Arthur Jeffrey Dempster (August 14, 1886 – March 11, 1950) was a Canadian-American

physicist A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe. Physicists generally are interested in the root or ultimate cau ...

best known for his work in

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...

and his discovery in 1935 of the

uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

²³⁵U.

Early life and education

Dempster was born in

Toronto Toronto ( , locally pronounced or ) is the List of the largest municipalities in Canada by population, most populous city in Canada. It is the capital city of the Provinces and territories of Canada, Canadian province of Ontario. With a p ...

, Ontario, Canada. He received his

bachelor's A bachelor's degree (from Medieval Latin ''baccalaureus'') or baccalaureate (from Modern Latin ''baccalaureatus'') is an undergraduate degree awarded by colleges and universities upon completion of a course of study lasting three to six years ( ...

and

master's degree A master's degree (from Latin ) is a postgraduate academic degree awarded by universities or colleges upon completion of a course of study demonstrating mastery or a high-order overview of a specific field of study or area of professional prac ...

s at the

University of Toronto The University of Toronto (UToronto or U of T) is a public university, public research university whose main campus is located on the grounds that surround Queen's Park (Toronto), Queen's Park in Toronto, Ontario, Canada. It was founded by ...

in 1909 and 1910, respectively. After spending a semester each at Gottingen and Munich, Germany, and two years at the University of Wurzburg under Wilhelm Wien he left at the outset of

World War I World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...

for the United States; there he completed his Ph.D. in physics, graduating summa cum laude, at the

University of Chicago The University of Chicago (UChicago, Chicago, or UChi) is a Private university, private research university in Chicago, Illinois, United States. Its main campus is in the Hyde Park, Chicago, Hyde Park neighborhood on Chicago's South Side, Chic ...

originally begun in Germany under Wien.

Academic career

Dempster joined the

physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...

faculty at the University of Chicago in 1916 and remained there until his death in 1950. He co-developed a double-focusing mass spectrograph in 1936 with the help of the Americans Kenneth T. Bainbridge and J.H.E. Mattauch of Austria. This apparatus allowed the measurement of the mass of atomic nuclei. During

World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

he worked on the secret

Manhattan Project The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada. From 1942 to 1946, the ...

to develop the world's first

nuclear weapon A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission or atomic bomb) or a combination of fission and fusion reactions (thermonuclear weapon), producing a nuclear exp ...

s. Dempster used a mass spectrometer of his design, in 1935 to find uranium-235, an isotope of uranium which is lighter than uranium-238 and allowed for the production of atomic weaponry, and later energy. The quantity of uranium-235 in naturally occurring uranium is only 0.7%. Dempster was elected to the

American Philosophical Society The American Philosophical Society (APS) is an American scholarly organization and learned society founded in 1743 in Philadelphia that promotes knowledge in the humanities and natural sciences through research, professional meetings, publicat ...

in 1932 and the United States

National Academy of Sciences The National Academy of Sciences (NAS) is a United States nonprofit, NGO, non-governmental organization. NAS is part of the National Academies of Sciences, Engineering, and Medicine, along with the National Academy of Engineering (NAE) and the ...

in 1937. From 1943 to 1946, Dempster was chief physicist of the University of Chicago's Metallurgical Laboratory or "Met Lab" which integrally related to the Manhattan Project and was founded to study the materials necessary for the manufacture of atomic bombs. In 1946, he took a position as a division director at the

Argonne National Laboratory Argonne National Laboratory is a Federally funded research and development centers, federally funded research and development center in Lemont, Illinois, Lemont, Illinois, United States. Founded in 1946, the laboratory is owned by the United Sta ...

. Dempster died on March 11, 1950, in

Stuart, Florida Stuart is a city in and the county seat of Martin County, Florida, United States. Located in southeastern Florida, Stuart is the largest of five Municipal corporation, incorporated municipalities in Martin County. The population is 17,425 accordi ...

, at the age of 63 of a myocardial infarction whilst on vacation.

Research

In 1918, Dempster developed the first modern

mass spectrometer Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is us ...

, a scientific apparatus allowing physicists to identify compounds by the mass of elements in a sample, and determine the isotopic composition of elements in a sample. Dempster's mass spectrometer was over 100 times more accurate than previous versions, and established the basic theory and design of mass spectrometers that is still used to this day. Dempster's research over his career centered on the mass spectrometer and its applications, leading in 1935 to his discovery of the uranium isotope ²³⁵U. This isotope's ability to cause a rapidly expanding fission

nuclear chain reaction In nuclear physics, a nuclear chain reaction occurs when one single nuclear reaction causes an average of one or more subsequent nuclear reactions, thus leading to the possibility of a self-propagating series or "positive feedback loop" of thes ...

allowed the development of the atom bomb and

nuclear power Nuclear power is the use of nuclear reactions to produce electricity. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced by ...

. Dempster was also well known as an authority on positive rays.

References

{{DEFAULTSORT:Dempster, Arthur Jeffrey 1886 births 1950 deaths People from Old Toronto University of Toronto alumni University of Chicago alumni 20th-century American physicists American nuclear physicists Canadian physicists Canadian nuclear physicists Mass spectrometrists Manhattan Project people Fellows of the American Physical Society Members of the United States National Academy of Sciences Presidents of the American Physical Society Canadian emigrants to the United States Members of the American Philosophical Society