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 2.GQ Geiger Muller Counter
 K40 detection
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jlp1528

USA
19 Posts

Posted - 05/23/2020 :  07:59:29  Show Profile  Reply with Quote
So here's the story. I've had the GMC-500+ for a few months now, and I've known for quite a while that anything with potassium in it contains potassium 40 and is thus slightly radioactive. I'm not sure why, but the banana is the classic example.

Considering this, I was under the impression that potassium permanganate, chemical formula KMnO#8324;, would contain enough potassium to be detectable with a Geiger counter. Unfortunately, the GMC-500+ CPM wouldn't budge from background. At this point I just figured I was wrong - that there wasn't enough potassium in this particular compound to matter - and moved on.

Here's where things get weird. For entirely unrelated reasons, I finally bit the bullet (money wise) and sprang for the GMC-600+ last week. Of course, this model is actually quite a reasonably priced detector for this level, but $300 is still $300. Anyway, this model *does* show an elevated reading on the potassium permanganate - over 200 CPM vs a background of about 40.

This is puzzling though, since the energy levels of potassium 40 decays are well over 1 MeV - several times higher than the stated minimums of 250 KeV beta and 100 KeV gamma in the GMC-500+ manual. So shouldn't both the 500+ and 600+ be able to read this stuff? Sure, the tube in the 500+ is both somewhat larger and much more sensitive, but if the decay energy is above the detection threshold of both, that shouldn't matter in terms of defecting the sample at all, right? What am I missing here...?
Reply #1

kotarak

USA
19 Posts

Posted - 05/23/2020 :  09:54:08  Show Profile  Reply with Quote
For what is worth, I just measured a 20Lb. bag of Potassium Chloride with my 600+ and I get around 470-500 CPM (my background is 42-45CPM).

I think, what you are seeing has to do with the sensitivity of the M4011 tube in 500+. This is a Chinese glass tube, somewhat comparable to the Russian SBM-20 in sensitivity.
On the other hand 600+ is using a US made and way more expensive LND 7317 which is practically the low-voltage version of LND 7311 - used by Ludlum model 3 and other high-quality counters).
I think it might be that the specs of M4011 are loose and the actual sensitivity to low-energy beta is just not comparable to the LND tubes (with a mica window, friendly to alpha particles but I am sure beta sensitivity doesn't suffer too)
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Reply #2

Damien68

France
241 Posts

Posted - 05/23/2020 :  13:06:55  Show Profile  Reply with Quote
We often talk about bananas because it is one of the fruits that contains the most potassium.

Pancake tube like one present in GMC-600+ was originally made to test samples and not specifically to test alpha radiation.
From sample, Each particle is emitted in a totally random direction. We should talk about solid angle and integration... but it will become complex. To simplify, we will say that the pankake tube will cover half of the surface around the sample to be tested, that is to say that if a particle is emitted by the potasium, it has One chance out of two to crossing the pankake tube and to be tetected, and an other chance going to the other side and to not be detected.
For traditional tubes for example the M4011 present in the GMC-500 + the active surface of the tube around the sample will be much weaker, also the probability that a particle emitted by potassium is intercepted by the tube is much lower, in proportion to the covered (by the tube)/uncovered ratio of sphere surface around the sample.

But the pancake tubes also have their shortcomings (directional, size, saturation ...).

Otherwise I did a quick calculation:
The activity of 1 g of pure K40 is 263 kBq, This means that one gram of pure K40 will emit 263 thousand particles per second.
On the other hand: 1g of standard potasium on earth contains 0.012% of K40.
So activity of 1 g of standard potasium is 263k x 0.012/100 = 31 Bq, This means that one gram of standard potasium will emit 31 particles per second.

for example I bought a 240 g box of diet salt composed of 40% potassium.
It must therefore emit 240 x 0.4 x 31 = 2976 particles per second.

