Charged Particle Scattering – By Sophia Pettitt-Kenney

Particle scattering is a phenomenon that happens all the time, and all around us, though we can’t see it. Essentially, particle scattering is what happens when a moving particle (or multiple particles) hits an obstacle in its path and “scatters” in another direction. If we imagine what’s going on in a nuclear reactor, it’s obvious we’re going to be seeing a lot of particle scattering. Simply put, when we run the reactor we’re putting gas into a vacuum chamber and bombarding it with a lot of energy in the form of high-voltage electricity. The particles inside the reactor are going to be zooming around inside the vacuum chamber bouncing off the walls and into each other.

There are lots of different types of scattering phenomena, but in this post, we’re only going to look at scattering phenomena of charge particles. Because we’re talking about charged particles here, it may be helpful to check out Kobi’s blog post on the Coulomb force if you’re unfamiliar with basic electromagnetic theory.

Probably the most well-known example of charged particle scattering is Rutherford scattering, sometimes called Coulomb scattering. Rutherford scattering occurs when alpha particles, which are positively charged, come very close to the nucleus of an atom, which is made up of positively charged protons and neutral neutrons. When it comes to Coulomb force opposites attract, so the positive alpha particle is repelled by the also-positive nucleus of the atom. This results in a deflection of the alpha particle’s trajectory which can be seen in the figures below. The amount to which an alpha particle scatters depends on a few factors including how much charge is carried by the atomic nucleus, how close the alpha particle gets to the nucleus and whether or not they actually collide, and where the negatively charged electrons in relation to the alpha particle and the nucleus. Discovered in the early 1900s, Rutherford scattering is what shifted the scientific thinking about atoms from the “plum pudding” model to the “planetary” model. Initially, scientists thought atoms were a sphere of positive charges with negative charges sprinkled in, kind of like chunks of plums in plum pudding. The discovery of Rutherford scattering made it clear that this could not be the case and that there had to be a high concentration of positive charge in the center of an atom with the negative charges orbiting around it, kind of like the solar system. This is because the degree to which the trajectories alpha particles shifted when they encountered the atoms they collided with was so great that it could not have been achieved with the weaker, less concentrated positive charge of a “plum pudding” atom, but it could have happened had the particles gotten near nuclei as we understand them today.

Another form of charged particle scattering is Compton scattering. Compton scattering involves a photon and a charged particle, it most commonly occurs with x-ray and gamma-ray photons and electrons, however. A photon is a light particle that has no charge and an electron carries a negative charge. When a photon is scattered by an electron, its direction changes and some of its energy is transferred to the electron it hits. Sometimes, the electron (or other charged particle) transfers some of its energy to the photon, this is called inverse Compton scattering. The discovery of Compton scattering was a huge breakthrough because it is a great practical example of the quantization of light. Normally when we think of light, we think about waves, but if light is purely waves and not particles, Compton scattering cannot be explained, but when we expand our idea of light to think about it like a particle, the result of Compton scattering seems quite obvious.

Compton scattering is inelastic, meaning that the kinetic energy of the particles is not conserved. However, at really low energy limits, we get Thomson scattering which is similar to Compton scattering except that the kinetic energy of the particles is unchanged, so it is elastic. Some inverse Compton scattering also falls under the umbrella of Thomson scattering. Thomson scattering can be very useful for measuring solar radiation, plasma in fusers, and in x-ray technology. Check back in later for Kiana’s post that will take a deeper dive into x-rays. It’s possible that we may see all kinds of particle scattering in our reactor but the most common by far will be neutron scattering. As mentioned before, neutrons are neutral particles which is why we didn’t discuss them too much in this post. However, it’s worth mentioning since we’re going to be dealing with it quite a bit. Neutron scattering, like other types of scattering, also happens naturally, but it will happen at a much much higher rate within the reactor.