Americium was the fourth transuranic element to be discovered; the isotope 241Am was identified by Glenn T. Seaborg, James, Morgan, and Albert Ghiorso late in 1944 at the wartime Metallurgical Laboratory of the University of Chicago as the result of successive neutron capture reactions by plutonium isotopes in a nuclear reactor. The luster of freshly prepared americium metal is white and more silvery than plutonium or neptunium prepared in the same manner. It appears to be more malleable than uranium or neptunium and tarnishes slowly in dry air at room temperature. Americium must be handled with great care to avoid personal contamination. The alpha activity from 241Am is about three times that of radium. When gram quantities of 241Am are handled, the intense gamma activity makes exposure a serious problem. 241Am has been used as a portable source for gamma radiography. Small amounts of Americium-241 has also been used as a radioactive glass thickness gauge for the flat glass industry and as a source of ionization for smoke detectors.