did i break my gieger tube? - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: September 03, 2017, 07:43:50 am
content: I have a SI 3 BG Geiger tube, which is apparently only reactive to high rad levels, which I did not know.I hooked it up directly to a 10mA power supply, and played with various voltages from 400-800 volts.It glowed red once (like a neon), and on 500V it sparks occasionally.I finally managed to decyper the markings, and I found its rated to 450V and its recommended to use with a 10Meg limiting resistor. I was taking 'measurement' by having it drag down the power supply and watching the spike on the oscilloscope.Is it still good? How hard are these things to destroy? I thought it was sparking due to background radiation when I ran it at high power levels.. it gets more sensitive right?I assume that the heavy discharges might have pitted the surface, so there is less of a gap in certain places (since the cratering effect tends to raise material around the lip of the crater).Has anyone managed to destroy one of these?http://www.rhelectronics.net/store/si3bg-geiger-muller-counter-tube.htmlDon't wanna spend the money on another one if I dont' have to.

author: Gyro
author_rank: Super Contributor
timestamp: September 03, 2017, 09:20:13 am
content: Hard to give a definitive answer but I would go with 'Yes you probably did', they are easy to damage.The life of GM tubes is normally specified in 'counts'. The life is determined by the quenching agent - either a halogen or organic compound which ensures that the avlanche discharge terminates when it detects a particle. That's when it is fed from the appropriate source impedance - importantly, both high resistance and low parasitic capacitance. The quenching agent eventully becomes 'used up' and the tube characteristics become unusable.It's hard to guess what you will have done to the chemical composition of the gas filling by putting it into uncontrolled discharge but it probably isn't good. The best advice is to put it into the correct circuit configuration (including proper current sensitive pulse detection to see when it is counting) and see if it still does anything.P.S. Their 'example circuit is lousy, I hope they don't test them that way for long! The correct way is to put the 10M resistor in the anode supply and use a current pulse detector (a darlington pair of small signal transistors would do) in the cathode circuit. The detector senses the current pulse as the internal capacitance GM tube recharges through the 10M resistor after a particle detection.

author: Fraser
author_rank: Super Contributor
timestamp: September 03, 2017, 04:36:51 pm
content: Having spoken to a chap who services Mini-Monitor GM based Geiger counters and survey meters, I can confirm that neglecting to fit the current limiting resistor causes an overload of the GM tube and destroys it very quickly indeed.Sadly what you did to that tube is highly likely to have destroyed it.I suggest you search for a SBM-20 or STS-5 tube on eBay. The STS-5 is usually quite cheap and it is a GM tube for everyday use. Expect to pay around $15 delivered for one.Fraser

author: CopperCone
author_rank: Super Contributor
timestamp: September 03, 2017, 09:22:02 pm
content: i hooked up the tube to the proper resistor, and I found a gamma ray standard laying around.It appears to still work. I am getting approximately no counts with the tube in air (its not supposed to be a sensitive tube) and I am getting a ~2 (AVG) -5 (record high) counts per second with a gamma ray standard.If I increase the bias past 400 volts, I get more counts.Is 'overvolting' gm tubes an acceptable way of getting higher sensitivity in exchange for life?You can really get alot of background counts out of it when its at a high voltage. Whats responsible for this?

author: Gyro
author_rank: Super Contributor
timestamp: September 03, 2017, 09:48:43 pm
content: Ah, that's something then. It is still capable of detecting something, but note that it's characteristics may have changed from the design parameters.All gm tubes have a 'plateau' region where their count rate remains stable and actually related to the amount of radiation that it is being exposed to. For a 400V tube this is probably about 50V long (the tube datasheet would specify this) but it should be centred about the specified nominal voltage, maybe 375 to 425V for example.Below the plateau there isn't enough voltage for the tube to support avalanche discharge in response to radiation (no counts). Above the plateau region the tube starts spontaniously discharging (ultimately it would reach the continuous discharge that you caused by having no resistor).If the plateau region no longer exists (ie. you can't find a stable count rate voltage range, as above) then the tube is beyond use due to either anode damage or depletion of the quenching agent.Basically once you get out of the stable plateau region you're not increasing the sensitivity, you're just getting lots of false counts (and damage).Try a google search, there is lots of information on GM tube characteristics and theory of operation.

author: Rbastler
author_rank: Frequent Contributor
timestamp: September 04, 2017, 07:06:02 am
content: I can recommend the SI22G and SBM19 Geiger tubes. They are cheap and quite sensitive. I build my Geiger counter around them. Very happy. The only drawback is, that they are relatively large.Sent from my A0001 using Tapatalk