How does a Geiger counter detect radiation, and why does it make that clicking noise?
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Certain kinds of radiation can knock the electrons off atoms, turning them into ions (charged particles). This can turn a gas that can’t conduct electricity into ions that can.
Geiger counters exploit this…they setup a tube of low pressure gas with a really high electrical voltage across the gas. The gas is normally an insulator (doesn’t conduct electricity), but if radiation comes through it ionizes the gas so that it becomes conductive and electricity can flow. That creates a big electric pulse that’s easy for the electronics in the counter to measure.
It’s also really simple to connect that pulse signal to a speaker. And the sound of a short electrical pulse through a speaker is…a click.
So the clicks are literally the electrical pulses released by each radiation particle zipping through the counter. It’s a simple, visceral, and effective way to tell the operator what’s going on.
When ionizing radiation strikes air, it knocks an electron free. Under high voltage, this electron will go flying and strike more electrons off of more atoms, causing an avalanche.
This avalanche allows an electrical arc to form in air. This arc is hooked up to a speaker and an arrestor, so it makes a click and then lets the air settle back to its normal state to await the next event.
Geiger counter detects ionizing radiation, which is radiation that can sufficient energy to detach the electron from an atom it hit. The way that works is to exploit that phenomenon.
The sensor tube is filled with an inert gas at low pressure so that the radiation can ionize. It also has a cathode and anode with a high voltage over them. The negatively charged free electrons will accelerate toward the positive anode because of electrostatic attraction. The positively charged ionized gas atom will move to a negatively charged charge.
When this happens the electron that is accelerated will hit other gas atom and ionizes them to. This is an avalanche effect, a form of amplification that makes it possible to get a detectable current. The principle is called https://en.wikipedia.org/wiki/Townsend_discharge
The ionized atoms will get electrons from the cathode and the electrons that hit the cathode will be absorbed by it. This is fundamentally the same as a current pulse that flows through the tube. The detector circuit counts the number of electric pulses.
The sound is this electric signal used to drive a speaker, It is a way to get auditory feedback of how much radiation it detects without needing to look at it all the time. This is typically something portable Geiger counter used because typical usage is to move them around so you can detect something radioactive and for a rough estimate listening to the sound is enough.
Looks like this has already been answered, but if you want slightly more information, [SciShow answered your question](https://youtu.be/qtvz8lH5zhk) a few years back.
The tube has gas inside it, and a small high voltage capacitor in series with the speaker. Normally, the gas is an insulator, no current flows. If you were to crank up the high voltage a lot higher, it actually could break down the gas in the tube, but that’s not what we want to do.
Radiation will make a track of ionized gas that IS conductive. If the track only covers half the gap, then it will require half the voltage to break down and conduct, and there may be enough voltage present to do that.
Once the gas breaks down and conducts, the capacitor discharges and the current makes a “tick” on the speaker due to a sudden *change* in current. The capacitor will discharge faster that the battery and high voltage inverter will recharge it.
But once the capacitor discharges far enough, it doesn’t have enough voltage to keep the gas ionized, and the gas de-ionizes and becomes an insulator again, current flow stops, which allows the capacitor to recharge and the speaker diaphragm falls back to the rest position.
Here’s a scary fact though- if you hit the tube with an obscene amount of radiation, it may actually STOP ticking! It can keep multiple overlapping conductive tracks present constantly, so the capacitor discharges and makes a tick ONCE, but cannot recover and recharge for another tick because the tube just becomes constantly conducting for an indefinite period of time. So it deceptively stops cycling and goes quiet.
… *too* quiet!
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.
Sub question: is it true Geiger counters need to be built with metal from shipwrecks pre 1945?
Heard that the background radiation from nuclear bombs caused contamination, so every bit of new metal is verrrry slightly radioactive, but it messes with the readings.
What is that mysterious ticking noise?
You know those bug zappers that look like a badminton racquet? That’s a good analogy to a geiger-mueller detector. There is a high voltage between two screens. When a bug gets between them, there is no longer enough insulating distance and a spark flies between the two screens, zapping the bug.
With a geiger counter, an ionized particle causes that high voltage discharge instead of a mosquito. The click is just a convenient way to know that audibly indicate that the discharge has occurred. The faster the discharges/clicks, the more radiation there is.
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