system
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human
ELI5: How do physicists shoot beams of small particles, like electrons, protons, and neutrons?

On occasion, when reading about some experiment, say the dual slit experiment, I'll read that a beam of electrons are fired at the slits. How exactly does one do this? I'm assuming you can't just go out, buy some, and stick them in a gun.
gpt
Electrons are easy; they're the social butterflies of the subatomic realm. They flit about very promiscuously. And because they have electric charge, they're easy to manipulate with electric fields.

The general process by which electron beams are created involves passing an electric current (which is a flow of electrons, essentially) through some kind of conductor that, when it gets hot, sprays electrons out. This phenomenon is called *thermionic emission,* and it generally means that the *thermal* energy of the electrons exceeds the *binding* energy of the electrons, so they escape from whatever material you're using.

Once the electrons get sprayed out of the hot cathode, their trajectories are "bent" by the presence of electric fields. The spray is therefore shaped into a collimated beam.

Proton beams are a bit more complicated, because protons don't come flying off of hot things the way electrons do. Generally the way proton beams are created is by taking hydrogen gas — which is just protons with electrons clinging to them — and stripping off the electrons with a strong electric field. Electrons are very light particles compared to protons, so it's possible to yank them away and send the protons shooting off in the opposite direction. From there, it's the same story: Shape the spray of protons with electric fields to create a beam.

Neutrons are even more complicated. They're electrically neutral, which means we can't touch them — literally. There's no way for you to accelerate a neutron. Instead, they have to be *made* by knocking hydrogen atoms together. Most hydrogen atoms contain just a single proton and a single electron, but some — called deuterium — have a neutron in there as well. If you smack two deuterium atoms together in the right way, they glomp together to make a helium atom, and eject the extra neutron. Do this in a very precise fashion, and you can create a beam of neutrons shooting out of a piece of metal. You can't control the beam the way you can with electrons or protons, because again, neutrons have no electric charge and can't be touched. But you get a beam nonetheless, which you can point at things to make a wide variety of interesting things happen.