In the experiment that was performed that initially split the atom, the explanation goes that you take a neutron and fire it into a heavy, unstable element like U-238 which splits into more neutrons and releases some energy etc etc etc.
We’ve all heard it.
However it seems to miss the part around how you obtain the initial neutrons. As far as I’m aware they don’t occur naturally on their own, so how did they obtain enough of them to split the atom in the experiment?
(I’m aware that for the bomb they combine too lumps of U238 to create a super critical mass, however I’m more interested in the initial experiment)
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When detonated, the bomb design makes sure there is a “super critical” mass of uranium or plutonium. This means getting the “fuel” together, compact enough, so that if a few neutrons gets things going, you would have an atomic explosion.
There is a chance that a random decay of the uranium (or plutonium) could liberate a neutron, or there could be a stray comic ray, but bomb makes don’t like to take chances. So they make sure there are plenty of neutrons at the exact right moment.
What the original bomb makers did was put an “initiator” at the center of the where the super critical mass will be (center of plutonium implosion), or where gun will fire uranium piece at target.
The initiator is a beryllium pellet, with a beryllium shell and polonium between the two. They do NOT touch, there is a tiny gap between the polonium and beryllium. Polonium is radioactive and continuously makes a lot of alpha particles (really high speed helium atoms). When the alpha particles hit the beryllium, they knock neutrons out that will start the reaction. Having a tiny gap, normally no neutrons are being made.
The initiator is set up so that when the bomb is detonated and the “fuel” (uranium or plutonium) are smashed, that crushes the initiator and mixes the beryllium and polonium to generate a lot of neutrons.
Today, this is done with a tiny particle accelerator tube (there are several types).
**tl;dr there are ways to geneate neutrons, on demand, at the exact right moment when the uranium or plutonium is super critical.**
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