Harry K. Daghlian, Jr

On 21 August, 1945 at 9:55 PM, young graduate student Harry K. Daghlian, Jr was working on an experiment at Los Alamos determining the critical masses of plutonium. With a 6.2 kg sphere of plutonium cradled on a table, he was placing tungsten carbide bricks around it, reflecting neutrons released from the plutonium back into it, releasing more neutrons. As he was about to place the fifth brick around the plutonium sphere, a burst of clicks from the monitors warned him the plutonium was about to achieve critical mass and he should not put this fifth brick in place.

Then he accidentally dropped it.

Partially-Reflected Plutonium Sphere Credit: United States Department of Energy

A guard in an adjoining room reported seeing a flash of light. Daghlian reported seeing his hand enveloped in a blue light as he quickly reached into the assembly to retrieve the brick. He then took the time to disassemble the assembly to a more stable configuration, and another graduate student drove him to the hospital.

Over the next 26 days, Daghilian suffered nausea, blistering along his hands and arms, dramatic weight loss, organ failure, loss of the epidermis along his arms and chest, and finally dementia before he finally passed away on September 15th 1945 at 4:30 PM.

On May 20th, 2000, a memorial was dedicated to Harry K. Daghlian, Jr in London Connecticut that reads:

A BRILLIANT SCIENTIST ON THE MANHATTAN
PROJECT. HIS WORK INVOLVED THE DETERMINATION
OF CRITICAL MASS. DURING AN EXPERIMENT GONE
AWRY, HE BECAME THE FIRST AMERICAN CASUALTY
OF THE ATOMIC AGE. THOUGH NOT IN UNIFORM,
HE DIED IN SERVICE TO HIS COUNTRY.

Further Reading:

Harry K. Daghlian, Jr.:
America’s First Peacetime Atom Bomb Fatality.

(HT BMF)