Conundrum wrote ...

Also, if you need to build a Geiger counter in a hurry, I figured out that it is possible to improvise one using one of those mini door siren alarms and the GM tube.
Voltage stability isn't brilliant but I used two 180V zeners in series to even it out.

Then use a high gain transistor connected base to GM cathode and emitter to Gnd, collector to low voltage circuit via LED.

There's a way to make one using an old monochrome CMOS camera and a piece of mica painted with silver demister paint.
Lightproof the rest of the camera casing and replace the crystal with a 4 MHz one to increase sensitivity.
I also found that putting an infrared LED on the back of the board near the sensor fools the onboard AGC into behaving itself.