Its better to describe things having diagrams so take a look below :).
The higher radioactivity is, the more pulses the device produce on the output., so it's working more or less like a GM counter.
The sensor is made from three photodiodes connected in parallel. They are polarized using four 12V batteries, the current taken the batteries is extremely low, so they will last very long.
First stage of amplification consist transimpedance amplifier, due to very low signal from photodiodes, feedback resistor of this amplifier have value of 1GOhm. Next stage is inverting amplifier.
The device have two outputs, either analog, where e.g. oscilloscope can be connected, or digital, with logic level of 12V. This digital output (high state when pulse occurs, otherwise low state) can be then collected and analysed.
Hardware was done using Kicad.
In order to store data, I used NUCLEO-F091RC with a small breadboard attached, this sub-project enabled me with possibility to watch the results in real time using a small LCD, but also send it further for later analysis. I rework now this sub-project, progress can be followed here: Hardware Data Logger
I created a simple script in R in order to plot the results. Below you can see a sample of results.
It's important to note that the device won't work without proper shielding from EMI and light, the chassis must be made from metal, and must be grounded.
I drilled a small hole where the photodiodes are and covered it with tin foil to allow easier penetration of radioactivity, otherwise thick metal chassis would block it.