The Ins and Outs of Geiger Counters, for Personal Reasons

There are times in one’s life when circumstances drive an intense interest in one specific topic, and we put our energy into devouring all the information we can on the subject. [The Current Source], aka [Derek], seems to be in such a situation these days, and his area of interest is radioactivity and its measurement. So with time to spare on his hands, he has worked up this video review of radioactivity and how Geiger counters work.

Why the interest in radioactivity? Bluntly put, because he is radioactive, at least for the next week. You see, [Derek] was recently diagnosed with thyroid cancer, and one of the post-thyroidectomy therapeutic options to scavenge up any stray thyroid cells is drinking a cocktail of iodine-131, a radioisotope that accumulates in thyroid cells and kills them. Trouble is, this leaves the patient dangerously radioactive, necessitating isolation for a week or more. To pass the time away from family and friends, [Derek] did a teardown on a commercial Geiger counter, the classic Ludlum Model 2 with a pancake probe. The internals of the meter are surprisingly simple, and each stage of the circuit is easily identified. He follows that up with a DIY Geiger counter kit build, which is also very simple — just a high-voltage section made from a 555 timer along with a microcontroller. He tests both instruments using himself as a source; we have to say it’s pretty alarming to hear how hot he still is. Check it out in the video below.

Given the circumstances, we’re amazed that [Derek] is not only keeping his cool but exhibiting a good sense of humor. We wish him well in his recovery, and if doing teardowns like this or projects like this freezer alarm or a no-IC bipolar power supply helps him cope, then we all win.

24 thoughts on “The Ins and Outs of Geiger Counters, for Personal Reasons

  1. Interesting thanks, would like to craft a modern detector sooner than later for beta radiation in my food lab or even for my kitchen to check if other foods than those linked below are emitting anything easy to test for. Especially some supplements from USA finding their way to Australia – weird as I understand in USA it is verboten to test dietary supplements for efficacy and even Safety since 1994 with legislation sanctioned by FDA – that is if the details are correct about legislation interpretation in terms of choice of vocabulary with possible exaggeration by documentary pundits on free to air tv etc. I think Dr Muller who is investigating this and published on SBS free to air tv network in Australia had his terms of reference correct, he has not been wrong before !

    In meantime, links to two foods that have proclivity to absorb radioactive isotopes all depending on soil constituents of course. Interesting in USA re issues with radon as it’s the product from radium decay so worth checking if any ther foods and production crops (corn?) grown in radon areas also pick up some radium…

    I’d be looking for a sensitive alpha tester too but, would need to be close range though. This hackaday’s post renewed my interest in looking more deeply into sensitive radiation detector methods, cheers.

    1. The UK Perrigo site was great about testing for radioactivity, where I don’t recall us performing in the U.S. or at any other global site, though we did heavy metals testing and I made sure to write and have an approved Standard Operating Procedure for imported raw materials that used the ICP’s and where needed ICP-MS to meet California Prop-65. The radioactive raw materials of concern were the sea based products and I forget which and what they were looking for. I think mainly was just gamma emissions and may have been from processing though I forget since didn’t work with specifically. They might have also tested beta and alpha emission also. Seems they had there own regulation and maybe for Australian products too. I was only briefed on during the initial site tour.

  2. Best wishes for a speedy recovery. While you have some time to think, why not think of how to get rid of the 555 if you are using a microcontroller for other aspects of the circuit? I have pondered building a geiger counter but have not done it yet, I have a collection of them. I have built strobes though and early on I decided if I was going to have a micro in it, I might as well have the micro do as much of the lifting as possible. Are you using the 555 to drive a transformer directly or do you have a transistor and or some isolation?

  3. Chaney Electronics introduced the SBM-20 Geiger Tubes to North America almost 10 years ago in very inexpensive circuits you can build, or buy already built. For example, the SBM-20 tube works on Beta and Gamma radiation. If you go to the hospital for nuclear testing, such as drinking the Barium Oxide solution, the SBM tube will detect that and ring at a distance of 30 feet from you.

    1. I think you meant Barium Sulphate as the oxide is an irritant and would release Ba ions in the gut which are unhealthy, the sulphate releases only negligible numbers of ions & in any case the transit time is shortened by mixing with chemicals. The barium meal is not by itself radioactive, radiation comes from the x-rays where Barium is far more opaque ie used as a contrast medium only to assess gut structure eg blockages, diverticulitis etc
      Most barium by far is stable and those that aren’t very small quantity. Your statement re 30 feet distance well outside anything I’ve known, please offer more info ???
      I’m guessing the measurement method very suspect eg nucleonic energy over sphere where radius is 30 ft would be farkin huge especially if gamma, if beta then some will absorb most over 10ft depending on isotope source and Barium is in no way in that range…

    2. The SBM-20 is not very good at detecting beta radiation, the metal walls of the tube make that very hard (most of the counts from beta is probably bremsstrahlung from the electrons crashing into the metal case :D)

      1. The metal walls are very thin in that tube and actually increase the sensitivity to most betas. Tritium betas might not be detectable, but it’s unusual to have them outside a sealed container. For ‘bremsstrahlung’, I think what you mean are secondary electrons. Most of the gamma sensitivity is going to be photoelectric effect in the metal case and the sensitivity to all but the very lowest energy would increase with thicker metal.

