Robert T. Young, 73, a physicist who was the chief consultant at the Harry Diamond Laboratories in Adelphi for 21 years until he retired in 1969, died Dec. 8 at a nursing home in Austin, Tex. He had cerebellar ataxia, a neurological disorder.

Dr. Young was a native of Grand Forks, N.D., and a graduate of the University of Montana. He earned his doctorate at Harvard University. He moved from Chevy Chase to New Hampshire in 1975 and to Austin in 1977.

From 1935 until 1946, he was a teacher at the Worcester Polytechnic Institute in Worcester, Mass., and a research associate at the Scripps Institute of Oceanography in La Jolla, Calif., and the Massachusetts Institute of Technology in Cambridge, Mass.

Dr. Young moved to Washington in 1946 and was a research associate for the Naval Research Laboratory and the National Bureau of Standards. He joined the old Advanced Group on Electron Devices, now the Harry Diamond Laboratories, at the Department of Defense in 1948.

He was a member of the American Physical Society and the Washington Academy of Sciences.

Survivors include his wife, Elvira Ogden Young of Austin; two daughters, Ellen O. Young of Dallas and Lucy F. Young of Washington; two sons, Robert O., of Norristown, Pa., and Richard N., of Windsor, Conn., and three grandchildren.