eli5 – how are neutrons “fired” ?


I recently went to see Oppenheimer. In the film they talk about neutron bombardment to split an atom.

How exactly do you “fire” neutrons? I assume its not with some sort of gun lol

Do you put some material in a confined spce and the neutrons fizz around, eventually splitting an atom?

What is done practically?

Explanations welcome, thx

In: 10

A reaction is used which produces neutrons. There are several ways to accomplish this such as unstable isotopes, fusion and neutron spallation.

In the early era of nuclear physics, neutrons were produced by bombarding beryllium metal with alpha particles produced by the decay of the radioactive element radium.

An alpha particle is the nucleus of a helium atom and beryllium is a very light element, so the impact of the alpha particle causes it to spit out a neutron. Later, researchers switched to radon as the source as it produces much more energetic alpha particles – but radon has a much shorter half life of just a few days and needs to be replaced regularly.

By the time research was going on into how uranium underwent fission and into the production of tiny samples of plutonium; the most effective way was to use one of the first particle accelerators, a cyclotron, to fire heavy hydrogen nuclei (deuterons) at beryllium to produce extremely energetic neutrons. This meant they could run experiments for hours, days and weeks – such as to produce the first microgram quantities of plutonium.

In the bombs themselves, neutrons were produced by mixing polonium – a ferocious alpha source – with beryllium in devices known as initiators at the point of detonation. The initiator for the Trinity test and the Nagasaki bomb was called ‘The Urchin’. Modern bombs use tiny particle accelerators which are ‘safer’ and more controllable.

We originally thought you couldn’t. Our first attempt to split uranium involved launching alpha particles at it (alpha particles have a positive charge, so we could repel them with another positive charge, this is how a normal particle accelerator works) but this just made plutonium.

What we do for neutrons is make a different nuclear reaction that fires neutrons, and we use that reaction’s energy to send the neutron at high speeds. We generally use a light element like lithium or beryllium, shoot alpha particles at it, and this causes it to eject a neutron. You would need to tweak the elements and alpha particle speed to get the resulting neutron energy you need. This setup is called a neutron source.

This is exactly how we start nuclear reactors. The uranium-235 fission releases 2 or 3 neutrons of its own when it splits, so the reaction is self-sustaining. We actually need to absorb some extra neutrons to avoid the reaction happening too fast. This is what the control rods are for. If left uncontrolled, you would have a meltdown in reactor grade uranium, in weapons grade uranium, this uncontrolled reaction is what makes it explode.

When I read “I assume it’s not with a gun”, I laughed because in Chernobyl, neutrons are described as the bullet there.

Wondering which part is the gun though.

It depends on what you are trying to do with the neutrons. There are several reactions that produce neutrons. For example, if an alpha particle is absorbed by beryllium, this ejects a neutron. So you could put a strong alpha emitter (like Polonium-210) next to a bunch of beryllium and they’ll start producing neutrons. In the center of the implosion bomb in the movie they have a tiny mixture of beryllium and polonium separated by a gold foil. When the implosion crushes the capsule, the beryllium and polonium mix, creating neutrons. Something similar was used in the early experiment that discovered fission — a mixture of materials that was known to produce neutrons (the Germans used a radon–beryllium source) was put near the uranium they wanted to irradiate, basically.

You could also do a version of this with a particle accelerator, which can spin lots of charged particles up to high speeds. If you accelerate a bunch of deuterium nuclei (“deuterons”) and smash them into beryllium, they’ll also produce a bunch of neutrons.

You could also make a very tiny, not very powerful nuclear fusion source. Nuclear fusion reactions generate a lot of neutrons.

If you want the neutrons you make to be mostly going in one direction, you put the neutron source in a box made of something that can absorb neutrons and then put a little hole in it, so that only neutrons going in one direction can go out of the hole.