how does one “obtain” neutrons to fire at atomic nuclei?

Question

so, i get that atoms can be split by colliding with neutrons, but how does one get a singular neutron?

In: 2

Answer 1

There are some nuclear reactions that produce neutrons. The classic one is an alpha particle colliding with a beryllium atom, which then ejects a neutron. So to make a neutron source (or “gun”), you need to put a strong alpha emitter (like radon or polonium or radium) next to beryllium. This will generate neutrons in random directions. To make it a “gun” you could put the whole thing in a box made of something that absorbs neutrons, and then cut a little hole in it, so that only the neutrons going in the direction of the hole would get out.

In the first atomic bomb, there was a little capsule in the center of the bomb that was made out of beryllium, polonium, and a little bit of gold foil separating the two of them. Around this was some more beryllium, with little spikes cut in it, [like this](https://blog.nuclearsecrecy.com/wp-content/uploads/2015/05/Urchin.png). Prior to detonation, the gold absorbed the alpha particles from the polonium and prevented them from reaching the beryllium. When the bomb was set off, it crushed the capsule, the spikes broke the foil, and the beryllium and polonium got mixed together. This created around 100 neutrons that were used to “jump start” the bomb’s chain reaction. There likely would have been neutrons already available, even if the capsule wasn’t there, because plutonium (the nuclear fuel) has some level of “spontaneous fission” that means neutrons are always present in it. But they wanted to make sure there was a “blast” of neutrons at just the right moment, to make sure the reaction got off to a good start.