The Lost Women of the Manhattan Project

When Howes and Herzenberg called Joan Hinton to request an interview in 1990, the 69-year-old Hinton told them she had just repaired her car on a solo cross-country bird-watching trip.¹ It was perfectly characteristic of Hinton, who was chronically curious and a natural problem solver.

In 1942, Hinton graduated from Bennington College in Vermont.² For her sophomore project she had created a cloud chamber, a particle detector that visualizes the passage of ionizing radiation. Hinton also spent time at Cornell University, and when its particle accelerator broke down, Hinton eagerly helped repair it — working alongside the very physicists who later recruited her to Los Alamos.³

Hinton flourished both within physics and beyond it. She was known as a dauntless skier at the University of Wisconsin, where she earned her doctorate in 1944, and was constantly in motion. This boded well for her Manhattan Project group, which was headed by Italian physicist Enrico Fermi. Fermi encouraged his young team to play as hard as they worked, leading them on daylong hikes and ski trips.⁴

Fermi’s team built two reactors to test uranium and plutonium. The first was a low-power reactor they made from scratch. When the team first tested it, they guessed how much uranium was needed for a self-sustaining reaction. There were no formal predictions; they just guessed “for fun,” Hinton recalled. “In the spirit of the test, many of the ‘big shots’ came down” — Hinton’s team worked down a cliff from the main laboratory — “and signed their initials on the graph at the place they expected it to ‘go critical,’” the state at which a nuclear chain reaction becomes self-sustaining.⁵

The second reactor was high-powered, stronger than the first and nicknamed “the water boiler” to describe the water that cooled it. Hinton helped design and build its control rods, piled beryllium blocks around its core, and assembled electronic circuits to measure radiation.

At Los Alamos, information was distributed sparingly. When Hinton discovered details of the Trinity detonation test, she snuck to a hill 25 miles from ground zero on the back of a friend’s motor scooter. When the bomb went off just before dawn, “it was like being at the bottom of an ocean of light,” Hinton described.⁶ “Then suddenly the sound reached us. It was very sharp and rumbled and all the mountains were rubbing with it.”

Hinton’s days weren’t always filled with wonder. The detonation destroyed the 100-foot metal tower that held the bomb and turned sand to glass, called trinitite.⁷ She and some colleagues sent the trinitite to mayors, asking if they wanted their cities to suffer the same destruction.⁸ When the U.S. dropped the atomic bombs on Japan, killing hundreds of thousands of Japanese civilians, Hinton was appalled. She advocated against military control of nuclear weapons and became an outspoken peace activist.  

Two weeks after the bombing, Los Alamos physicist Harry Daghlian dropped a piece of tungsten carbide brick directly on a plutonium core.⁹ When he pushed off the radioactive brick, his hand went numb. Hinton drove him to the site hospital, where the doctors took all the metal objects out of his pockets and asked Hinton to measure their radioactivity. “They were so ‘hot’ the counters just jammed,” Hinton said.¹⁰ Within a month Daghlian was dead. For Los Alamos scientists, this event personally underscored the devastating impact of nuclear weapons. As Hinton recalled: “When 150,000 Japanese disappeared into a huge lethal mushroom of radiation, Harry’s death helped bring the lesson home.”

For two years after Los Alamos, Hinton studied at the University of Chicago with Fermi and other Manhattan Project scientists. But she was frustrated with the militarized direction physics had taken. She traveled to China in 1948, where her brother was a dairy farmer with his friend Erwin Engst. She found that designing and building farm equipment quite suited her. Hinton married Engst and they remained in China, raising their three children outside Beijing.