Dr. Rubby Sherr, 99, a Princeton University physics professor who helped develop the atomic bomb and witnessed its first test, died Monday, July 8, at the Quadrangle, a retirement community in Haverford, where he lived since 1998.

The test took place near Alamogordo, N.M., on July 16, 1945. The United States dropped the first atomic bomb in wartime, over Hiroshima, Japan, on Aug. 6, 1945.

"His major contribution," son-in-law Robert Hess said, was "the portion of the device positioned at the center of the bomb, designed to spread the nuclear chain reaction rapidly throughout the fissile plutonium material."

Dr. Sherr was the coinventor of the device, known as the Fuchs-Sherr Polonium-Beryllium neutron initiator, Hess said. Coinventor Klaus Fuchs was convicted of espionage in Britain in 1950.

When he looked outside the bunker that had protected him from the blast, Dr. Sherr told a Princeton publication, "[I] thought, 'This is the greatest scientific experiment of all time' - it was certainly the biggest.

"Then the horror sank in that the thing had actually worked, followed by relief that the atmosphere hadn't ignited, as some had feared it would."

Born in Long Branch, N.J., of Lithuanian immigrants, he graduated from Lakewood (N.J.) High School, earned a bachelor's degree in physics at New York University in 1934 and a doctorate in physics at Princeton in 1938.

In 1942, he joined the MIT-Harvard Radiation Laboratory, where he helped develop an airborne radar for detecting vehicle traffic.

"He spoke of testing the device while crammed into the rear fuselage of a small Army aircraft, flying high above a long, straight stretch of highway somewhere in New England," daughter Frances Sherr Ross said.

"The experience so terrified him," she said, "that a decade passed before he flew again."

He became an assistant professor of physics at Princeton in 1946, associate professor in 1949, and professor in 1955.

From 1955 to 1971, he was principal investigator for an Atomic Energy Commission contract that supported "experimental and theoretical research in low energy nuclear physics," making him responsible "for the operation of the Princeton 18 Mev cyclotron," his son-in-law said.

His work resulted in the Princeton AVF cyclotron in 1970, Hess said.

Since September 1936, Dr. Sherr published more than 100 articles in scientific journals.

While in New Mexico for the atomic-bomb research, "he learned fly fishing and remained an avid fisherman until the 1990s," his daughter said.

Dr. Sherr and his wife, Rita, played hosts in Princeton to folk-song artists and writers "and occasionally battled with Alan Lomax over the proper lyrics for folk songs they both knew," his daughter said. Lomax, who died in 2002, was a nationally renowned collector of such music.

Late in life he developed a fondness for growing orchids, his daughter said.

Besides daughter Frances Sherr, Mr. Sherr is survived by daughter Elizabeth Sherr Sklar and a granddaughter. His wife died in 1997.

A private life celebration has been set.

Donations may be made to the Quadrangle Board Education Fund, 3300 Darby Rd., Haverford, Pa. 19041.

Condolences may be offered to the family at www.levinefuneral.com.

Contact Walter F. Naedele at 610-313-8134, wnaedele@phillynews.com, or follow on Twitter @WNaedele.