The simplest of all gas-filled radiation detectors is the ionization chamber, today I will describe my first attempts to build and use this device, I will also say a bit about my further ideas regarding this subject. The device is presented on below images.
The circuit is simple, it has 2 (3 in later version) NPN transistors in Darlington configuration, the emitter is plugged to a mass, the collector and Vcc is plugged to a digital voltmeter, the base is plugged to an electrode that is in a metal can, the can is plugged to Vcc. Vcc=5V (12V in later version). Input resistance of the multimeter is 10MOhm. In later version I plugged a capacitor 220nF in parallel to the multimeter, it was used to filter noises. A can, together with the electrode makes an ionization chamber, rest of the circuit is used to gain a signal from the detector.