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 2.GQ Geiger Muller Counter
 GMC 320plus and Americium 241
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seltech

USA
4 Posts

Posted - 03/06/2015 :  09:45:23  Show Profile  Reply with Quote
Hello all. New member here and new GMC owner. So I've received my new GMC 320plus which I'm very impressed with. I decided to see if it would pick up anything from my smoke detectors at home, which I didn't expect it too since the Americium 241 is a alpha emitter. To my surprise I'm getting around 300cpm sitting over the shielded Americium source. Am I detecting that 1% of gamma that can come from the source or am I missing something in my understanding all together?

I'm quite a nuclear noob if you will. So I apologize if its has been discuss in the past.

Thanks for any input!
Reply #1

ZLM

1241 Posts

Posted - 03/06/2015 :  10:45:37  Show Profile  Reply with Quote
Yes. The GMC-320+ detect Gamma from smoke detector. It is pretty sensitive to Gamma ray.

There are some Gamma radiation leakage from ionized type smoke detector.

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Reply #2

seltech

USA
4 Posts

Posted - 03/06/2015 :  14:13:53  Show Profile  Reply with Quote
Ok that makes sense then, thanks for clearing that up for me ZLM!
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Reply #3

sztajmes

11 Posts

Posted - 03/07/2015 :  18:52:42  Show Profile  Reply with Quote
Hi
Strange for me.
I getting nothing with mine 320 Plus.
Any particular model of smoke ?


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Reply #4

ZLM

1241 Posts

Posted - 03/07/2015 :  19:05:35  Show Profile  Reply with Quote
Only ionization smoke detector has Americium 241. However, the Gama leakage amount depends on the what type of shield used to cover the Americium 241. The manufacture always do their best to prevent the Gamma leakage.

See this:
h**p://www.homedepot.com/p/Kidde-Battery-Operated-Ionization-Smoke-Alarm-with-Test-Button-44037402/100154867?N=5yc1vZbmh8
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Reply #5

sztajmes

11 Posts

Posted - 03/07/2015 :  19:07:43  Show Profile  Reply with Quote
Looks like I have really great smoke detectors ...
Mine are 40$ and up....

Edited by - sztajmes on 03/07/2015 19:19:19
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Reply #6

seltech

USA
4 Posts

Posted - 03/11/2015 :  08:25:27  Show Profile  Reply with Quote
My smoke detectors are Kiddie ionizing types. Oddly enough I can't get nearly the same count I was getting the day I tested. I was seeing 300cpm for 30 minutes. Then when I tested it the next day, I was bearly getting 100cpm.

I think I'm going to buy all photoelectric detectors now
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Reply #7

ZLM

1241 Posts

Posted - 03/11/2015 :  10:39:14  Show Profile  Reply with Quote
Since the Gamma ray from smoke detector is considering leaking, so the position will be an important factor. The reading may be vary depending on the postion and distance you tested.
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Reply #8

dexterslab

United Kingdom
1 Posts

Posted - 03/18/2015 :  05:01:02  Show Profile  Reply with Quote
The americium source is a very small point inside the detector maybe about 3mm round so you will have to get the counter in a very specific spot to get the maximum reading.

You can see the source in this video, it was removed from an old smoke detector.

https://youtu.be/Xm9PS91gJfw?t=8m3s
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Reply #9

Alchemy2

Canada
89 Posts

Posted - 04/23/2015 :  10:07:37  Show Profile  Reply with Quote
Hi guys,

Yes, the stock 320+ WILL read the 60keV gamma from Am-241. Typical Kidde (what I have sourced for Am-241 and alpha sources) detectors use a small 1/8" dia (~3mm) AmO2 pellet housed in a steel rivet. Gamma and Alpha are fairly unidirectional for sources like this. The housing has a small shield which typically cuts gamma counts by 2-3x, soldered on the circuit board to provide part of the photoionization circuit used to detect smoke particles.

Usually the AmO2 (Americium(IV)Oxide) is such that activity ~0.9uCi or ~33.3kBq.

If you want a nice alpha source, these work well once removed, but use good tools and care to pop the rivet out of its plastic housing. A hammer on wood works well

Once removed, the source can be shielded in a small lead pig, and alpha is blocked by a standard piece of paper. gamma from this source is easily shielded with 3mm lead sheet, or even a folded sheet from lead fishing weight strips.

I housed my one source used for trials and tests with my other commercial sources by drilling a 4.5mm hole in the middle of a 1"x4mm thick acrylic disk (available on eBay for cheap) and used some CA glue to affix to the disk. Just take care not to drip the crazy glue or cyanoacrylate (CA) adhesive onto the top of the source exposed. If you do, it can be cleaned with Ronson lighter fluid on a Q tip.

I then made this label for the source:



Image Insert:

10.68 KB

The more I learn, the more I realize I do not know!
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Reply #10

Alchemy2

Canada
89 Posts

Posted - 04/23/2015 :  10:19:42  Show Profile  Reply with Quote
For reading alpha off these sources, you need a different tube. The SBT-11a is an ideal choice with these units. THey can be attached without any mods other than maybe shielded cable if you run more than 6-8" from the unit. I modified one of my 320+ units to have the SBT-11a direct to unit, and another to run a 14" coaxial cable to a SBT-11a probe I made using BNC connectors.

THe SBT-11a can then be tweaked a bit to yield best sensitivity by adjusting the small surface mounted trim pot on the board. THe SBT-11a is a nice quiet tube, and my favorite for general purpose use on these 320+ units (I have three to date). I'd estimate that it has around 70-75% of the sensitivity of the more costly LND7317 and similar tubes. (which also require higher voltages to run, close to 500V instead of 390-400V for these SBT-11a tubes, FYI).

These tubes will get many counts off a 0.9uCi Am-241 smoke detector source, typically in the 20000CPM range when 5mm over the source. This is a mix of alpha and gamma from the source. You have to then cover the source with paper to get only the gamma contribution. Efficiency is over 5-8% for alpha for these tubes typically. Around 2-3% for gamma.

The more I learn, the more I realize I do not know!
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Reply #11

donghelan

Japan
61 Posts

Posted - 12/08/2018 :  06:54:18  Show Profile  Reply with Quote
@alchemy2 I wonder if a reading of 50cpm with that chinese 4011 tube is close to normal?
The background here in Yokohama is 14-15cpm. I put the dismantled smoke detector source about 1cm above the middle of the tube.
So results is an extra 35cpm from Americium. Hopefully I can use it for reference and verification purpose of several tubes I have on the shelf like LND712 and SBM20.

[EDIT 20181209:2240JST:] See graph of 2 peaks of 47 and 50CPM caused by Americium attached.

Image Insert:

89370 bytes

To bug or not debug, that is the Q

Edited by - donghelan on 12/09/2018 05:44:47
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