Geiger–Müller Tubes is a technology that has been embraced by many physicists and educators since the early 20th century when measuring or detectining Ionising Radiation.
Despite the evolution of alternative Ionising radiation detection methods such as photomultipliers or SiPM coupled with a scintillator. A Geiger–Müller tube persists as the most economical and accessible technology for Do-It-Yourself (DIY) radiation detector projects.
A Geiger–Müller tube (GMT) typically functions at a high voltage around 400V to 600V DC or even higher, depending on the manufacturer. There are many low-cost versions available, such as surplus ex-Soviet models (as below) and other more modern manufacturers, most of which operate within the range of 350 to 450V. Despite this elevated voltage, these tubes consume very little power, operating in the microamp region since they operate in series with a high-value resistor typically around 10 Megaohms.
Setting the correct voltage though is crucial when operating a Geiger–Müller Tube (GMT). When an ionizing particle passes through the GMT, it ionizes the gas inside, causing the resistance between the Anode and Cathode to decrease rapidly before returning to its initial state. This phenomenon occurs within a specific voltage range. If the voltage is too low, ionization will not occur; conversely, if the voltage is too high, the ionization may not be properly quenched, potentially damaging the tube and compromising accuracy. Therefore, maintaining the appropriate regulated voltage is essential for ensuring optimal performance and accuracy when used with multiple GMT.
Pulse Shapping
Another critical aspect of a Geiger–Müller tube circuit involves the conversion of the pulse generated by a Geiger–Müller tube (GMT) down to a usable level used by the logic circuit for counting. At the juncture where the high-value resistor ~10M and GMT connect in series, a voltage drop will occur with regards to ground. At this point a low-value capacitor ~20pF is coupled to the input of an Inverting Schmitt Trigger which has been bias High (1) with another high value resistor ~100K at the logic circuits supply voltage +3.3V meaning the output of the Schmitt Trigger will be Low (0). When the negative pulse of the GMT is seen by the Schmitt Trigger it will register as Low (0) and and the output will be High (1), for the duration negative traveling transition.
