Gas filling of Geiger tube

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Antonio Zanardo
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Joined: Tue Feb 27, 2018 8:48 am

Gas filling of Geiger tube

Postby Antonio Zanardo » Tue Feb 27, 2018 10:13 am

Hi all,

I got from my sypplier a badly crushed soviet CTC6 GM tube. As it was unusable, I opened it up to see how it is made inside.

The attached picture shows the whole interior of it.

The inspection reveals that the gas contained into the tube may stays there provided a sealed chamber is formed by a tight crimping of tube at the end caps.

Together with the crushed tube I received more undamaged CTC-6 and SBM-19 tubes. A couple of them however have one or both caps that are not firmly crimped, thus they can be moved by twisting them a little bit by hand.

This fact led me to conclude that the gas of those tubes has leaked out through minute tube-caps fissures, as there is no other sealing, thus making those tubes unusable.

However, when I tested them, they resulted working pretty well, in contrast to what I expected.

Do you have any explanation for that odd behaviour? Maybe the gas has not completely leaked out and some of it still stays inside the tubes? Or what?

Thank you

Antonio Zanardo
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tubo geiger.jpg

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mightyohm
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Re: Gas filling of Geiger tube

Postby mightyohm » Tue Feb 27, 2018 10:35 am

Antonio, can you post a closeup photo of the end caps?

Antonio Zanardo
Posts: 5
Joined: Tue Feb 27, 2018 8:48 am

Re: Gas filling of Geiger tube

Postby Antonio Zanardo » Tue Feb 27, 2018 1:06 pm

Hello Jeff,

here are two more pictures.

On left side of the first image, you can see the negative cap with a small glass tube having a hole through wich the wire passes. The wire is much thinner than that hole, its diameter being 0.5 mm. This cap is electrically connected to the GM tube body.

On the second picture, again on the left side, you can see inside the small tube a piece of central wire and a small spring . The spring function is to keep the entire wire isolated, tense and centered on the tube axis. The rest of wire is not visible as I have cut it away.

The other cap on the right of both pictures is the isolated positive cap.

I hope to have been clear enough. English is not my mother tongue, so I have some difficulties to explain properly all details.

Antonio Zanardo
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DSC01693.JPG
DSC01691.JPG

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mightyohm
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Re: Gas filling of Geiger tube

Postby mightyohm » Tue Feb 27, 2018 1:57 pm

Antonio,

Great photos! Based on your photo I think there might be a junction between the plastic end cap and the metal seal. See where the arrow is pointing in the photo below. Is it possible that on the "bad" tubes, only the plastic portion of the cap is rotating and not the metal sealing surface?
2018-02-27_13-54-27.jpg

Antonio Zanardo
Posts: 5
Joined: Tue Feb 27, 2018 8:48 am

Re: Gas filling of Geiger tube

Postby Antonio Zanardo » Tue Feb 27, 2018 4:18 pm

Jeff, you are absolutely right!

Following your hint, I cut the cap near the point you indicated with an arrow. I found that the clearer part which supports the small glass tube is a metal little cup, and the circular line you indicated is where the tube is welded at that cup.

The darker part is the bakelite insulator, wich is bonded to that cup. Those soviet GM tubes are very old indeed, their cement has lost its sticking power and the bakelite insulator gets loose. This fact however cannot affect the tube welding, so the tube itself remains leak proof.

Well, the mistery is solved. Thank you so much for your smart hint.

I have a lot more questions about the Geiger tube as I am attempting to make some prototypes, but it's too late now and I'll post them next time.

Antonio Zanardo

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mightyohm
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Re: Gas filling of Geiger tube

Postby mightyohm » Tue Feb 27, 2018 4:31 pm

Antonio,

I am glad that I could help. Good luck with your project.


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