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 2.GQ Geiger Muller Counter
 CS137 10 Ci check source expected CPM?

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Bobakman Posted - 12/04/2022 : 10:40:05
CS137 10 Ci Check Source Circa 1985
For the above check source which is approximately 7 years into its second half life are the below readings as expected using a GCM-600+?

Edit: Images Video links removed.
6   L A T E S T    R E P L I E S    (Newest First)
Bobakman Posted - 12/06/2022 : 20:55:44
Understood. Thanks again for the information and direction. Ill see when I can
Redo the assessment and post the results. It may be a bit though.
My better half has other pending projects one is some antiquing with the 600+ LOL!

ullix Posted - 12/06/2022 : 01:57:12
(One request: next time you post a picture shrink it to no wider than 800px. Ultra-wide pics make reading really hard !)

$20 for such a source? Sigh. Here I can't get one at all; one needs a certification. And being a nuclear physicist is by far not enough :-(

I am not sure to fully understand what you did as posted on this Acrobat site. You put a 1 inch steel plate between Counter and source, and the paper and alu in addition? Can you put the results in a table?

The 4 instruments basically show all the same thing, CPM between 1000 and 1500. For this coarse approach, I take this as "all the same".

It would be consistent with the assumption of a complete absorption of all beta, and marginally reduced counts for gamma. You now also have a greater distance between counter and source, so your initial 279k/14k would also be lower. I suggest to remeasure, keeping the new distance, but nothing in between.

A time course would be welcome. GeigerLog can do the job (please, with a FET setting of 60!!!).

This article h**p:// has some nice details on gamma absorption in steel. For Cs-137 they measure approx 3.5 fold reduction over 1 inch.

The order of materials in the sandwich is important. The creation of Bremsstrahlung increases with the Z of the atom. Lead makes more Bremsstrahlung than carbon. So you begin with carbon to stop the electrons, and whatever Bremsstrahlung is created is absorbed in the metal. Using a sheet of plastic (a few mm) followed by steel (1mm) is preferred, but paper and alu is more easily available in households and almost as good.

This sandwich approach is also taken in nuclear institution, which run e.g. accelerators, to minimize cost and volume of shielding. Wouldn't be welcome to protect people from beta, but fry them in Bremsstrahlung ;-).

Bobakman Posted - 12/05/2022 : 11:49:40
Thank you ullix for your suggestions and "Potty Training" article.
Please see attached link below (Removed See Edit below) to a PDF of the results of the additional assessment testing for my CS137 check source.
I just realized I had the paper and aluminum out of order I hope the assessment still yields some value none the less.


Edit: Removed deleted file link
Bobakman Posted - 12/05/2022 : 08:00:51
See attached original purchase invoice for the source. Unfortunatly it does not give much if any
insight. I do remember it originally came wrapped inside a small sheet of lead long recycled.
Don't remember if an instruction sheet was included if it was it's long gone as well.
I will try as you have stated some other measurements using additional shielding.
I searched the web using the catalog number 62-103 on the invoice and came across some Fluke radiation instrument documents and it appears it was sold by them as a biomedical check source one of their instrument manuals states in using it the following manner:"Test the unit for proper operation by placing a check source (10 uCi, 137Cs, P/N 05103) on the top of the front panel in the position indicated with the label down. The alarm should activate in the low (2.5 mR/h) setting."
And from another fluke manual linked below: "To check overall consistency in operation of the PRIMA 7, a license free radioactive check source such as catalog number 62-103 containing approximately ten microcuries of 137Cs should be used. To check for proper operation with this source, place the source (label side down) directly over the target area indicated on the top of the front panel. Because the check source is not a calibrated source, the reading should be, on most instruments, between 10 and 15 mR/h. The main use of the check source is to verify that instrument operation is consistent from day to day. Typical readings from day to day will be from 11 to 13 mR/h with most readings at 12 mR/h."

Links for these instruments and other DOC's are below:

Thanks for your reply.


Edit: PO Image removed.




ullix Posted - 12/05/2022 : 01:58:16
A challenging question, and very surprising results.

Cs-137 has a pure(!) beta decay. It is only the resulting Ba-137 which decays further with the 662 keV gamma line, which is taken as a characteristic Cs-137 spectrum, and e.g. used for calibration purposes.

My assumption so far had been that the any such source is packaged in a way (in a steel container) that no beta can escape, because the steel is thick enough to absorb all beta. Apparently this is not the case. I think the CPM=280k contain a lot of beta, while the other side with CPM=14k might be closer to a pure gamma source. However, since one side is as "leaky", I would not trust the other side to be "clean".

Do you still have the specs for this source, or can you find them? What guidance does it give for use?

But you can investigate further by making your own tests. Please see the Appendix in my "Potty Training" article for understanding

Make an absorber as a sandwich of a metal plate (~1mm, Alu, copper, iron,...) and a stack of copy paper (some 20 pages). Place it in the order counter + metal + paper + Cs-source.

Use the Cs-source in both orientations, as you did before.

The betas should be absorbed much stronger than the gammas. So, my prediction is that the counts on the high count side will be reduced much stronger than on the low count side, and finally the counts on both side being much closer.

If not already, the try it with a thicker/thinner absorber (more/less paper, more/less metal thickness) to find the point where betas are near fully absorbed and gammas still strong.

For the count prediction additional problems arise: The sensitivity of the tube is given by the manufacturer as 348 CPM/(Sv/h), but for a Co-60 source, not Cs-137. So you have to compare the two spectra, and mathematically fold it with the spectral sensitivity. Unfortunately the latter is unknown, so you are left with guessing :-(

Then you also need to account for the spatial angle of the (almost) point source Cs-137 toward the larger detector.

Impress us all and give a reasonable prediction ;-))

Bobakman Posted - 12/04/2022 : 12:41:43
I did come across this document. Has a lot to filter through though.


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