[Swlug] DIY Geiger counter

Fri Mar 7 20:57:24 UTC 2025

P.s. to my previous email: The "AM-like" subcircuit that I was talking about is known as an "envelope detector", and it is used in simple AM demodulation, not even "-like", it is exactly that, diode--cap--resistor (see attached schematic from Wikipedia).
https://en.wikipedia.org/wiki/Envelope_detector 

At Z+0000=2025-03-07Fri17:00:58, Rhys Sage via Swlug sent:
> Thanks for everybody's responses so far. I think what I'm going to have to do is to experiment.
> 
> Reading up on radiation, it seems that the emissions are rapid enough that during any particular sampling session. Using an ATTiny13 with its 1mhz clock speed, the samples should contain enough readings to represent an accurate radiation count.
> 
> The interesting thing is that using the ADC on the ATTiny13 I might get not just a frequency count but a strength count. I''ll have to experiment.

    For a particular particle at a particular frequency, there is a direct relation between the frequency count of particles/events and the amount of energy or strength that that corresponds to.  In other words, each photon of light of a particular wavelength has an energy, often measured in electron-volts.  I think beta and alpha varies depending on the "speed" of the electron or helium nucleus but a particular decay reaction produces particles of the same speed/energy.  So if you are able to detect a frequency count and also a strength count, then you might be able to read something into whether you are dealing with alpha, beta, or gamma by looking at the ratios between these 2 counts.

> What I'm aiming at is a handheld Geiger counter that can give a good indication of the radiation count in whichever of the count methods I eventually employ. I had thought of using Americium 241 from a smoke detector as my test sample. The problem with that though is that plastic blocks the alpha emissions. My detector crystal is in a plastic housing so probably no alpha reception.

    That's a good point.  So does just about anything -- paper is sufficient to block most of an alpha source, iirc., as is skin, and of course anything thicker.  It can travel through air, but only really short distances.  The Geiger counter sensors that I remember from school Physics, housed in light-blue plastic, had a fine mesh on the end.  In hindsight, this must have been to enable alpha detection, because gamma would not have needed the air holes, and I don't think beta would need them either.

> I did find out that the Americium 241 is an extremely hazardous substance which could be fatal even in the small quantity in a smoke detector.

    Yes it is -- it is extremely dangerous if it gets into your blood stream because blood vessels walls are also very good at absorbing alpha radiation, so it eats at your capillaries from the inside.  Causes a lot of internal bleeding if ingested or inhaled in sufficient quantity, and it does not take a lot to do this apparently.  Very nasty.

> I don't think 'er indoors has seen an episode of Criminal Minds in which it's used so I'm still safe for the moment.

    Not seen that but there's an episode of The Simpsons / Itchy & Scratchy that always springs to mind when I think of the effects of alpha radiation on capillaries.

> I was going to use a pair of BC547s, a capacitor and a resistor as the amplifier for the BPX61. Then I read around and the OPA134PA amplifier was recommended as producing less electronic noise though I can't imagine what noise a simple setup of BC547s would produce.

    Were you going to use the BJTs in emitter-follower formation?  I was going to suggest that as being potentially faster than an audio opamp, but then thought better of it.  While it would be true that if all transistors and their connections were the same, fewer transistors would equate to quicker response and better bandwidth, that's not the case when comparing discretes with an IC, unfortunately, because the lead length and loop area between the leads becomes the limiting factor for the discretes.  Opamps have several transistors, but they are tiny and within micrometres of each other, maybe even nanometres depending on the generation of the technology.  So I'm not sure whether 2 discretes might be better or worse than a single good opamp.  It will depend how short you can get the leads of the discretes, as well as minimising the loop area of any changing current paths that the signal invokes.

    It also depends how good the discrete BJTs are in terms of bandwidth.  I tend to use discretes if I find an opamp is not needed, but I can't yet remember how the various discretes compare.  I have to look it up each time.