I emptied the salt shaker in a ziploc bag, then I put my 500+ on it, I then have an elevation of only 60 CPM and I speak well of CPM.

In theory we should have 2976 * 60 = 178560 particle emited per minutes, but most of it must not leave the Ziplock, certainly it must be intercepted by other nucleus or atomes, and of all the particles that come out of my bag, the tube of my 500+ will detect only a few percent



Mastery is acquired by studying, with it everything becomes simple

Edited by - Damien68 on 05/23/2020 22:39:45
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Reply #3

jlp1528

USA
19 Posts

Posted - 05/23/2020 :  16:31:11  Show Profile  Reply with Quote
I did wonder if the simple increase in size could be a factor. Good points on this and my other post that the energy level specs might not be accurate. Thanks for the detailed info and examples! Still learning here; I'll stop learning when I'm dead. Maybe. ;)
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Reply #4

ullix

Germany
492 Posts

Posted - 05/25/2020 :  00:23:19  Show Profile  Reply with Quote
If you wonder about Potassium, it is time for some "Potty Training"!

See my article on this subject here: https://sourceforge.net/projects/geigerlog/files/Articles/GeigerLog-Potty%20Training%20for%20Your%20Geiger%20Counter-v1.0.pdf/download

And if you are wondering about Going Banana with Potassium, I also show how this can be done: https://sourceforge.net/projects/geigerlog/files/Articles/GeigerLog-Going%20Banana-v1.0.pdf/download

And all of this done with a plain old GMQ 300E+ and my program GeigerLog, available on the same site.

You just need a bit of patience; getting the statistic good enough takes time.
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Reply #5

Damien68

France
241 Posts

Posted - 05/25/2020 :  01:28:19  Show Profile  Reply with Quote
a larger tube gives better observation and allow measurements to be made more quickly in proportion of his size, but will not fundamentally change the result.

A pancake is better for sample test because it is very directional, but for an ambient measurement the traditional tube is better.


something else:
if you are comfortable with math you can look at the following article:
https://en.wikipedia.org/wiki/Solid_angle

It is important to know that for geometric reasons, the number of particles detected (intercepted by the tube) is a function of the square of the distance between the source and the tube (following the solid angle value formed by the active surface of the tube, and a center located at the source location).
This is largely why as soon as we remove the counter from the source, the detection decreases quickly.
There is also the fact that not all particles go very far.

Simulation bellow show a 10x10x10mm sample source with a SBM-20-1 and a LND7317 GM tube both at 10mm from the source.

we can see that in the case of the classic tube, the brown / orange cone is much narrower in the direction of its projection with respect to the thickness of the tube. in reality, it should even be a little narrower than that because of the tangent effects.

Mastery is acquired by studying, with it everything becomes simple

Edited by - Damien68 on 06/04/2020 23:40:54
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Reply #6

jlp1528

USA
19 Posts

Posted - 06/11/2020 :  13:42:01  Show Profile  Reply with Quote
Thank you for the information. After failing to trigger the GMC-500+ with KCl ("No Salt") I decided to try yet another potassium compound, another one I may actually use at that: KOH, aka caustic potash, aka put this in water to make Drano. The increase in CPM is minimal and slow, but it is definitely there. :)
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Reply #7

kotarak

USA
19 Posts

Posted - 06/11/2020 :  15:16:12  Show Profile  Reply with Quote
quote:
Originally posted by jlp1528

Thank you for the information. After failing to trigger the GMC-500+ with KCl ("No Salt") I decided to try yet another potassium compound, another one I may actually use at that: KOH, aka caustic potash, aka put this in water to make Drano. The increase in CPM is minimal and slow, but it is definitely there. :)



If you are in US, just go to Home Depot or Lowes and look for the
"Water Softener" section. The regular softener is NaCl but they also sell KCl softener in 40 LB bags. If you go with your counter and they are well stocked, you'll have the chance to measure the activity of hundreds of pounds of KCl.
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