U.S. National Bureau of Standards (NBS) low-temperature research group. The group was headed by US physicist Ferdinand Graft Brickwedde (1903-1989, centre) from 1928 to 1946. In 1932 this research group was the first in the US to liquefy helium gas. Helium has to be cooled to a temperature of minus 269 degrees Celsius to become a liquid. In 1931, working with Harold Urey and others, Brickwedde helped produced the hydrogen samples used to observe its heavy isotope, deuterium. This discovery resulted in the 1934 Nobel Prize in Chemistry for Urey.