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Today in History: Louis Slotin succumbs to demon core radiation

5/21/2026

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Summary

Story summary

On May 21 1946, Manhattan Project physicist Louis Slotin was conducting a criticality test on the 6.2 kg plutonium-gallium “demon core.” A slip in the experiment caused the sphere to go prompt critical, releasing a burst of ionizing radiation that delivered a lethal dose to Slotin, marking the second recorded criticality accident.

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Story summary

On May 21 1946, Manhattan Project physicist Louis Slotin was conducting a criticality test on the 6.2 kg plutonium-gallium “demon core.” A slip in the experiment caused the sphere to go prompt critical, releasing a burst of ionizing radiation that delivered a lethal dose to Slotin, marking the second recorded criticality accident.