let's talk about it. Dr. Louis Slotin fatally irradiated himself (and exposed a number of other persons to significant radioactive bombardment) with the "Demon Core" on this day in history, 21 May 1946. but what's the context?
the Core itself was a plutonium sphere—not a solid sphere of pure plutonium, but an assembly of two hemispheres formed out of nickel-plated plutonium alloyed with about 1% by weight of gallium, which was found experimentally to stabilize Pu metal in its least dense, most workable δ allotrope. its mass was about 6.2 kg and it was originally intended for a third atomic bomb, intended to be dropped on Japan in late August 1945. I quote Maj. Gen. Leslie Groves (from the Wikipedia article on the Demon Core:)
The next bomb of the implosion type had been scheduled to be ready for delivery on the target on the first good weather after August 24th, 1945. We have gained 4 days in manufacture and expect to ship the final components from New Mexico on August 12th or 13th. Providing there are no unforeseen difficulties in manufacture, in transportation to the theatre or after arrival in the theatre, the bomb should be ready for delivery on the first suitable weather after August 17th or 18th.
a bombing date of 19 August 1945 was set, but Japan's capitulation after the first two war crimes—er, nuclear bombings—was so swift that the planned third bomb was never assembled, and the Demon Core remained at Los Alamos.
the Core was designed to be very slightly subcritical, with a safety margin of only 5¢ below delayed criticality—which requires some explanation. in a critical assembly of fissionable material, there are two types of neutrons involved in sustaining the nuclear chain reaction. prompt neutrons are released virtually instantaneously from the fission of a nucleus. but this fission also produces unstable nuclear fragments that continue to break apart over time, releasing more neutrons in the process; these are delayed neutrons. therefore it it possible to define two thresholds in a critical system: a higher threshold of "prompt criticality" in which the quickly-released prompt neutrons are sufficient to sustain a chain reaction; and a lower threshold of "delayed criticality" in which the slower release of delayed neutrons participates in the chain reaction. the difference in reactivity between these two thresholds is assigned the value of one dollar, and you can thank Dr. Louis Slotin for the terminology. hence the Demon Core was designed to be only slightly below the lower criticality threshold; it needed to be subjected only to a modest increase in neutron flux to attain criticality.
two days after the planned third bombing of Japan, on 21 August 1945, Armenian-American physicist Haroutune "Harry" Daghlian Jr. (https://en.wikipedia.org/wiki/Harry_Daghlian) accidentally did just this: he was conducting an experiment in which he was surrounding the Demon Core with blocks of tungsten carbide, which act as a "neutron reflector", i.e. the material is effective at scattering neutrons and, when bombarded with neutrons, it sends some of them back towards the neutron source. a mass of plutonium spontaneously emits neutrons; surrounding the Pu metal with a neutron reflector, therefore, subjects the metal to greater neutron bombardment and thus increases fissile reactivity. Daghlian was monitoring neutron flux from the Demon Core while assembling the tungsten carbide blocks around it, so he knew that he was approaching the limit of criticality; but tungsten carbide is very heavy, Daghlian dropped a block at the worst possible moment, and twenty-five days later he was dead.
one other man was present—Private Robert J. Hemmerly, sitting at a desk a short distance away—and he received a sublethal dose.
and then, nine months later, on 21 May 1946, it happened again. apparently the Los Alamos scientists were still willing to fool around with hand-built, hand-monitored critical assemblies, particularly Dr. Louis Slotin—whose reckless behavior around nuclear materials caused Enrico Fermi to predict that Slotin would kill himself within a year (and Fermi turned out to be correct.) I have yet to discover myself what the purported purpose of Slotin's Demon Core experiment was; there was a small crowd of onlookers, which suggests that maybe Slotin was showing off just a little by edging the Demon Core. "see? neutron flux goes up, neutron flux goes down. neutron flux goes up, neutron flux goes down. scary huh?"
Slotin's neutron reflector was a lightweight beryllium shell with a hole in it, easily gripped with one hand as you can see in the photograph, and he could ease it down onto the Demon Core with a screwdriver, using the blade to furnish just that little bit of safe distance between the beryllium and the Pu core, presumably while keeping an eye on a neutron counter to stay on the safe side of criticality.
until the screwdriver slipped out, and the rest is history. the Demon Core was eventually melted down and the plutonium reused.
apparently...it had once been named Rufus.
~Chara of Pnictogen
