Measuring Radioactivity – The Geiger-Muller Tube

Radioactive emissions cause ionisation when electrons are chipped off atoms, rather like bullets chipping bits off stonework. The electrons are negatively charged, so if we could sweep them up with a + voltage and measure the current they generate, we get an idea of how many electrons were chipped off by the radioactivity in a particular time. This page with animation tells you all you need to know. Here’s a picture.Image edited from Wikipedia

The radiation passes through a thin window, ionises the inert gas inside, producing an avalanche of electrons which are attracted to the central positive anode. The electron current goes through a resistor and the voltage across it is a measure of the count rate. A calibrated counter records the count rate.

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