I have found somewhere the following scheme for an arduino based geiger counter. (The arduino main purpose is to count the pulses from the tube) As far as I can understand it, the transistor would amplify the small current coming from the tube and the arduino would then detect a low logic level when a ionizing radiation is passing. My doubt is about the capacitor: why is it there? I have tried to build the circuit simulating the passage of radiation by pressing a button. I tried to remove the cap from the circuit too, and yet the detection is noticeably more accurate with the capacitor in. I guess it is because the capacitor is like a filter for "false pulses".Can someone please explain me the role C1?
It's there to stretch the pulses into something the arduino can see.
AVR microcontrollers can't reliably detect pulses shorter than one clock period.
the capscitor in combination with the pull-up resistor will stetch the pulses out to ballpark 100 microseconds which should be easily detectable.
C1 acts as an integrator. The current out of the geiger tube is amplified by Q1, then the collector current is integrated on C1. The current is a pretty sharp pulse of short duration so it will look like a voltage step. The voltage step then exponentially decays back up to the baseline (~5V) with the time constant C1*Rpullup (~103 ns).
The capacitor works as an output voltage stabilizer: a random short "firing" of the transistor doesn't discharge the capacitor much, and hence doesn't register as pulse.