Is it essential where the detector is placed?


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Profile Saenger
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Message 140 - Posted: 30 Jul 2011 | 17:37:21 UTC

Should it be somewhere in the open air? Or near the window? Or could it as well be the computer cabinet in the basement?
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Kenneth Larsen
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Message 141 - Posted: 30 Jul 2011 | 17:57:36 UTC

I believe gamma rays pass pretty much unhindered through most stuff, so as soon as your walls are not dense lead or meter-thick concrete I don't think it matters.

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Message 143 - Posted: 30 Jul 2011 | 18:10:16 UTC

Depending on where you live, i the basement could be a high concentration of Radon, in south-east Germany (Erzgebirge) that's often the case.

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Message 145 - Posted: 30 Jul 2011 | 18:38:50 UTC - in response to Message 143.

But Radon generates alpha rays...

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Message 147 - Posted: 31 Jul 2011 | 5:52:40 UTC - in response to Message 145.

But Radon generates alpha rays...

Every particle emission, alpha or beta, is accompanied with gamma rays.
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Message 219 - Posted: 17 Aug 2011 | 9:15:00 UTC - in response to Message 147.
Last modified: 17 Aug 2011 | 9:16:06 UTC

Notice: I'm not expert for this kind of measures, anyway ...

a suggestion to the admins: for a better post processing of the raw data, could be useful add ASAP more informations to the hosts DB (ASAP because I see that changes in this DB are not followed by the users, example, the QCN project added to sensor DB the altitude over sea, or over ground, but almost no user has inserted valid data in the new field).

- sensor place: indoor, outdoor
- date of manufacture of the tube (to take into account the tube decay)
- tube model/ID (new kind of sensors in the future ...)
- counter model/ID (new kind of sensors in the future ...)
- type of initial calibration/test for the tube

and so on.

Alessandro Freda
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Message 220 - Posted: 17 Aug 2011 | 11:00:46 UTC - in response to Message 219.

Just another bit about sensor placement info ...

I read here:

http://en.wikipedia.org/wiki/Gamma_ray#Shielding

"gamma rays that require 1 cm (0.4″) of lead to reduce their intensity by 50% will also have their intensity reduced in half by 4.1 cm of granite rock, 6 cm (2½″) of concrete, or 9 cm (3½″) of packed soil"

So may make sense add infos about the sensor placement.

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Message 237 - Posted: 29 Aug 2011 | 16:36:53 UTC - in response to Message 220.

Again another bit about sensor placement info :)

If the sensor is placed in a not well ventilated cellar, it could detect also the variations of the radon. I think that to correctly measure the radon gas also a alfa geiger detector is needed, btw the gamma tube could measure the radon variation instead (or combined) with the external gamma variation.

Another item to add to the above list of infos for the user sensors DB:

- a list of date/time position variations

could be interesting, because local sources changes could modify the background (tiles, stones, concrete ...)

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Message 244 - Posted: 2 Sep 2011 | 12:12:50 UTC

SBM-20 is almost immune to alpha radiation, it barely detect alpha source when it's touching the tube.
There's also another variant of the tube, SBM-20-1 which has a layer of lead/tin foil - it's completely immune to alpha even at touching range.

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Message 248 - Posted: 17 Sep 2011 | 16:35:39 UTC - in response to Message 244.

Don't forget that gamma can be at varying energy levels too right? Kind of like X rays?

Let me hit you with 12 volts, then touch you with 138KVolts ... notice the difference? Granted a crude comparison but I think it works.

oh and I just joined the project today, now to find out where to get one of these sensors so I can set it up.

Aaron

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Message 437 - Posted: 30 Oct 2011 | 19:16:34 UTC

I just tried an experiment. I partially shielded the box that my GRS is in by putting an old PC PSU under it, and another on top (lots of steel casing!). I can just about see a drop to about half of my normal readings. I've just uncovered the GRS again, so should be able to see a dip in my chart followed by a return to normal shortly :)

I did this because my workshop is full of large metal objects (lead-acid batteries, a safe, metal boxes etc) and I'm trying to find a place to put the GRS. I may have to move it into the house to get away from the problem, but then I'm likely to run into problems with lots of electrical equipment :/

I did consider mounting the tube in a weather-tight box and mounting it on the roof of my workshop, but I'd only feel happy with armoured cable and things then get a bit cumbersome to manage :(

Actually, thinking about it, I could probably site it in my office and run a 'long' (4 or 5 metres) USB cable and put it on a window sill away from electronic stuff. Have to google the max USB cable length ...

Al.


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Message 442 - Posted: 30 Oct 2011 | 19:57:41 UTC - in response to Message 437.

Chertsey, I have seen USB cables up to 15 feet long. (Roughly 5 meters) In fact I use one of them for my printer. As for power requirements for your project, I am not entirely sure how bad any voltage drop on it or data transfer might be affected. I don't think data would be too bad an issue if it can still support a printer in my instance.

They do make powered or amplified USB cables too. If you do use a long one, I would recommend running it off one of the USB ports right on your mother board and not from a hub unless it's externally powered for each port.

Aaron

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Message 454 - Posted: 30 Oct 2011 | 22:33:08 UTC - in response to Message 437.
Last modified: 30 Oct 2011 | 22:35:29 UTC

Al,

What about removing the tube from the box it's now in and using long wires between the tube and the box. I think the length of the wire from the tube to the box is not important because even with a small voltage drop there will still be sufficient voltage at the tube. That wire might be cheaper than a long USB cable and perhaps allow you to site the tube in the attic above your workshop, out of the weather but away from whatever you have in the workshop. I guess the first thing to do would be to check the minimum voltage requirement for the tube.
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Message 469 - Posted: 2 Nov 2011 | 19:58:54 UTC - in response to Message 454.

