The simplest of all gas-filled radiation detectors is the ionization chamber - it's filled with regular air (usually it isn't hermetic) and don't need high voltage to operate. Today I will describe my first attempts to build and test this device, I will also say a bit about my further ideas in this area.
The circuit is simple, a can together with an electrode makes an ionization chamber, rest of the circuit is used to gain the signal. It contains 2 (3 in later version) NPN transistors in Darlington configuration, the emitter is connected to the mass, the collector and Vcc is connected to a digital voltmeter, the base is connected 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 to filter noises.
In articles I've found on the Internet, a radioactive material was used to prove that the device works. Such materials can be bought normally because theirs radiation is really small and safe. My goal was different, I wasn't interested in checking if radioactive samples are radioactive because obviously that's true, I wanted to check if regular things that can be found at home have different level of radioactivity.
It doesn't work! My version of ionization chamber doesn't work with such small levels of radioactivity. I have been trying with different samples including: ground, natural crystal of salt, my hand, vegetables that I found in the kitchen and all other stuff that I could imagine. The results have been always the same.
I will make a different sensor from a burned-out bulb and metal foil, this will solve a problem of vapor in the air (the vapor decreases resistance between electrodes). I will also try to use TL072 instead of the transistors, this could increase sensitivity. Probably it would be good to use FET transistors, but I've always broken them during soldering (I'm not good at ESD) and I don't have them now.
EDIT: I tried with the TL072, results were the same.
I will share my further adventures with ionization chamber in next posts.