The Secret Life of Louis Slotin
1910 - 1946

There have been few research projects shrouded in more mystery than the Manhattan Project was in the 1940's.   Dr. Louis Slotin (University of Manitoba B.Sc. 1932 and M.Sc. 1933, Ph.D. King's College, London University, 1936) was one of a select group of elite scientists invited to Los Alamos to work on the Project aimed at outrunning the Nazis' bid to create an atomic bomb.

Slotin, who specialized in triggering devices, worked quietly beside other great scientists like Oppenheimer, Teller and Fermi in Chicago, Oak Ridge, Tennessee and the desert town of Los Alamos until 1946.   Then in 1946, Slotin died a hero's death and was buried in a lead coffin in Winnipeg following a lab accident. (This is inaccurate - see below)

This is how the story goes: Slotin and the others were gathered in a laboratory performing an experiment known as "tickling the dragon's tail."   The experiment involved creating the beginning of a fission reaction by bringing together two metal hemispheres of highly reactive, beryllium-coated plutonium.   The trick was to bring the hemispheres close enough together without allowing them to touch.   But on one fateful day - May 21 1946 - after successfully "tickling the dragon's tail" dozens of times before, the hemispheres touched, generating a vast flux of radiation.

Slotin's reaction was to use his hands to separate the hemispheres.   His body shielded the others from the neutrons that emanated from the plutonium.   While the results proved fatal to him nine days later, he is credited with having saved the other seven scientists from an agonizing death.

"Slotin's personal sacrifice undoubtedly saved those who were in the room with him.   For himself, there was no hope of recovery, something he must have known at once," said Robin Connor, a professor of physics at the University of Manitoba

For further reading, see Louis Slotin And The Invisible Killer, and A Tribute to my Uncle, Dr. Louis Slotin.

These links have some very interesting additional information, photographs, and personal recollections.   Note, however, that some inaccurate statements have slipped into these documents (including the lead article above):

1. "a blue glow and a wave of heat swept through the room."
2. "They [regularly] pushed the hemispheres together till they were close enough to produce the beginning of a chain reaction, which they detected by the blue glow set up in the air around the hemispheres by Cerenkov radiation (ionisation of the air itself).   Then they pushed the hemispheres apart with the screwdrivers. "
3. "Instantly a blue glow, stronger than the spring sunshine, filled the laboratory.   There was a wave of unbearable heat.   Some present experienced a dry, prickly sensation on their tongues - a sign of excessive radiation."
4. "Then in 1946, Slotin died a hero's death and was buried in a lead coffin in Winnipeg"

Dr. M. Attas, Dr. D. Chen, J. Franta, Dr. A. Dion and others helped clarify some of these points.   Drs. Attas and Chen are experts on Cerenkov radiation, having developed a Cerenkov Viewing Device (CVD) for use in non-proliferation verification work.

• A blue glow (Cerenkov radiation) may be seen only if charged particles travel, through a transparent material, faster than the local speed of light.   Usually the only particles capable of speeds approaching the speed of light are electrons with an energy of at least 0.2 MeV.   Anything heavier (e.g. protons or alpha particles) would require enormous energy (in the order of GeV).

  The speed of light in a vacuum is 3.00x10⁸ m/s.   The speed of light in dry air is 99.97% of the speed of light in a vacuum, and this difference is usually too small for visible Cerenkov radiation to occur.   At very high fluxes of high energy electrons, however, it is conceivable that Cerenkov radiation could be measured by sensitive instruments.

  The speed of light in water, however, is 75% of the speed of light in a vacuum (that's why light refracts when passing through water, but that's another discussion).    Thus Cerenkov radiation is visible in water surrounding a strong radiation source, such as an operating reactor.   It is possible that Cerenkov radiation could be visible with a very high flux of high energy electrons in moist air.   Note, however, that Slotin was in Los Alamos (in the New Mexico desert) at the time of his death.  

  Thus there probably was no blue flash filling Slotin's laboratory, but rather a Cerenkov radiation flash may have occurred in the fluid (mainly water) within the eyeballs of those present.   The experimenters certainly did not use a blue glow to detect "the beginning of a chain reaction" as a regular experimental procedure - if they had done so, they would have induced criticality and suffered high personal doses!

• Slotin never specialized in triggering devices.   His duties at Los Alamos were confined to criticality testing, first with Otto Frisch's uranium Dragon experiments, and then with plutonium cores.

  The criticality accident that killed Slotin was performed with two hemispheres of gamma-phase plutonium coated with 5 mils of nickel.   The total core mass was 6.2 kg and the density was about 15.7 g/cm³.   The plutonium core was surrounded by two hemispherical Be shells.   The core was the same one that was involved in a criticality accident nine months earlier, on August 21, 1945, and the experimenter in that case died 28 days afterwards.

  

  In Slotin's accident (re-enacted above), the techniques involved in creating a metal critical assembly were being demonstrated to several people.   The top and final hemispherical beryllium shell was being slowly lowered into place; one edge was touching the lower beryllium hemisphere while the edge 180^o away was resting on the tip of a screwdriver.   The Be shells were normally separated by spacers / shims, but this separation did not provide an adequate increase in neutron multiplication for demonstration purposes.   Thus Slotin removed the shims and attempted to maintain the separation with a screwdriver blade - the use of a screwdriver was never part of the standard procedure.   He held the top shell with his left thumb placed in an opening at the polar point.

  The screwdriver slipped and the experiment went prompt critical - i.e. the fission rate rapidly increased due to prompt (immediately released) neutrons, and did not require the delayed neutrons that are released from some fissioned nuclei.   Slotin's left thumb was in the polar hole of the upper Be shell and, in a knee-jerk reaction, he raised his left arm and let the shell fall to the floor.

  The yield of this excursion was 3 ×10¹⁵ fissions.   The eight people in the room received doses of about 2100, 360, 250, 160, 110, 65, 47,and 37 rem.   Slotin died nine days later.

  The above details and photograph were taken from an excellent report, titled "LA-13638 - A Review of Criticality Accidents, 2000 Revision", found at http://www.orau.org/ptp/Library/accidents/la-13638.pdf.

• It was unlikely there was any "wave of heat or prickly sensation" induced by the radiation.   The heat energy produced by the criticality was small - the damage done to Slotin's body was from ionizing particles and energy.   If there was a prickly sensation, it was probably a psychological response (e.g. cold sweat, mouth going dry) because Slotin and those present understood all too well what had just happened.

• Slotin was not buried in a lead coffin.   That would imply he had been contaminated by enough radioactive material to become a hazard to others.   Slotin's coffin had a liner, which was required by law for any corpse that crossed an international border.

  People may have confused Slotin's coffin with those used for the three victims of the US army reactor (SL-1) criticality accident in 1961.   In that case all three men were heavily contaminated - the nurse who tended one of the men (before he died) received a significant dose herself,

Other tid-bits on Dr. Slotin's life include being one of the fifty one witnesses of the start-up of "Chicago Pile 1", the first man-made reactor, on December 2 1942.