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 demon’s core was mass of radioactive material and it emitted radiation. Imagine a ball and it is emitting radioactive particles.
Now the experiment was that you place the ball inside a beryllium enclosure which is a radioactive particle reflector meaning if a radioactive particle hits it it gets reflected backwards. Imagine radioactive particles emitting from the ball hitting the enclosure and reflecting back to the core.
Now when the enclosure is part open and part open some radioactive particles reflect back and some pass through the opening.
The experiment was to keep reducing the opening so that more radioactive particles are reflected backwards and measure the activity of the core.
Why measure the activity of the core?
Because there were trying to measure how many radio active particles hitting back the core is required to make the core go super critical.
Supercritical meaning as more and more radioactive particles are reflected backwards and hit the core after a certain amount a chain reaction starts which causes more radioactive particles to be emitted from the core and more getting reflected backwards and in turn more getting emitted.
They were trying to measure just how much they can reduce the opening before this chain reaction starts that is the core goes super critical.
They were testing radioactive criticality, which is the point where the environmental conditions become favorable to sustain a nuclear chain reaction. In that experiment, they had a sample of radioactive material that was refined to the point of being *just barely( sub-critical (it was radioactive, but couldn’t sustain a chain reaction on its own, and was originally intended to be used in a third Nuke). The shielding around it (two hemispheres of metal separated by a small gap) was designed to reflect neutrons back into it, thus increasing the chance of making a nuclear reaction happen. As long as there was a gap, *just* enough neutrons escaped that the reaction wouldn’t run away.
Someone accidentally removed the screwdriver that had been maintaining that gap. The two halves of the shielding closed entirely, and the core went supercritical. Everyone around was given radiation poisoning.
>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.
The screwdriver was the only thing used to keep the two halves of the beryllium neutron reflector from closing over the radioactive core. The experimenters were manually adjusting the separation of the halves to test radioactivity levels, which rose and fell based on what percentage of the incident neutrons were being reflected at the core.
When the screwdriver slipped, and the two halves closed, the reaction immediately went supercritical and released a staggering amount of ionizing radiation.
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