demon core

a geiger counter is needed to balance and not cause a chain reaction
the chain reaction is accompanied by a colossal release of radiation
and for this the counter can help a lot if you set it up normally but you see that the Geiger counter slightly increases the radiation level, then it is advisable to try to set it up again so that there is no chain reaction

Vavilov-Cherenkov effect (later) When a charged particle moves at the speed of light, for example, or more than the phase velocity, this creates a blue tint

but the blue glow is not always there, it can be seen if you don't place the object on the plutonium correctly or press it too hard, which can also create a blue glow or drop something on the plutonium because with any even almost weak contact the charge can be in a critical and supercritical state
in order to stop the chain reaction, you need to lift the object that fell on the plutonium, because of this the plutonium can calm down and be in a normal state, and in order to put it down normally and not to cause a critical condition you need to have a Geiger counter if you installed it as if it were normal but the counter does not count then you need to remove it and repeat so as not to cause a chain reaction

the core that killed two people from the incident

On 21 August 1945, a young scientist, Harry Daghlian, conducted a neutron reflection experiment alone. The nucleus was placed inside a block structure made of tungsten carbide, a neutron reflector. The addition of each new 4.4 kg block (the total mass of the blocks was to be 236 kg) brought the assembly close to criticality. While attempting to install the next block, Daghlian dropped it directly onto the plutonium, which put the assembly into a supercritical state. Although the block was immediately removed, Daghlian received a lethal dose of radiation (about 5-8 Sv) and died of radiation sickness 25 days later

On May 21, 1946, physicist Louis Zlotin conducted an experiment at the Los Alamos laboratory to study the properties of nuclear fission using a neutron reflector in a subcritical assembly, while demonstrating the experiment to seven other scientists. Due to its extreme danger, the experiment was called "pulling the dragon's tail." The experimenter placed the nucleus between two beryllium hemispheres (acting as reflectors) and manually lowered the upper hemisphere onto the nucleus, holding it with his thumb over the hole in the upper part. As the hemisphere was moved up and down, sensors recorded changes in the activity of the assembly. The only preventing thing the hemispheres from closing was the tip of a flat screwdriver, which the scientist held in his right hand. Zlotin had conducted this experiment more than ten times before. Enrico Fermi had previously told him and other employees involved in the experiments that if they did not stop their dangerous experiments, they would "die within a year"

As the upper hemisphere was being lowered, Zlotin's screwdriver slipped, and the hemispheres closed, completely enclosing the plutonium. The assembly instantly went into a supercritical state, but Zlotin quickly knocked the upper hemisphere onto the floor, stopping the chain reaction and possibly saving the lives of the other people in the laboratory. Zlotin received a large dose of radiation — about 1,000 rads (mainly from fast neutrons) — and died nine days later from acute radiation sickness in the intestinal form. The others present received doses estimated in the 1940s to be 0.4–3.6 Sv.

demonic core—the name given to a subcritical mass of plutonium weighing 6.2 kg, which was involved in two accidents at the Los Alamos laboratory in 1945 and 1946. In both cases, radioactive exposure resulting from an unplanned transition to a supercritical state led to the death of scientists, after which the assembly received its name. The design consisted of two halves of plutonium spheres in the delta phase, covered with a layer of nickel, with a total mass of 6.2 kg and a density of 15.7 g/cm³.