GM tube pulse scope capture - Page 1
Forum section: electronics / projects

author: erikj
author_rank: Contributor
timestamp: December 02, 2022, 09:49:43 am
content: I did scope captures of what the signal from a Geiger-Müller tube looks like, have been looking for this myself and could not find it online so decided to post it here.The tube is a SBM-20 driven with 415V.Current is is measured over a 10k "current sense" resistor.Setup:The signal is a very fast ~2us pulse of ~200uA followed by a slower discharge that takes ~100usZoomed in:I hope someone will find this useful

author: lasmux
author_rank: Regular Contributor
timestamp: December 02, 2022, 11:02:23 am
content: Interesting, I guess this works in a similar way to a PMT or SPAD? It definitely has a similar signal response and circuit (quench resistor, sense resistor). Although it looks like the maximum count rate is only around 10kHz. I wonder if you could build an actively quenched counter like you can with SPADs...

author: RoGeorge
author_rank: Super Contributor
timestamp: December 02, 2022, 11:22:12 am
content: Quote from: lasmux on December 02, 2022, 11:02:23 amThinking about this as we speak,erikjis it possible to repeat the measurement with the tube kept 10-20*C above the room temperature?I expect a warmer tube will produce shorter pulses. If so, then the max counting frequency can be increased by placing a heating element nearby, and only power the heater when the tube is needed to count faster.

author: lasmux
author_rank: Regular Contributor
timestamp: December 02, 2022, 11:49:17 am
content: How would a hotterQuote from: RoGeorge on December 02, 2022, 11:22:12 amWhat physical mechanism would be behind that? It's an interesting thought.Edit. seems the opposite is true.link

author: erikj
author_rank: Contributor
timestamp: December 02, 2022, 12:24:40 pm
content: Quote from: lasmux on December 02, 2022, 11:02:23 amSounds about right, IIRC I have seen figures of a few kHz maximum count rate for similar tubes. Removing the high voltage and maybe shorting the tube might work for quenching, but I have no beta/gamma/x-ray source to really test this.Quote from: RoGeorge on December 02, 2022, 11:22:12 amThat is interesting, I will see if I can find a way to apply a bit of heat to the tube. I can also try cooling it in the fridge I guess.Ambient temp was around 20 deg C for this measurement.

author: lasmux
author_rank: Regular Contributor
timestamp: December 02, 2022, 12:35:40 pm
content: Careful with condensation and high voltages if you cool it...One of my projects is an actively quenched SPAD, I tried cooling it with a peltier device and had to have it in an airtight container with a strong dessicant to stop condensation causing a short.

author: David Hess
author_rank: Super Contributor
timestamp: December 02, 2022, 01:31:18 pm
content: Why are the pulse heights different?Quote from: lasmux on December 02, 2022, 11:02:23 amA GM tube operates like a SPAD but not a PMT. A PMT has a fixed linear gain which multiplies the input but a GM tube or SPAD undergo unbounded avalanche multiplication and must be quenched to reset.A GM tube or SPAD may be operated at a lower voltage for proportional counting, but with less sensitivity than a PMT.

author: erikj
author_rank: Contributor
timestamp: December 02, 2022, 01:57:21 pm
content: Quote from: David Hess on December 02, 2022, 01:31:18 pmThey are not from the exact same pulse, after some scope menu mishaps I had to take another capture for the zoomed in one.There seem to be a small variance in peak amplitude in different pulses, overall shape is very similar though.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 02, 2022, 07:09:16 pm
content: Quote from: lasmux on December 02, 2022, 11:49:17 amI was thinking a higher temperature means higher Brownian motion, so the gas molecules have higher moving speed, and thus the ionization path produced by a particle will be broken/difuzed faster, so the tube would be able to count faster.Though, it seems the outgassing is the determinant factor there, interesting link, thank you.

author: jwet
author_rank: Frequent Contributor
timestamp: December 02, 2022, 07:42:28 pm
content: As far as "active quenching"- yes, this is common in high quality and military survey equipment today. In the early 80's, there was a system called "Time to Count", where the tube was biased up by a half bridge and the time to a first count was measured. This was repeated to get good statistics. This eliminated a lot of pileup, dead time effects and increased effective dynamic range several decades over simple counters to about 6 decades. In early 90's, I and others patented a system called extended range counting which improved on the method and was able to get 8-9 decades from a single tube. This is nice as you can go from background to deadly rates. Also tube life is greatly extended in these schemes as each chemical quench shortens the life of the tube (consumes quench gas). You can dig up stuff online or from the PTO using "time to count" and "extended range counting".

author: iMo
author_rank: Super Contributor
timestamp: December 02, 2022, 07:54:23 pm
content: Quote from: jwet on December 02, 2022, 07:42:28 pmWhat does it mean "decades"? The number of counts/min, or, the tube life time in pulses?

