My first revision of this project was just a single PCB. To protect the sensitive detector from electromagnetic radiation I wrapped it in tape: First Black electrical tape to block light and then Aluminum tape to shield it. A hole was left in the aluminum tape as it would also block most of the beta radiation.

To simplify the assembly process and make it prettier my second revision is made with a stack of four PCBs. The stack of PCBs acts as a fielding cage around the analog section. By painting the edges black and covering the openings with black tape all light can be blocked. Unfortunately this also blocks parts of the nice front side. I'm planning to redesign one of the middle PCBs to put the tape layer below the top to make it look nicer.