Mini Instruments 6-70 Radiation Meter / Geiger Counter

[dexters_lab]

i picked this up off ebay recently and have been poking around with, powered it and had a scope around a little to just verify that it works but it's puzzling me because it both appears to work but also not.

The meter is old, you can see from the picture it had 'new' batteries in 32 years ago so probably dates from the 70s.

Inside there are two sections, one is clearly the GM / analog meter driver board which is connected to a logic / display board with a simple two wire interface (10v falling edge pulse). The digital board basically gives you a reading over a specified time selected by the adjustment knob.

Powering it on the meter will begin to move as i would expect for background radiation, on my scope i can see randomly spaced pulses at reasonable intervals.

BUT there is no different response when i bring a radioactive source near it, i have some uranium glazed Fiestaware that will give me 4000 counts/min on my other GM counters.

If i disconnect the probe the pulses go away and there is a good 415v coming from the driver which is all as i would expect.

Unfortunately the probe is sealed and the sticker worn off so i have no idea of the internal GM tube but it is massive. The meter is in uR so should be pretty sensitive.

I dont know much about how GM tubes fail, but i would have thought they just go deaf and dont produce any output? If anyone can clarify?

Seems odd that it seems to be showing activity, but not responding to something radioactive.

Any thoughts??

[SeanB]

Could be a tube that has lost gas fill, and this combined with old and noisy carbon composition resistors coupling the noise into the counter. Could also be a breakdown of the insulation on the tube outer that is conducting current to the centre pin.

[Leadfootin]

Try this site for more G-M tube info, some circuits near bottom of page.  https://sites.google.com/site/diygeigercounter/gm-tubes-supported

Is the case grounded or connected by a resistor, typically 1 meg? Try connecting a 5pf thru a 1 meg to ground and take a signal directly off the 1 meg. A random count should appear, effected by the Fiestaware. Large tubes exhibit fairly large background counts.

To measure tube voltage accurately add 90 meg in series with your voltmeter, check for accuracy (Fluke 97III). These tubes are extremely sensitive to drive voltage and too high may cause complete loss of sensitivity due to gas remaining ionized.

[dexters_lab]

ok, just been playing further...

i have the probe connected up to my mighty ohm counter, this normally has a SBM-20 tube which runs just under 400v if memory serves correctly

it did nothing until i tweaked the voltage up slightly and i'm getting an output around 160CPM, if i keep turning up the volts it does stop outputting as too when i drop the voltage. So the probe does seem to be behaving normally.

...yet nothing seems to affect its reading

maybe it is an insulation breakdown issue as Sean suggests, hmmm

[dexters_lab]

i took apart the probe today and found its a gamma only gm tube mounted in a srpb phenolic tube, the srpb seems to block most beta particles. This is why my fiestaware uranium source didn't do much.

the tube was also wrapped mostly in lead shielding which would block some gamma rays, which is most odd.

So i need a gamma source to test it properly, but i think the tube is working

[dexters_lab]

just noticed something interesting

i was just handling the metal shields that i found around the geiger tube and started to think, it's not heavy enough for lead!

just did some measurements, each piece is 92x95x1.5mm and weighs 79.9grams, if it were lead it would weigh more like 140grams

assuming it's not an alloy, the closest match i can see with a 2% error is Lanthanum 

[dexters_lab]

doh, probably Tin most likely