        1. No, I do not mean secondary electrons… Bremstrahlung (I have no idea why Americans can’t simply use the term “braking radiation”) are X-ray produced by electrons accelerating (or decelerating) enough, like “crashing” into metal…you know, the exact same way all X-ray tubes work ;-)

  4. I would love to find a source for a modular scintillation counter vs fragile glass Geiger tubes and high voltage; I imagine an optocoupler but without a led and substitute in something that scintillates when it interacts with energetic particles.
    I had a chance around the 2003 Iraq war to look at but not rip apart a sealed keychain fob sized military COTS radiation detector called Nukealert which I see is still available. It was pretty much a lithium battery, a low power microprocessor, the scintillation counter, and a piezo speaker which beeped out the threshold levels form 1-10 beeps from pretty safe to write a quick goodbye letter.

    1. A normal photodiode is way too insensitive to work with scintillators of any kind, don’t waste your time.

      search for “SiPM”…simply put, it’s a solid state alternative to PMTs (which are vacuum tubes with all their downsides) and most will happily run with slightly less then 30V.
      read this –
      pretty much all you’ll need, just needs the scintillator housing slightly modified to allow beta radiation to enter and probably play with the code to make it suit your needs.

      As for the scintillators themselves – ebay is full of cut plastic ones, those respond both to gamma/X-ray and betas. They’re cheap, durable and DIY friendly. If you have a couple of hundred to thousands of $$$ burning a hole in your pocket, you can get huge pieces of it, over 3ft long, several inches thick and wide.
      You can try to get an inorganic ones, but those tend to be in short supply and thus expensive, some don’t like water, which makes it difficult to use them.

      For alpha, you’d need a ZnS(Ag) film (again, ebay is your best bet), either on a piece of plexiglass to be alpha-only, or coupled to the plastic scintillator to get a multi-purpose detector ;-)

    2. Unless the tech has changed, and it may have, last I knew photo diodes were not near sensitive enough to pick up individual photons from the scintillation material, the best tool for the job is still a photo multiplier and you are right back with pretty much the same HV requirements.

      1. It definitely has ;-)
        “Silicon photomultipliers” (SiPM) are an array of avalanche photodiodes with a bit of passive circuitry to allow them to work in Geiger mode, all on silicon chip. They come in either 2 or 3 lead versions, so no 13+ lead devices like PMTs ;-)
        Behavior is very similar to photomultipliers (probably the reason for the naming), but operational voltage is usually below 30V.
        Being a solid-state device, it’s virtually indestructible by mechanical shock, it doesn’t care about magnetic fields, they’re small and light and depending on the application, can count insanely fast.

    3. The Nukalert is a Gadolinium OxySulfide scintillator and a CdS photoresistor. It’s not even a photodiode. The minimum detection, 100mR/hr is probably about 4000 times natural background. They talk about a patent a lot and never give a number, but I did find a good technical description. My favourite line from this isn’t technical though, it’s “Although the thresholds are printed on the label to three significant figures for ease of understanding, the threshold accuracy is generally plus double to minus one half. ”

  5. I bought one of these mightyohm kits a few years back, just for a fun build and to have in case of max max style post apocalypse situation. Real nice kit, I never exposed it to a source of beta/gamma radiation so it always beeped once every 1-2 seconds for background radiation… A nice mod to make to it is tp change the sound from a micro controller/piezo beep to a proper analogue ‘giger noise sound’. Use an inductor, amplifier circuit next to the tube, really gives that authentic disconcerting crackle when particles hits the tube… you know… for the max max situation.

  6. A few years ago, I played with one of the cheap kits that has an MCU set up to count events.The counts were very inaccurate because HV power supply collapsed when the tube started conducting. The detection circuit would count several bogus events as the HV power supply built back up. On my “real soon now” list of things to do has been to build a better power supply that won’t collapse when the tube conducts. It has not happened yet.

    1. This sounds odd. A small GM tube is only a few pf and it shorts during a pulse, a lot like a spark gap. If the voltage is right the tube may not be self quenching or the resistor is wrong. A pulse should never deplete the PSU capacitor.

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