Louis H. Hempelmann

Louis Henry Hempelmann (1914-1993) was Director of the Health Group at the Los Alamos site of the Manhattan Project. He remained at the forefront of radiobiology research even after the war, and was a close personal friend of J. Robert Oppenheimer.



The son of a doctor, Hempelmann went to college and medical school in the city of his birth, at Washington University in St. Louis. He graduated in 1938 and specialized in internal medicine. After his internship and residency, he joined the Mallinckrodt Institute at his alma mater, which was developing a cyclotron for medical uses. While affiliated with the Institute, he received a fellowship to go to the University of California, Berkeley, which already had a cyclotron of its own. Hempelmann worked with at Berkeley with John Lawrence and met John’s brother, Ernest Lawrence. While on the fellowship, he also met Oppenheimer and soon-to-be wife, Kitty.

In early 1943, Hempelmann got a call asking him to meet with Oppenheimer. After the two met, Hempelmann agreed to join the Los Alamos site and head what would be known as the “Health Group,” a staff that dealt with radiation and safeguards and reported directly to Oppenheimer. John Lawrence had recommended him for the post. Before he arrived at the site, Hempelmann recruited a classmate from medical school, Jim Nolan, to handle the site’s general medical needs. The pair were the first physicians at Los Alamos. Hempelmann would continue to recruit graduates of Washington University to staff the Health Group. In June 1943, mere months after he started at Los Alamos, he married Elinor Pulitzer.

His initial duties involved confronting many areas of science with which he was not familiar. When he arrived, he participated in a “crash course” series of lectures on atomic science that the site’s many experts were giving to bring each other up to speed. He also had to deal with unfamiliar areas of medicine, such as diagnosing sick animals or acting as the (self-described) “world’s worst” ad hoc anesthesiologist.



Because Los Alamos started off as a weapons design site, initially little radioactive material passed through the laboratory. Hempelmann’s lab started small, with just him, a lab technician, and assistants doing blood cell counts (including Kitty Oppenheimer and later Laura Fermi). As the amount of radioactive material at Los Alamos increased, the Health Group struggled because, as Hempelmann described, the field of radiobiology at the time was “unknown” and “primitive.”

Radiobiology research was initially expected to be conducted at other Manhattan Project sites. In the Health Group’s infancy, Hempelmann visited the sites at Chicago and Berkeley to understand their research. He also visited a radium dial plant in Boston to observe the safety procedures they had in place. An accident in 1944 involving chemist Don Mastick swallowing an amount of plutonium spurred the development of research at Los Alamos. Hempelmann even encouraged, in a memo to Oppenheimer, the injection of plutonium into human subjects for experimentation. However, he later denied knowledge of the controversial human plutonium injection experiments that were eventually carried out.

The big issue for the Health Group was safe handling of plutonium. Working with different research groups individually, Hempelmann helped develop hazard reduction and tolerance standards. He also helped develop the nasal “swipe” and urine sample detection methods for plutonium. Occasionally, he also had to deal with logistics for handling incoming shipments of radioactive materials.

Hempelmann also helped plan for the Trinity Test in 1945. He headed a group examining the possibilities of fallout, and led the safety planning for the “100-Ton Shot”. While the main safety planning for Trinity was deputized to Nolan, Hempelmann took over when Nolan left on a secret mission to deliver components of the Little Boy bomb to Tinian Island. Leading up to the test, Hempelmann expressed concerns about how weather conditions could affect fallout. He also had to deal with the aftermath of the test: fallout ended up at higher-than-expected levels. This affected the health of some livestock in the area, but Hempelmann found no adverse affects on humans in the area, even after following up months later.



After the war, Manhattan Engineering District Medical Director Stafford Warren, who worked with Hempelmann throughout the war, commended his work in a memo to the director of the newly created Los Alamos National Laboratory (LANL):

He has done an exceedingly good job. Many men owe their lives to Dr. Hempelmann’s sound judgment and the practices which he instituted in a new endeavor. There are no men trained in the field nor even in industrial medicine by which to replace him if he is permitted to resign.

Hempelmann continued working at Los Alamos until 1948, and he often returned to the house near Santa Fe that he and Elinor purchased until his death. Before leaving LANL, he also helped with safety planning for Operation Crossroads in the Pacific.  

From Los Alamos he moved on to professorships at Harvard and at the University of Rochester, where he eventually chaired the Radiology Department. He also worked with the Atomic Energy Commission. Much of his research built on his work on the Manhattan Project. He published the first paper on the fatal nuclear accidents from the project, and conducted a years-long study (along with Wright Langham and, later, George Voelz) following up with several workers who had the most serious plutonium exposures. He also researched numerous other subjects, usually focusing on radiobiology in an public health context. He continued to publish well into the 1980s, and died in 1993.

Hempelmann maintained a close relationship with the Oppenheimers, which began at Berkeley but blossomed during the war. He recounted, years later, going to parties and on horse rides with “Oppie.” Their friendship continued after the war. For example, Oppenheimer’s children stayed with the Hempelmanns during their father’s security hearing, and one of them, Peter, spent part of his honeymoon at their house in Rochester. Years later, he fondly recalled Oppie’s management skills, intelligence, courteousness, and humor.


Louis H. Hempelmann's Timeline

  • 1914 Mar 5th Born in St. Louis, Missouri.
  • 1938 Graduated from Washington University in St. Louis Medical School.
  • 1943 Jan Recruited by Oppenheimer to work on the Manhattan Project.

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