In 1955, Nels joined the staff at Argonne National Laboratory as a physicist. He discovered the use of ultrasound there in 1957. At the time, Nels was working on a scanner for the production volume testing of reactor fuel elements. In an experiment, he substituted his own arm for the fuel element and adjusted the sensitivity of the recording unit so that it was possible to discriminate between flesh and bone on the electrosensitive paper.

"X-ray pictures with sound waves?" a science magazine asked on Dec. 9, 1957. "Argonne National Laboratory is demonstrating it can be done, though the pictures aren't yet of standard quality. W.N. Beck of Argonne's Metallurgy Division pioneered the technique..."