Arthur Jeffery Dempster

Arthur Jeffery Dempster was born on August 14, 1886 in Toronto, Canada. His parents were James and Emily (Cheney) Dempster. As a young man he had a wide field of interest, wining multiple scholarships in different subjects. He went to the University of Toronto, earning bachelors and masters degrees in 1909 and 1910 respectively. His first scientific publication was a paper on Darwin's tidal theory.

In 1911 Dempster went to Germany where he first spent a semester each at the Universities of Munich and Gottingen, then went on to spend two years at the University of Wurzburg studying under Wilhelm Wien. Wien was studying the deflection of positive ion beams by electric and magnetic fields. Dempster began to work on a Ph.D. thesis but his studies were interupted by the outbreak of the first world war. Being a British subject, Dempster was forced to flee Wurzburg and left on the last train carrying civilians before the general mobilization. Another Canadian student who made the choice to stay spent four years in an internment camp. Dempster decided to finish his doctorate at the University of Chicago, where he finished in 1916, graduating summa cum laude.

After briefly serving in the Army during World War I and being naturalized as a U.S. citizen he returned to the faculty of the University of Chicago in 1916 and was made full professor in 1927. Dempster remained at the University of Chicago until his death in 1950. Dempster continued studying positive ion rays and using the properties of these rays in 1918 he developed the first modern mass spectrometer. In 1912 J.J. Thompson had developed a mass spectrometer which he used to show that stable elements have can have multiple isotopes, but it was Dempster who perfected it. His mass spectrometer was over 100% more accurate than Thompson's.

A mass spectrometer is an instrument that seperates chemical species by their atomic weight. A sample put into a mass spectrometer is first vaporized and then ionized (electrons are removed making positive ions). After ionization it is seperated by atomic mass by means of electric and magnetic fields. Because the heavier elements are less easy to move using electric or magnetic fields they can be seperated from lighter elements. Using this instrument in 1935 Dempster discovered uranium-235, an isotope of uranium lighter than uranium-238, which made possible atomic energy. Uranium-235 has only a 0.7% abundance in naturally ocuring uranium.

From 1943 to 1946 Dempster was the chief physicist of the University of Chicago's Metallurgical Laboratory, a laboratory specialy purposed to develop the materials neccessary for the production of atomic weapons. Honors won by Dempster include election to the National Academy of Science, a prize from the American Association for the Advancement of Science, and the Lewis Award from the American Philisophical Society.

Dempster died from a heart attack while vacationing in Florida on March 11, 1950.

References:

Allison, Samuel King; "Arthur Jeffrey Dempster: 1886-1950"; Biographical Memoirs; National Academy Press (1952)

"Arthur Jeffrey Dempster, Physicist, 63, Dead"; New York Times; March 12, 1950

Arthur Jeffery Dempster Wikipedia Article