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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
I have one of the second type (SBM20) in my geiger counter detects Americium alpha and some other long range/penetrating (gamma?) 'rays' detects Uranium glass (junk shop glassware) etc. It's stable and seems repeatable. about 1 'click' per 2 seconds background count. don't know if that's good or poor, it's what came in my eBay Russian Geiger counter years ago. easy to modify for connection to a digital counter. the smps is poor .. acoustically & electrically noisy - doesn't seem to matter with a large plateau.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Sulaiman wrote ...
about 1 'click' per 2 seconds background count.
We must always take care to distinguish the true background level from an apparent background figure from which the tube's shielded background count has not been subtracted.
The 'shielded background' also called 'own background' is the figure obtained when the tube is placed within standardized shielding of 50mm Pb + 3mm Al.
A significant proportion of this 'own background' is made up from alphas and betas emitted by 210Pb and 210Po in the solder*
Own background figures roughly follow the sensitivity of the tube, so that low sensitivity tubes may only have a shielded background of a few cpm, while very sensitive types may have own backgrounds of several hundred cpm.
I have the subjective impression that Soviet Cold War era GM tubes were quite heavy on the solder in some designs, leading to a high spurious own background figure, which must be taken into account when working up the results.
* K. Bunzla and W. Krackea Natural radioactive contaminants in solder Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment Volume 238, Issue 1, 15 July 1985, Pages 191-192
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I used the same tube here, It worked alright and could detect radium watch hands and Americium buttons. I don't remember what I paid for the tube at the time, but I'm sure it was less than £14.47. That's pretty steep given the huge quantity. I think buying anything Geiger counter related this close to the Fukushima incident is foolish.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
Apologies for xposting from LPF, but this is interesting.
(this was after I found that my expensive vintage GM tube was defunct)
What causes the degradation?
If its as simple as corrosion of the window then surely backfilling the module with inert dry argon gas from a welding cylinder would work?
Making the module airtight might be a bit of a pain though, my thought was to UV glue a window in place halfway down the aixiz then press fit the decanned diode, solder on wires and backfill with argon. Then UV glue the back of the can to ensure no air can leak in.
As the construction of this is remarkably similar to a Geiger tube a mica or graphite end window can be substituted for the glass to make a $1 replacement for a $100 tube which fits into any old surplus case. Replace the diode with a thin metal tube i.e. from a printer ink refill kit, fill up gap with non conducting UV glue and vacuum it down then backfill with the required mixture of halogen and argon.
Sealing it off would just be a matter of a single drop of LMP alloy i.e. 70C BiSnPb with a pre-poked hole for backfilling, then once checked it would be sealed off permanently using a brief induction heater pulse.
Reckon this would work guys, I had a look at commercial GM tubes and they are constructed in much the same way.
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Really cheap geiger tubes
