When I was playing with arduino geiger counter, apart from of course breaking the tube, I struggled with counting the results.

On one hand it's trivial - counts (ticks) per second. However, this (of course!) can be very spiky. I ended up using pretty simple EWMA to smooth the results for user interaction. Anything really works, short decay is fine.

Then the really fun bit, was trying it with more serious radiation source, and guess what.... interrupt per tick, is... really bad! I was easily able to overwhelm the arduino, too many interrupts. Fun project to understand interrupt masking.