ModeronNeutron Moderation / Elastic ScatteringSticker

Moderon is a pool hustler, a proton with almost exactly the neutron’s mass, and he waits on the table.

A neutron fresh from fission flies at about 2 MeV. To keep a chain reaction going in a thermal reactor it has to be slowed to about 0.025 eV, roughly 2200 m/s, where uranium-235 catches it far more readily. The neutron has no charge, so the only way to slow it is to let it bump into nuclei.

The physics is the pool table. When a moving ball hits a resting ball of the same mass head-on, it stops dead and the other rolls off with all its energy. That is the first line on this page with A equal to one. Glancing hits take less; on average a collision with hydrogen takes away half the energy, so getting from 2 MeV to thermal takes about 18 hits. Heavier targets do worse. Deuterium needs about 25, carbon about 115, and uranium-238 about 2172, because a light ball bouncing off a heavy one keeps almost all its speed. That is why reactors use water and graphite.

In October 1934 Enrico Fermi’s group in Rome found that neutrons made their samples far more radioactive after passing through paraffin wax, which is rich in hydrogen. Fermi realised that the hydrogen was slowing the neutrons, and that slow neutrons are captured more easily. Edoardo Amaldi, Oscar D’Agostino, Fermi, Bruno Pontecorvo, Franco Rasetti and Emilio Segrè published the work in 1935, and Fermi received the 1938 Nobel Prize.

So Moderon doesn’t shoot. The neutron rolls in, and the hustle is nothing but conservation of momentum and energy.

Same mass. You stop. I roll.