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Making a Geiger-MÃ¼ller Counter

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uzzors2k

Sun Jul 22 2007, 12:47PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I plan on making a Geiger counter so I can sense x-ray leakage from my future x-ray setup. I have a few questions about the tubes though. I've sighted out two different tubes, the STS5 and the FHZ74V. I've found very little information on them besides pictures. First of all which would be best suited for detecting x-rays/gamma rays? I assume both will work, but it seems that some tubes are made for alpha and others for beta radiation. Shouldn't x-rays be capable of exciting any tube?

Info on the FHZ74V.

I found nothing on the STS5, besides a picture from the seller's webshop.

Sulaiman

Sun Jul 22 2007, 01:59PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The tube in bottom photo looks like the SBM-20 in my russian GM counter
perfect for x-rays.

Is it worth making a GM counter when they're so cheap on eBay?

c4r0

Sun Jul 22 2007, 03:09PM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

Yes, it's SBM-20 or similar on the second photo and it's good for x-rays detecting. You just need about 400VDC to power it. Number of pulses in 40s measure time is equal to radiation "power" in uR/h (1uR/h = 0.01uSv/h). You sholud get about 15 pulses (in 40s time) from natural background radiation. If you want I can send you schematics of a few russian geiger counters using this tube

uzzors2k

Sun Jul 22 2007, 03:35PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

That would be great c4r0.

I can't find any cheap Geiger counters on ebay atm, and I can buy the GM tube from the same place as the HV rectifiers which I'll use for x-ray tubes. What I save in shipping costs will make up for it.

c4r0

Sun Jul 22 2007, 04:01PM

Registered Member #151 Joined: Sun Feb 12 2006, 02:53PM
Location: Poland
Posts: 153

Ok, so this is the simplest and the most useful one - russian dosimeter BELLA:

It simply counts the pulses and displays it. Uses one SBM-20 tube

This one is more complicated - also russian POLARON-PRIPYAT:

Uses two SBM-20 tubes.

And here is an old (1960s) Polish Army radiometer DP-66:

It has three diferent tubes becouse of wide measure range (0,5mR/h - 200R/h), one of the tubes is SBM-20.

Also these websites should help you:

Sulaiman

Sun Jul 22 2007, 04:30PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Tube specifications
]1185121847_162_FT28803_sbm20.pdf[/file]

uzzors2k

Sun Jul 22 2007, 05:13PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Wow, thanks guys. I'll order a SBM-20 then, and tell how the counter turns out.

Simon

Mon Jul 23 2007, 12:34AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Uzzors wrote ...

Shouldn't x-rays be capable of exciting any tube?

GM tubes are often not so sensitive to gamma and X rays but it should work.

uzzors2k

Fri Aug 17 2007, 07:50PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Well, he was all out of SBM-20s so I had to buy a FHZ74. I put together a simple circuit which provides a regulated output, and pops a speaker when the output is grounded (tube ionization). This gives the cool crackling sound familiar from games and movies.

1187377776 95 FT28803 Geiger Counter Schematic

Anyway, it's not acting like I expected it to, which is somewhere around 15 pops per 40 seconds.With the tube voltage set to 575V I get constant crackling, as though I were in the middle of Chernobyl. If I set the tube voltage down to 420V the crackling stops altogether. I don't even get occasional pops from background radiation. Is this normal behavior? I haven't tried the counter near a known radioactive source yet, maybe the counting starts again or increases even more. I'm not sure what the different voltages in the datasheet are for, but "arbeitsspannung" (working-voltage) must be what I need to drive the tube at. Will the tube only function properly at the working voltage, close to 500V? Or is the excessive crackling due to the circuit design?

]FHZ74.pdf[/file]

Sulaiman

Fri Aug 17 2007, 09:21PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I'm not sure but I think that your GM tube is for high radiation environments
50 r/hr is high (500 r over 5 hours is lethal according to wikipedia)
and I think the spec. gives 0 to 5 impulses per minute for background radiation
much less than the SBM20.

The constant crackling at 525 V is due to over-voltage breakdown in the tube (bad)
420 V should be good though.

Hopefully someone can give a second opinion?

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