https://en.wikipedia.org/wiki/Demon_core
In regards to the above, what was the actual purpose of using the screw driver, what were they testing?
In:
>The standard protocol was to use [shims](https://en.wikipedia.org/wiki/Shim_(spacer)) between the halves, as allowing them to close completely could result in the instantaneous formation of a critical mass and a lethal power excursion. Under Slotin’s own unapproved protocol, the shims were not used and the only thing preventing the closure was the blade of a standard straight [screwdriver](https://en.wikipedia.org/wiki/Screwdriver) manipulated in Slotin’s other hand.
A shim is just a thin wedge of some material designed to act as a spacer between two objects. In this case, they are keeping two beryllium spheres separated. Instead of using shims as designed, the experimenter used a screw-driver instead.
The purpose of keeping the spheres separated was to prevent the demon core (made of plutonium) from going critical. Critical, in terms of radioactive material, is the point where there is enough of a radioactive material in close enough proximity that the radioactive emissions are high enough to start a nuclear chain reaction. Basically the emissions cause the radioactive material to split (at an atomic level) apart releasing more radioactive emissions causing more material to split apart, etc. This cause big boom.
Why were they doing this? Basically they were trying to get the demon core as close to the critical point as possible. The purpose of the beryllium spheres is they act as neutron reflectors. “Neutrons” are part of the radioactive emissions as described above. Normally radioactive material will send neutrons in all directions. The purpose of a reflector is to bounce them back toward the radioactive material. The closer the reflectors get to the material, the more neutrons are reflected back, increasing radioactivity and bringing it closer to critical.
So they have a ball of radioactive plutonium (the demon core) that is sending neutrons in all directions spontaneously. They want to surround the plutonium with beryllium which reflects those neutrons back into the plutonium, causing more radioactive emissions. The closer the two beryllium spheres get to the plutonium, the more neutrons are reflected back and the higher the radioactivity gets. If they touch, the radioactivity reaches a point known as critical and the thing becomes a nuclear bomb.
The only thing stopping that outcome was the tip of a flat-edged screwdriver in the hands of a scientist.
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