author: KE5FX
author_rank: Super Contributor
timestamp: December 02, 2022, 08:10:26 pm
content: Looks familiar, but with a different recovery characteristic.

author: jwet
author_rank: Frequent Contributor
timestamp: December 02, 2022, 10:10:43 pm
content: "Decades" expanation.GM tubes have no trouble counting at low rates, a few counts per second. As you've seen in the scope plots, the "recovery" time is a few uS up to about 10 uS. The arrival time is also random. When you get close to the count rates that are 1/recovery time- pulses start to "pile up" as the tube is not active during quench. This is called dead time. If you keep increasing the radiation, the tube won't ever get a chance to recover and will actually show a zero count rate eventually- kind of bad and a major problem with counting GM tubes mehtods. A good meter will detect this condition and indicate full scale or over-range etc but many cheap ones don't. Another problem is that you'll wear out the tube pretty fast. A tube has a lifetime measured in total counts, it might be 10^12 total counts but its finite. The quench gas gets used up a little on each recovery.For a standard counting GM circuit, you can count from a few counts per second (background) up to something like 1/20 of the deadtime rate- about 5 Khz without things getting non linear. This is a 3.5 decade dynamic range. This is fine for many purposes but can give false low readings in high fields and forces you to have 2 meters, a background responding meter and a high range meter. Every one likes background responding meters because you can easily tell they're working. A high range meter will never read anything unless its in a high field- you can never be sure its working. It's "noise floor" is above background so it will just read its minimum level.If you use techniques like time to count, etc. that setup the bias on the tube and wait for a single count- you can avoid a of the dead time problems and get an equivalent dynamic range of 5 decades, 1 CPS to equivalent of 1e6 CPS- this is a 1 uS of average time to count- 6 "decades". Some counting statistic corrections need to be thought through to give accurate results.More advanced techniques like Extended range counting that I worked on and patented can improve this further to 9 or so decades by using some fancy statistics and electronics that operate the tube even beyond the time to count range. Watching the tube coming out of quench on a fast rising edge and looking at the statistics of a count happening in this slew time. The time is very small- about a microsecond. Its somewhat complicated- you can read the patents from the early 90's- SAIC/Leidos was the assignee and Ken Valentine(RIP) and John M. Wettroth (me) are listed as inventors on most. Dust off your fancy math texts, it does gets deep- all I can remember today is that it relied on Parseval's theorem. Have fun.The Extended Range Counting Technique (ERG) was invented under a contract for the US Navy for the Multifunction Radiac set- the current "end all be all" survey instrument in the fleet today. The idea was to give a sailor one instrument that would just read what was there from background up to rates that would kill you in an hour. All are possible on a Nuclear Warship or Sub. I haven't thought of a better way to do this since the early 90's so maybe this is it.What everyone would like to have is meter that reads properly in any field. Generally there are low rate meters like GM tube friskers that are used at low rates and high rate techniques like Ion chamber meters that measure average ion current in a non avalanching gas for high rates. Victoreen has made very low range Ion Chamber meters (455) that do a pretty good job of measuring background and are probably the closest to a perfect meter- probably 6 "decades". In general, most techniques cover a a few decade range and used together they give the full range required. It takes a fair amount of sophistication to keep all this high range/low range in mind in a Nuclear emergency. You would like to give a basic user something that "just worked"."All" sensors have some working dynamic range - its especially pernicious with radiation detectors because the rates are random and the consequence can be dire.I revised this later to correct some inconsistencies and perhaps make it clearer. A rich topic.

author: antenna
author_rank: Frequent Contributor
timestamp: December 02, 2022, 10:16:16 pm
content: Would something like this work to quench a GM tube?When the tube fires, the NPN avalanches (putting more current on the tube, I know) for just long enough for a fast RF mosfet to turn on and shunt the tube HV to ground quenching the tube. R1 and R2 probably being equal, R3 much lower than R1 and R2, and R4 just small enough to relaibly turn on the mosfet.Maybe even a small resistor in the source of the mosfet to take the pulses from. The output would be buffered and of a known low voltage.

author: jwet
author_rank: Frequent Contributor
timestamp: December 02, 2022, 10:27:37 pm
content: It would kind of work but at high rates, it couldn't recover. It quenches plenty fast but recovers with the RC of the large R's and the tube C. If counts are coming in fast, it won't ever get biased back up, it will get hit during the time that you're charging back up. Time to count uses active pull up and pull down with HV transistors. You want to get the tube back up in a microsecond or so to be useful and extend the dynamic range. Slewing 500V in a fraction of a microsecond is a challenge and why the stuff is/was patented. The quench gasses in these electronically quench tubes are also either absent or customized for electronic quenching.