In 1938, how did the scientists bombard uranium with neutrons ( what device did they set up ), and how did they know they split the uranium atom?
I know I am just one of a million asking nuclear physics questions after the release of Oppenheimer, but I am just curious
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In the 1930s they had linear accelerators that would shoot hydrogen or helium nuclei into metal foil targets. The particle collisions would split off neutrons, which could collide with uranium on the other side of the foil target.
In the specific 1938 experiment they used a mixture of radium and beryllium. Radium emits alpha particles and beryllium emits neutrons when bombarded with alpha particles.
Nowadays they use what is called neutron spallation sources (there are many great explanations of this process on the internet)
They understood Uranium was split as it was the only way to explain that they found a bunch of lighter atoms in the sample that weren’t there before.
The Hahn-Meitner (et al.) experiment basically involved putting a small mixture of radium and beryllium inside some paraffin, and putting a bit of uranium near that. Radium emits alpha particles, and when alpha particles hit beryllium, they create neutrons. The neutrons bounce off of the paraffin atoms and lose some of their energy, which increases the chance the uranium nuclei will absorb them.
So that is how they exposed the uranium to neutrons. The rest of the experiment was a lot of nuclear chemistry, which Hahn was the world expert on. Basically after exposing the uranium to neutrons, they saw that there was gamma radiation coming out of it that was of an unusual frequency. So far this was an adaptation of an experiment by Fermi some years earlier, and he said that this new radiation indicated that he had made a transuranic (heavier than uranium) element.
What Hahn wanted to do was to figure out, what exactly is giving off that new radiation? So he performed chemical reactions on the uranium that were known to remove certain chemical elements. So they removed the uranium itself, and ended up with “left over stuff” that wasn’t uranium, and this still gave off the radiation. Then they removed other elements, one by one, until they got to barium, and found that when they removed the barium, the “new radiation” went with it. So Hahn concluded that somehow by exposing uranium to neutrons, they created radioactive barium.
Which made no sense to him, because most nuclear reactions only change the nucleus of atoms by a nucleon or two, and barium is really far away from uranium on the periodic table. So he sent these results to his physicist collaborator, Lise Meitner, who by then had fled Germany because of the Nazis. And Meitner (and her nephew, Otto Frisch), looked at these results and puzzled over them, and concluded that the only way this made sense was if the uranium had been split into two pieces, of which one was barium. And that if that happened, a lot of energy would be released. Frisch came up with the name “fission” to describe this splitting.
So it is an interesting experiment — one part physics, one part chemistry. The results were unexpected; the idea that a heavy nucleus could just break apart so easily (a wimpy, low-energy neutron) was initially a little hard to believe. But soon physicists (notably Bohr and Wheeler) were able to figure out what was going on, that the heavy uranium atom was on a sort of edge of stability, and the neutron could push it over that edge.
The gym teachers puts all the uranium atoms (nerds) on one side of the gym and all the linear accelerators (jocks) on the other side and has a big net full of neutrons hanging over center court.
About knowing to split uranium, they didn’t. They were initially shooting neutrons at uranium in order to create new atoms, and discovered “splitting” atoms that way.
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