Thread by @AtomicAnalyst

76 years ago tonight at Los Alamos, 24-year-old graduate student and Manhattan Project physicist Harry Daghlian, Jr., was conducting a risky criticality experiment alone when he accidentally dropped a 4.4 kg (9.7 lb) tungsten carbide brick on a 6.2 kg (13.7 lb) plutonium core.

The brick increased neutron reflectivity back into the core, instantly causing it to go supercritical and flood the wooden shack with radiation. Although Daghlian quickly pushed the extra brick off the core with his right hand, he still received an estimated dose of 510 rem.

Daghlian was taken to the Los Alamos hospital and his symptoms were treated. As he slowly and painfully succumbed to acute radiation poisoning, he methodically described his condition to observing doctors. He died 25 days later, the Manhattan Project's first radiation fatality.

Army Private Robert Hemmerly was on guard duty that night in the shack as Daghlian performed his last experiment, but he was seated about 12 feet away with his back turned, reading a newspaper. The 29-year-old saw a bright blue flash and received an estimated dose of 50 rem.

Although Hemmerly's white blood cell count was elevated for a few days and he felt more tired than usual for about two months, he did not suffer from radiation sickness. He fathered two more children in 1947 and 1948 and died from acute myelogenous leukemia in 1978, at age 62.

Nine months to the day after Harry Daghlian's accident, another Los Alamos physicist conducting a similar criticality experiment with the same plutonium core suffered the same fate. The so-called "demon core" was subsequently melted down and reused.

Left: Herbert Lehr and Daghlian load the assembled plutonium core of the "Gadget" into a Plymouth sedan to drive it to the Trinity shot tower on July 13, 1945.

Right: Louis Slotin (standing left) and Daghlian (seated) help to fully assemble the "Gadget" before the Trinity test.

Here's another remarkable photo of Slotin and Lehr standing in the canvas tent at the base of the shot tower next to the now uncovered tamper plug containing the 13.6-pound core of the "Gadget" prior to its insertion into the casing.