false hits created by the electronics. I remember reading about this. It's a thing some manufacturers specify. (the noise)

GMC-300E+ V4.20 with sbt-11 alpha tube

My statements are to be taken as stuff-a-hobbyist-says.

Edited by - Distelzombie on 01/08/2017 15:07:29
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Reply #9

GBG12

Canada
23 Posts

Posted - 01/08/2017 :  13:57:52  Show Profile  Reply with Quote
Each GM tube manufacturer publishes a noise factor that is inherent to the design. A discussion on this is at this url: www.uradmonitor.com/topic/what-the-heck-are-we-counting/. There are links to 3 data sheets showing the inherent ('own' or 'self') noise, measured in a heavily shielded environment. The numbers are higher than what users typically read with no shielding, so it would appear that these published values are maximum, not typical or actual. The tube voltage would have an effect; there is a permissible range and a recommended voltage. Individual units may not be adjusted to the recommended voltage, so actual noise counts may vary for the same tube model. One poster at that url used 2 m of water as a shield and got half the value as above the water. I got similar results as you with a fully enclosed 2 mm lead box, but I did not build a thicker 6-sided box. This source www.blackcatsystems.com/GM/articles/alpha_beta_gamma_radiation.html states 1/2" of lead stops half the gamma radiation, so it would take quite a few inches to truly measure the inherent noise of a detector.

Edited by - GBG12 on 01/08/2017 13:59:44
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Reply #10

ullix

Germany
47 Posts

Posted - 01/10/2017 :  02:57:31  Show Profile  Reply with Quote
#6 ZLM gave a better explanation of what I meant to say regarding instrument background, thanks.

I just happened to look at this manual http://www.gqelectronicsllc.com/userguide/GMC300EPlusV4UserGuide2.00.doc where I found on page 5 this spec:
quote:
Instrument Background: < 0,2 pulses/s

I guess this is what I was looking for. It then means that out of my 17.1cpm background, up to 12 cpm are instrument background, hence true background is 5.1 cpm?

But as several of you have pointed out, this is not trivial to measure. High shielding with lead may have the problem that lead may contain impurities of higher Z decay chain, which contribute gamma. (As far as I can see, if it were pure lead, there would not be gamma decay detectable with GQ Geigers). And this still leaves aside the potential for "electrical interference/emf"; the Faraday idea might come in handy at times.

However difficult to determine - you need to know your system and take its properties into account.

Which brings me to another topic found in the specs:
quote:
Range of gamma radiation energy, MeV 0.1 to 1.25
Range of registered X-ray radiation energy MeV 0.03 to 3.0

What on earth does this mean? Gamma and X-ray are physically the same thing, and X-ray is mostly seen as around 10-100keV. Creating 3 MeV with an X-ray setup would require a 3 Mega-Volt tube. I never heard of that. (I once experimented on a 4MV van-de-Graaf, which needed its own building!) But while it is unusual though not false to call Gamma radiation of 3MeV X-rays, it surely becomes a false spec, when you at the same time specify sensitivity to Gamma of only up to 1.25MeV.

What was meant here?
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