How does a Geiger counter detect radiation, and why does it make that clicking noise?
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Lots of good comments about Geiger-muller tubes, but what about scintillation detectors?! There’s another really cool method of detecting radiation that uses special types of crystals to convert high energy photons like gamma rays and x-rays into light! So when an x-ray or a gamma ray strike the crystal it emits a short burst of light that can be detected with very sensitive detectors. One method is to use an old school device called a photo multiplier tube. These tubes are set up in a way that when a photon of light strikes it, it converts the photon into an electron and then uses some finely tuned parts and high voltage to multiply that electron. The cool thing about photo multiplier tubes is that they can detect single photons!
So what happens in a scintillation detector is: an x-ray or gamma ray strikes a scintillator crystal which then emits a photon in the visible spectrum. That photon is then detected by a photo multiplier tube as a very small pulse of electricity, and that pulse is what causes the speaker to click and the needle of the radiation meter to “jump”!
These days the photo multiplier tube can be replaced with a very sensitive photo diode which can make for much smaller radiation detectors, but Geiger tubes are still cheaper.
The coolest thing about this method of radiation detection is that you can actually use it to determine what isotope emitted the gamma ray! Gamma and x-rays can carry different levels of energy, and different elements emit gamma rays of different energies when they decay. The energy of the gamma ray will determine how bright the flash of light is when it hits the scintillation crystal, and brighter flashes will create larger pulses on the output (higher energy gamma/x rays cause the scintillator to emit more photons which makes the output of the photomultiplier bigger). So if instead of using the pulses to tick a speaker, you measure the pulses with a computer, you can determine the element that made the original gamma ray based on the size of the pulse! This is called gamma ray spectrometry.
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