    Even the diodes that I suggested, at least the 1st stage one need to be a so-called "fast" diode, maybe like the ones used for ESD protection (TVS?).  I think you can get TVS caps as well.  But as always, it's worth seeing whether what you have works before looking for dearer components.

> I do have an oscilloscope though I'm not brilliant at using it due to the lack of information in the Chinese instruction leaflet. 
> 
> There are a couple of other ideas I have for the scintillation detector. Once I have the handheld unit working, perhaps a LORA unit though the websites seem to indicate nobody uses LORAWAN nearby. I could also tie it with some of my other ideas/projects. I have the kit needed to use solar power to generate hydrogen from water and I have built a shortwave CW transmitter. Those could be tied together to make a hydrogen balloon that carries a small solar panel, radio transmitter and scintillation sensor then float around the world broadcasting radiation readings, annoying radio hams the world over but providing useful information. I'm not sure how leak proof or practical the big lawn and leaf bags would be as hydrogen balloons but that's all just future thoughts.

    Hydrogen is the leakiest gas, being just a proton and an electron.  I recently read that it how sealed-lead-acid batteries eventually "wear-out" even charged correctly -- it's because the temporary hydrogen that builds-up during charging is able to slowly leak through the solid plastic housing, even if no venting occurred.  The rate of charge is limited by how fast the recombination catalyst can work to recombine the hydrogen back into the electrolyte, but even if you are within the designated current, there is a tiny amount of "wear" occurring each charge just due to the fact that a tiny amount of this temporary hydrogen is able to escape through the solid plastic, and then the lack of hydrogen creates a chemical imbalance that allows excessive sulfation.  (I also determined that my mum's car battery was not damaged for being too low voltage, as long as it was charged with limited current rather than fixed voltage (e.g. a car alternator would have killed it very quickly).)

    Given that a helium-filled rubber balloon leaks to the point of losing buoyancy after a few days, and helium is less leaky than hydrogen, my guess is that it would be difficult to find a material that contains the hydrogen for long enough to get around the world.  Airships did it about a century ago but they had more volume per balloon material area, and it was a much thicker material, and they were powered so could move a lot quicker across oceans.

    Might be a really rubbish idea, but if you have solar panels on board...  Any chance it would be possible to collect a tiny amount of rain and do electrolysis on it just enough to sustain the balloon's pressure?  I thinking that given that batteries increase in pressure while they are being charged, it's not unreasonable to question whether it might be possible to /sustain/ the pressure in the balloon using a trickle of electrolysis each day.  But it would not be possible to ditch the heavier oxygen without losing pressure (and without the extra weight and complexity of a pump).  It would be simpler to collect both the hydrogen and oxygen into the balloon (ignoring the fire hazard for now) which I think is still lighter than air, so would have some buoyancy.  Oh, but problem, how would rain get in if under pressure of the balloon?  Probably can't avoid a pump.  Oh, also, the hydrogen will leak faster than the oxygen, meaning that it will get heavier over time and probably sink.

    Hmm, but if there was some can of tiny pump then you could ditch the oxygen and collect a few drops of rain, and it would not take a lot of water to make enough hydrogen to keep the balloon going.  I can see it being possible to make such a balloon going for as long as something crashes into it or the pump wears-out, but it would not be easy.  How do they do those high-altitude weather balloons?  Do they rely on helium?  They look expensive, so it wouldn't surprise me if they use huge amounts of helium.

    Does it even make sense to detect the amount of radioactivity in the air that you are moving with?  Aren't you just going to get the same reading over and over again, pretty much?

    Speaking of ideas/projects -- do you know much about weather radar?  That's something that I would like to do at some point but I have no idea how.

Kind regards,
James.
-- 
Wealth doesn't bring happiness, but poverty brings sadness.
Sent from Debian with Claws Mail, using email subaddressing as an alternative to error-prone heuristical spam filtering.
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