Sam Cohen invented the neutron bomb and then spent decades trying to persuade the U.S. to deploy the controversial weapon designed to kill people without damaging buildings.

Mr. Cohen, who died Sunday in Los Angeles at age 89, worked on the Manhattan Project that developed the first nuclear weapons. Later, he was a consultant at the Rand Corp. in Santa Monica, Calif., where he conceived of the neutron bomb in the early 1950s.

The...