[Swlug] DIY Geiger counter

[Alan Gray]

I have no particular expertise, but this what I found in a quick trawl. It sounds an interesting project.

Although info is out there, much is ambiguous. The crystal appears to be sensitive to alpha, beta and gamma radiation. As you might expect  a strike. causes a sharp rise in output with a decay that depends on what hit it. 

I've seen decays mentioned of under 20nS that I assume refer to gamma. Elsewhere it was said particles gave delays of 680nS and 3340nS depending on type. My assumption is that a heavy alpha particle ploughs into the crystal atoms causing a cascade of particles giving the longest decay period. The lighter beta does much the same, but causes a lighter cascade whilst the gamma gives a minimal cascade. 

In all cases the decay is followed by a longish (5ms+) of low level "afterglow". 

Many comment that the crystal is hygroscopic i.e. absorbs water. Be aware also the non thallium doped version is noted as being temperature sensitive and works best when cooled. 

The op amp seems designed for audio so I am not sure how it will fare with a pretty much instantaneous rise and 20nS decay. I suspect you may need to rethink the early stages.


Regards 
Alan Gray 



On 6 March 2025 16:33:23 GMT, Rhys Sage via Swlug <swlug at mailman.lug.org.uk> wrote:
>I'm in the middle of a project - to build a Geiger counter. It's a fun project so far.
>
>I have all the bits I need (I think) but as with all projects there's a lot of experimentation and changes made on the fly. 
>
>I'm using a Thalium-doped Cesium Iodide crystal in combination with a BPX61 photo diode. For those that don't know- the BPX61 is slightly more sensitive to green light. The crystal emits blue/green light when struck by gamma radiation. Whether it emits light when struck by Alpha, Beta or X-Ray, I don't know. I do know there are other crystals more suited to those.
>
>The BPX61 output will feed into an OPA134PA amplifier and then that will in turn feed into a microcontroller. Given that the events generated by radioactive decay are very brief, would microcontroller speed make much difference? I could use an ATTiny13/ATTiny85 or something small like a micro RP2040 or an ESP32 C3 Super Mini.
>
>With a regular Geiger counter,does the system store the events somehow or does it just rely upon measuring maximum pulses when a pulse happens during a sampling period?
>
>Rhys Sage
>
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