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Understanding Current Signals Induced by Drifting Electrons

Consider an electron drifting in a gas toward a collection electrode. A common misconception is that the electron produces a detectable signal only upon arrival at the electrode. In fact, the situation is quite the opposite. The electron induces a detectable current in the electrode as soon as it starts moving through the gas. This induced current vanishes when the electron arrives at the plate. To illustrate this phenomenon experimentally, we use a gas-filled parallel plate ionization chamber and a collimated $^{241}$Am alpha source, which produces a track of a fixed number of ionization electrons at a constant distance from the collection electrode. We find that the detected signal from the ionization chamber grows with the electron drift distance, as predicted by the model of charge induction, and in conflict with the idea that electrons are detectable upon arrival at the collection plate.