“Kai” is in the process of building a Geiger counter based on BPW34 photodiode and explains his first step for the sensor circuit.
The common approach to protect the photodiodes from light is to use one layer of tin foil and connect it to ground. This should also protect the circuit against electromagnetic radiation. I’ve started with something different and dipped the diodes three times into liquid rubber (Plasti Dip). My hope was to at least allow some beta particles to reach the semiconductor material.
Building a PIN Diode Geiger Counter – [Link]