The voltage drop is not important, as the current is very low.
The problem is the level of induced noise, if the wires are too long you'll get a lot of false signals.

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Message 470 - Posted: 2 Nov 2011 | 20:28:00 UTC - in response to Message 469.

That makes sense. Wouldn't using inexpensive armored wire of the kind they use for mains wiring in buildings eliminate the noise? Or run the wires through electrical conduit?

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Message 475 - Posted: 3 Nov 2011 | 9:17:29 UTC - in response to Message 470.

Shielded wire has another disadvantage: larger capacitance, if it gets too long it may act like a capacitor connected to tube socket. Also, if you use shielded wire, you need to pick one with high enough voltage rating. Once I used cheap one (from microphone I believe), everything worked fine for a couple of weeks and then suddenly the cable started to produce false readings.

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Message 479 - Posted: 3 Nov 2011 | 19:09:00 UTC - in response to Message 475.

Decided not to mount the tube externally (even though I've found some spare armoured mains cable left over from running power to the pond and fountain) as it will mean I can't easily move the whole thing around.

Anyway, it seems to be working OK where it is, and I've found a way of putting it in my office with just a 3 metre USB cable.

Still got to put it in a proper box. Although I've grown quite attached to my transparent sandwich-box arrangement. Adds a nice glow to my workshop at night :)

Al.

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Message 646 - Posted: 18 Dec 2011 | 20:55:30 UTC

Information about detector placement (indoor, outdoor) should be definitely provided on sensors map. Even more, two detectors in one location: one indoor and one outdoor measurements should be handled by software. Of course, at first stage most important is outdoor radiation. Indoor is "second line of defence".

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Message 647 - Posted: 18 Dec 2011 | 21:46:49 UTC

It's true...
I recommend that if the sensor is placed indoor then it should be as close to the window as it can - at the windowsill.

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Message 751 - Posted: 16 Feb 2012 | 21:28:58 UTC - in response to Message 647.

to lengthen the line between the Geiger counter and the controller must:
- Take the wire with thick insulation and a thin copper core.
- Twist them together. step is about 5-10cm.
- Put them in the screen (option - wrap the foil).
- Screen is isolated from the environment (heat shrink tubing)
- At the end where you need to connect the Geiger counter. Geiger are connected to the wires. screen is not podsoedenyat.
- The wires at the other end connected to the controller according to the polarity and the screen to GND.

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Message 1991 - Posted: 1 Sep 2013 | 0:46:02 UTC - in response to Message 437.

* The max USB cable length is 15 feet --unless you get something special (with built in powered repeaters?).

* However, if you have something like a GMC-200 or an "Inspector" (best block beta and alpha on that one for network use) --with a typical mini-jack pulse output, then you can use it with the intended "data cable" (comes with a driver and it has a powered chip in the USB end) and extend the pulse/audio end out at least 40 feet (about a 1% response roll-off at higher CPMs when I tested an Inspector).

I suspect that the GMC-200 and its data cable could be extended much further, since it works at lower impedance.

* Pulses for data do have limitations. The Inspector's square wave is about a millisecond wide, so Poisson distributions on the high side of average will start "piling up" and overlapping some of the pulses above 5000cpm (--which levels we'll hopefully never see).

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Message 1992 - Posted: 1 Sep 2013 | 1:41:41 UTC - in response to Message 647.

As to placement of an SBM-20 tube device, I can only speak from my experience with a Radex-RD1503, since March of 2011. (I'm new here, but am a member of Radiation Network, patched in with a Medcom "Inspector" GC. I post separately at:
> http://webpages.charter.net/123goto/map.htm

~ At 85cm above the ground, I seldom see a difference from inside readings with the 1503. I see greater (but not significant) variations with sensor orientation and placements. Furthermore, blocking the G-M tube side of the 1503's "view" of the ground with aluminum made no difference.

~ In general, I find it very hard to significantly attenuate the gamma count, not that it matters (in my opinion), since we're interested in each station being compared to its own baseline. The important thing (IMO) is to locate your GC where it doesn't have to be moved, the weather can't effect it, and where the geometry of other things in its immediate vicinity will not change.

~ The 1503's SBM-20 tube does have beta sensitivity at high enough energies, which you can prove to yourself by blocking the beta from a sealed^ packet of potassium chloride (ie: "salt substitute") with a 3mm or so thickness of aluminum.

^ KCl readily migrates into the air --perhaps partly driven by ionization, but via the air's moisture content for sure, so don't let it foul/contaminate your Geiger counter. Bag that fine white powder.

~ My understanding of the understanding in Germany --of their 1800 station system (all uniformly isolated, meter high, outside sensor placements) is that they're monitoring the ground plus any gamma from fallout which might settle in the vicinity of a monitoring station --per:
> http://odlinfo.bfs.de/

They frequently see a little bump with rainfall, which might be about hard beta.

~ If we wanted to have a separate network/reportage doing outside air monitoring, I believe it could be done with a setup like:

> http://www.blackcatsystems.com/GM/acc4.html

--and a bagged-with-desiccant GMC-200 (rated -50 +60 С)

(I'm retired and have zero affiliation with GQ-Electronics or any other business interest.)

Craig

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Message 2034 - Posted: 20 Sep 2013 | 7:06:12 UTC

Please look at my sensor reading.

It was moved at 18/09/2013 about 70 cm up in same location, just from behind of power wirings/ PC case which makes kind of shielding. Average redings goes up, say +10%.
Earlier I realize there was influence on the sensor from heat/strong sun radiation as well.
What's mean: location of the sensor should be described. Maybe we can do some improvement on our database?

Acvtual reading:
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