25 Days of Particles: Day 3

Neutron

Classification: hadron, baryon, fermion
Fundamental: no
Mass: 939.6 MeV = 1.67e-27 kg
Interactions: Weak, Nuclear/Strong, Gravity (all due to constituents)
Spin: 1/2

Paired up with the protons in the atomic nucleus is the neutron. It has about the same mass as a proton, but lacks any electric charge. This makes the neutron effectively invisible to most research techniques. Physicists like to use electric and magnetic fields to pull particles out of the atoms they come in and to move the freed particles around, but only electrically charged particles pay any attention to these fields. The neutral ones continue on their way until they smash into enough stuff to loose all their energy or until they trip over something they can interact with via the weak force. While you can use both of these effects to "find" neutrons, neutrons are more able to ignore these than charged particles can ignore electric fields.

This made discovering the neutron more difficult than discovering either the electron or proton. With the discovery of the proton and successful measurement of its charge and mass, a new puzzle popped up in describing what was in the atomic nucleus. Typically, an atom with atomic number X contains X protons and X electrons, but the mass of that nucleus corresponds to the mass of 2X protons. So everyone knew there had to be some additional mass in the nucleus that wasn't contributing to the overall charge. Rutherford, after discovering the proton, postulated that there could be some particles without electric charge in the nucleus, but couldn't prove it because he couldn't detect such particles. The other prevailing theory was that there were additional protons and electrons in the nucleus that were canceling out each other's electric charge.

But that led to another problem for the atomic researchers of the time. Particles have spin, and when you combine them into atoms, atoms have the spin that comes from vector-adding the component spins, and atomic can be measured. The nitrogen-14 nucleus had a measured spin of 1, which must be the net spin of all its constituent particles. But if that nucleus contains 7 protons producing the correct charge and another 7 protons plus 7 electrons to produce the right mass, that gives you 21 spin-1/2 particles. Spins can only be either up or down, and there is no way to vector-add 21 halves to get one.



But the people studying radiation at the time had started to see that they could get something to fly out of nuclei that wasn't charged but could carry a fair bit of energy along with it. Over the course of several years, conflicting theories about what this stuff was battled back and forth, but by the early 1930s came around, it had been proven that nuclei couldn't contain electrons and that the neutral stuff was a particle. It was dubbed the neutron, and the spin issue was solved (you can add 14 halves to get one).

Now, particle physics doesn't do much directly with neutrons. We can't play with them like we play with protons and electrons, and to make life more complicated, they aren't stable once you pry them out of the nucleus. Neutrons left to their own devices last on average about fifteen minutes before decaying, or breaking down into smaller and lighter particles. The neutron prefers to decay into a proton, an electron, and another particle we haven't met yet, in a process called beta decay. So neutrons don't get used for particle physics experiments much.

They matter a whole lot for nuclear experiments, though. Neutrons are major players in producing the chain of fission reactions necessary for nuclear reactors. They are also used in radiation treatments of cancerous tumors. Sometimes the quiet ones can have the biggest impact.