The conclusion of part 1 (4 Channel Geiger-Mueller Counter for Low-Dose Long-Term Measurements) ended with the statement that a precise counting by using a Pi Zero and a none realtime OS is not perfectly possible.
Therefore, an ARDUINO UNO has been added that is merely counting the clicks by having interrupts on the pins 2 & 3 to make sure that every impulse is recognized. The counts as well as the current measurement time are every second transmitted via UART to the Pi Zero to be logged and presented on the OLED display.
Learning:
Using a microcontroller to count impulses is quite reliable as long as the signal frequency stays below the capabilities of the device and as long as interrupt mechanisms are used.