Robert Loevinger, 89, who, in his 20 years as chief of the National Institute of Standards and Technology's dosimetry section, ensured that medical radiation therapy was safe and accurate, died Nov. 6 while visiting his daughter in San Diego. He had Alzheimer's disease.

Dr. Loevinger, a veteran of the Manhattan Project, was known in his field as "Mr. Dosimetry" for his career efforts to make radiation treatments for millions of cancer patients reliable. Dosimetry is the science of measuring doses of radiation.

He outlined, using a new X-ray measurement in the early 1980s, how to calculate proper cancer-patient dosages in radiation therapy. He also figured out how to calibrate X-ray machines in hospitals around the world and established a national network of radiation-therapy calibration laboratories to keep the machines accurate.

"The few good things that I have done have been pencil-and-paper and slide-rule accomplishments, with very limited help from computers or other modern technology," Dr. Loevinger said in 1995, upon receiving the William D. Coolidge Award from the American Association of Physicists in Medicine. "I am not inveighing against technology -- I am not a modern Luddite -- but I am saying that my limited skills are obsolete. I assume that present and future generations will be more au courant than I have been."

Despite that humble statement, virtually all radiation therapies in the United States today are traceable for their accuracy to the measurement and calibration programs led by Dr. Loevinger, according to NIST.

Born in St. Paul, Minn., he graduated from the University of Minnesota and received a master's degree in astronomy from Harvard University in 1938 and a doctoral degree in physics from the University of California at Berkeley in 1947.

During World War II, he worked at Berkeley on the Manhattan Project, the government's effort to develop a nuclear weapon. His job was to take high-speed motion pictures of the first atomic bomb test in Los Alamos, N.M. In Hollywood, he processed aerial films taken of the Hiroshima explosion. He later joined scientific organizations that promoted the peaceful use of nuclear technology.

Dr. Loevinger started his career in medical physics at Mount Sinai Hospital in New York, followed by stints at the Royal Marsden Hospital in London, Stanford University Medical Center in California and the International Atomic Energy Agency in Vienna, Austria, where he was dosimetry section chief and set up a series of standard dosimetry laboratories around the world.

In 1968, he joined the National Bureau of Standards (now NIST) as dosimetry group chief. He was responsible for dosimetry standards, research and new calibration services, as well as coordination and comparison with foreign calibration laboratories. He officially retired in 1988 but worked there for another four years.

He was awarded the Department of Commerce Silver Medal in 1980, the Federal Laboratory Consortium Award for Excellence in Technology Transfer in 1985, the Health Physics Society Distinguished Scientific Achievement Award in 1993 and the Farrington Daniels Award from the American Association of Physicists in Medicine.

Dr. Loevinger lived in Rockville for more than 30 years before moving to Gaithersburg and, last year, to Silver Spring. A dedicated family man, he cared for three nieces after his sister died and attended to in-laws in nursing homes. He loved classical music but became hearing-impaired early in life and could not continue to play his cello.

His wife of 52 years, Ruth Sonja Schimmel Loevinger, died in 2004.

Survivors include three children, Nancy Loevinger of San Diego, David Loevinger of Silver Spring and Neil Loevinger of Newton Center, Mass.; and three grandchildren.