On August 21, 1945, the demon core claimed its first victim. Harry K. Daghlian, Jr. was at Los Alamos, a secret laboratory in New Mexico where scientists worked feverishly on the equally secret Manhattan Project. To the outside world, at least those with sufficient security clearance, it was known simply by the mailing address – postbox 1663.

This particular night, Harry was working alone in the lab, stacking heavy blocks of tungsten carbide bricks around a core of plutonium. The bricks were to act as a neutron reflector, which would hopefully cause plutonium to reach the critical threshold at a much lower mass.

Criticality was, well, critical to making a nuclear bomb. It was that particular point when moments of nuclear fission supply the energy for more fission. At that point and beyond, fission runs away with itself and becomes self-sustaining, exuding radiation as it goes.

Just as he was about to place the final brick, intent on the delicate construction like a small child building their first block tower, the neutron counters sounded a warning. He froze. The numbers showed that if he added that one last brick, the plutonium would become supercritical. Slowly, heart pounding in his ears, he moved his hand back. As he did, the brick slipped through his fingers, fell, landed right in the center with a thud like a nail in a coffin.

What was it that made him drop that brick? Was it the beads of sweat that coated his hands as he realised what could happen? Was it that infallible fear-fulfillment that sends learning bicyclists directly to the obstacle they want to avoid? Or was it indeed a demon?

Either way, that final brick flipped the system into a critical reaction and radiation began bursting forth.

Harry panicked and tried to knock the brick off. No luck, it was a good four kilos. With a deep, shaky breath and a sour taste in his mouth, he disassembled the bricks as fast as he could until the reaction stopped.

By that time, Harry had received about 510 rem of neutron radiation, resulting from 1016 fissions. Unfortunately he became the first known fatality resulting from a criticality accident, and died less than a month later from acute radiation poisoning.

The demon core smiled.

Nine months later to the day, physicist Louis Slotin was tickling the dragon’s tail. It was an extremely high risk experiment to find the point when a sphere of plutonium, the same one which claimed Harry’s life, would become critical from the position of neutron reflectors – in this case, two half-shells of beryllium. Plutonium was placed inside one half, like a yolk inside an eggshell.

As the top half was raised and lowered, machines measured the activity from the core. Louis was showing seven other scientists how it was done.

“Lift it up and the activity is reduced,” said Louis, demonstrating. His thumb was crooked inside the thumbhole of the beryllium, allowing him to hold it rather like a bowling bowl. “Watch as I carefully lower the beryllium.” He slowly moved it down, down, down. The scintillation counters started to beep faster. Louis had done this almost a dozen times before, and had helped assemble the Trinity core, the first detonated atomic bomb. It never failed to amaze him, but he didn’t particularly like his work with bombs, and was training a replacement, Alvin Graves, to take over while he went back to biophysics.

“If you allow the beryllium spheres to close completely, the plutonium will reach critical mass,” he cautioned, brandishing a flathead screwdriver with his other hand but keeping both eyes on the core. “To get it as close as possible to that point, I have found a simple screwdriver does the job quite well.” He put the screwdriver tip between the beryllium as he lowered his hand further, until only a tiny space separated the two halves.

At that point, the unseen demon flexed. The screwdriver slipped.

The watching scientists later described a blue light and a wave of heat, and Louis tasted something sour in his mouth. Immediately, as if on reflex, he jerked his hand back to break the beryllium sphere and end the reaction. His body, crouched over as it had been during the experiment, had shielded the other scientists from most of the radiation.

At once they all left the building, Louis vomiting as soon as he was outside. He was rushed to hospital, but even numerous blood transfusions couldn’t save him.

Louis had received the equivalent radiation of someone standing one and a half kilometres from an atomic bomb blast and died nine days later in the presence of his parents.

After the accident, all future criticality testing was ‘hands off’ and scientists worked through machines like the Godiva devices.

As for the demon core, it was detonated later that year during Operation Crossroads testing at Bikini Atoll, which remains uninhabited to this day.