While researching the inner workings of Geiger-Müller counters, I stumbled upon Rad Pro.
Introduction to Malware Binary Triage (IMBT) Course
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It’s an open-source project developing firmware for cheap GM meters.

You get yourself a GM meter and replace its firmware with Rad Pro.
For some meters (like FNIRSI), it’s as simple as mounting the meter as a USB storage device and copying the Rad Pro firmware to it. Others require a programmer (like Bosean).
Article Link: Quickpost: Rad Pro | Didier Stevens