Re: How do neutrons accelerate in particle accelerators if they aren't charged?

Hello Tim,

You're right! Our usual particle machinery cannot accelerate neutrons and other neutral particles. However, we can create neutrons which are traveling very fast.

The best way to do this is via something called "spallation". Basically, when a high-energy proton (protons are charged, and easy to accelerate!) crashes into a nucleus, it will knock out a big spray of neutrons, protons, nuclear fragments, pions, muons, etc., all mixed together and traveling forward (in the same direction as the original proton) at high speed. (The verb "to spall" means "to split or chip; to detach small pieces")

Spallation isn't very useful unless you can sort out the neutrons from the rest of the spray. Fortunately this isn't too difficult. To sort out charged particles, pass the spallation products through a large magnetic field; charged particles will be deflected. You don't need to worry about other neutral particles (pions, lambdas, whatever) since they tend to decay within millimeters of the collision. The only thing in your "beam" a few meters away are the neutrons.

A neutron beam, at a neutron spallation facility like the Los Alamos Neutron Science Center, consists of exactly this: the collected neutron leftovers, from a batch of billions of high-energy protons smashing into a thin piece of metal. There are some interesting subtleties: most interestingly, you might notice that a "batch" of neutrons in this beam will contain neutrons in a wide range of energies. The experimenter can figure out the energy for a particular neutron by timing it. The first neutrons to arrive (timed from the well-known moment of the proton bunch impact) are the fastest and highest-energy; the last to arrive are the slowest and thus lowest-energy. (There's no way to generate a monoenergetic beam; you just generate a "broadband" one and pick out the data from the arrival-time-window of the neutron-energy you are interested in.) There are also interesting issues with the choice of target, with steering the beam (which I think is marginally possible with diffraction off crystals), and so on.

I described something similar in this answer, whose question asked how to create neutrino beams.

And I should mention that there's another very low-tech way to generate neutron beams of lower energies: drill a hole the shielding of a nuclear reactor core. The fission process continually boils off neutrons, and you will find yourself with more of them than you know what to do with. Research reactors like the one at Oak Ridge are built expressly as a source for this sort of neutron beam.

Good question! Check out the national lab Web pages above for more details.

-Ben