Manhattan Project Doctor Was Pioneer In Radiobiology Field

Louis Hempelmann, Manhattan Project Badge Photo. Courtesy/Historical Society

By Heather McClenahan
Los Alamos Historical Society

Imagine being assigned a job in a field that doesn’t yet exist in a town that doesn’t show on any maps. That was the experience of Dr. Louis Hempelmann, the physician appointed as the medical director for the Manhattan Project laboratory in Los Alamos.

“I was supposed to be the medical director for the laboratory, with an emphasis on radiation,” Hempelmann told Martin Sherwin, co-author of American Prometheus, the Pulitzer Prize-winning biography of Robert Oppenheimer. “I didn’t even know that there was such a thing as occupational medicine. I didn’t know anything about safety engineers, or all that sort of business. We operated in the most primitive way, really.”

A physician from St. Louis, Hempelmann first met Oppenheimer at the University of California during a fellowship to study medical uses for the products from a cyclotron. Oppenheimer asked him in early 1943 to head the “Health Group”, a staff that dealt with radiation and safeguards.

“In that first year, I really didn’t have too much to do in the laboratory,” Hempelmann told Sherwin. “I mean, there weren’t any radiation hazards yet. All the other occupational hazards were minimal.”

In fact, Kitty Oppenheimer, who had a degree in biology, worked for Hempelmann, assisting with blood counts, a way of determining radiation exposure.

“She was awful bossy,” Hempelmann said. “She went to Robert one time, not in my presence. She said she didn’t think the blood counts meant anything. She could not see why we did it.”

Hemplemann explained to Robert that, although Kitty may have been right at the time, conducting the counts was official Manhattan Project policy, and he was going to continue with it.

Kitty quit work when she became pregnant with the Oppenheimer’s second child, and Laura Fermi, wife of Nobel Prize-winning physicist Enrico Fermi, took over the blood count work. Hempelmann said she was a much more pleasant person.

As more radioactive material arrived from the processing plants in Oak Ridge, Tenn., and Hanford, Wash., Hempelmann’s workload increased. In August 1944, when chemist Don Mastick accidentally ingested some plutonium that had ejected from a sealed tube, Hempelmann had to pump the scientist’s stomach and recover the radioactive material. (Incidentally, reports in the past have stated that the 10 milligrams of plutonium in that accident were all the laboratory had on hand, but subsequent research has shown that Los Alamos had nearly 50 grams in August 1944.)

Planning for the Trinity test also kept Hempelmann busy. He told test director Kenneth Bainbridge that the medical department had to anticipate all the possible dangers to the “health of scientific personnel, residents of nearby towns, and of casuals, to provide means of detection of these dangers, and to notify proper authorities when such dangers exist. It is also necessary to obtain records which may have medico-legal bearing for future reference.”

Subsequently, Hempelmann would be in charge of reporting on the two fatal post-war radiation accidents at the laboratory, the deaths of Harry Daghlian and Louis Slotin.

Hempelmann is almost as famous for his spouse as for his work. He married Elinor Pulitzer, granddaughter of the famous newspaper magnate Joseph Pulitzer, in June 1943. In fact, already assigned to Los Alamos, Hempelmann had to get special permission to go back to St. Louis for the wedding. Elinor, heiress to a multimillion-dollar fortune, served at a secretary on the Manhattan Project.

After the war, Hempelmann worked at Harvard and then went to the University of Rochester, where he taught radiology and served as chairman of that department from 1960 until his retirement in 1979.
The Hempelmanns remained friends with the Oppenheimers and their children. They kept the children with them during Robert Oppenheimer’s security clearance hearing in 1953, and years later, Peter Oppenheimer would spend part of his honeymoon on the Hempelmann’s farm near Rochester.

While a 1993 obituary in the New York Times noted that “Dr. Hempelmann’s most renown study was of the accidental radiation exposure of workers in an uncontrolled fission reaction at Los Alamos in 1946,” colleagues Robert W. Miller and Thomas R. Koszalka wrote after his death, “he is best known for his monumental follow up studies of thyroid cancer among infants,” studies that eventually saved the lives of thousands of babies.

Find Hempelmann’s interview with Sherwin at

American Prometheus was co-authored by Kai Bird.