That was very understandable even for me lol Thanks. But what is the material? Plutonium? why does a certain amount more cause critical mass? thanks in advance. What an interesting subject
“But what is the material? Plutonium?”
I’m not sure, it depends on what kind of bomb they were building.
“why does a certain amount more cause critical mass?”
It’s just the nature of radioactive material. Nuclear radiation is actually the particles giving off mass and energy, as they are decaying into a different kind of atom. A radioactive element is inherently unstable, and the way they achieve stability is to shed off little bits of themselves until they are a different element that is stable.
Now, normally that radiation just shoots off in all directions and is gone. When you put enough radioactive material together in just the right configuration though, some of that radiation will be absorbed by the other nearby radioactive atoms. When that happens, they can’t simply hold on to that additional mass an energy, because they are too unstable. So they have to “re-emit” the radiation that struck them, along with the radiation that they would be already be emitting.
In the right configuration, that creates a chain reaction, one ray hits an atom, that atom emits two more rays that hit two more atoms, which then emit four rays, that hit four atoms, etc, and it just continues until all the material that can keep the reaction going is used up.
Uranium was used for Little Boy on Hiroshima.
Plutonium was used for Fat Man on Nagasaki.
Both incidents described above resulting in deaths were on a plutonium core. Critical mass was not achieve by creating more plutonium above subcritical. It was achieved by reflecting more neutrons into the plutonium core.
The first incident used tungsten carbide bricks to reflect neutrons that would have normally escaped back into the plutonium.
The second incident used beryllium half spheres, also to reflect.
In both incidents, the material dropped was the neutron reflecting material into direct contact. The amount of neutrons reflected back into the core greatly.
You could think of it as a mirror near a light, the amount of reflected light increases as the mirror approaches.
More neutrons reflected back into the core versus being cast away from the core decrease the amount of mass needed in the core to sustain fission reaction.