Henri Becquerel

Henri Becquerel was born on December 15, 1852 in Paris, France. He was born into a family of scientists. His grandfather, Antoine Cesar Becquerel, invented an electrolytic method for extracting metals from their ores and his father, Alexander Edmund Becquerel, a professor of applied physics was known for his work on solar radiation and phosphorescence. Becquerel attended the Ecole Polytechnique in 1872 and the Ecole des Pontes at Chaussees from 1874 to 1877 where he studied engineering. After graduation he worked as a engineer for the Department of Bridges and Highways. In 1876 he became an assistant teacher at the Ecole Polytechnique. In 1895 he became the chair of physics. He also worked as an assistant naturalist at the Museum of Natural History. When his father died in 1891 he assumed his position as the professor of physics at Museum of Natural History.

Becquerel's research covered a number of physics topics. His first work was the rotation of polarized light rays using magnets. Next he began researching phosphorescent materials following in the footsteps of his father. He studied phosphorescent materials by exposing them to sunlight and then exposing them to photographic plates. He was conducting an experiment with uranium. First he would expose it to sunlight and them place it in with a photographic plate. He noticed that the uranium fogged the picture produced. He found he could block the fogging by inserting coins between the uranium and the photographic plate. An experiment run on February 27 and 28, 1896. The weather was overcast when he exposed his uranium salts and so he expected only a faint fogging on the developed photographic plate. Instead he was surprised to find the fogging as severe as it would be from uranium exposed to the sun. This radiation produced by the uranium did not need to be illuminated with sunlight to be produced. The new rays that were produced by the uranium were named Becquerel rays.

Becquerel determined that the rays ionized gasses and could be bent by a magnetic field, unlike x-rays which had recently been discovered by Wilhelm Rontgen. Becquerel determined that the particles emitted by the uranium salts were identical to the electrons discovered by Joseph John Thomson. Becquerel named the particles beta particles. For his discovery of spontaneous radioactivity Becquerel shared the 1903 Nobel Prize in physics with Marie and Pierre Curie. Becquerel had given the breakdown products of his uranium salts to the Curies, who discovered polonium and radium in them. Other honors won by Becquerel include election to French Academy of Sciences in 1889. He was made an officer in the Legion of Honor in 1900.

Becquerel died on August 25, 1908.


References:

Steinman, Rebecca; "Henri Bequerel"; in Biographies of Atomic Scientists

Henri Becquerel Nobel Biography

Henri Becquerel Wikipedia Entry

Lise Meitner

Born on November 7 1878 Lise Meitner played an important part in the discovery of atomic fission. She was the second woman to obtain a doctoral degree in physics from the University of Vienna.

At the time many male scientists did not approve of women in science and her career was hampered (including her being omitted from the Nobel Prize in 1944 for her work in discovering the nuclear chain reaction of atomic fission according to some historians).

Because of her Jewish ancestry (she had been baptised a Protestant in 1908) she was forced to flee Germany in 1938 (after the German annexation of Austria). In Holland she was unable to get a position so she went to Stockholm, Sweden where she got a position in Manne Siegbahn's laboratory.

Working in Sweden she corresponded with her cousin Otto Hahn who she had worked with before she fled Germany and clandestinely traveled to Copenhagen in November 1938 and after corresponded with Hahn to design a set of experiments that Hahn would later carry out that would provide evidence of nuclear fission of uranium.

Nuclear fission is the process by which the nuclei of large atoms break down into smaller nuclei. Large nuclei (with high number of protons) will break down into smaller nuclei because the large amount of electrostatic repulsion (positive protons repelling each other) overcomes the strong nuclear force which holds the nucleus together. Meitner (along with her nephew Otto Frisch) discovered why no elements larger than uranium naturally occur; the electrostatic repulsion of so many protons is so great it overcomes the strong nuclear force which holds the nuclei together. She was able to explain the large amount of heat energy produced by a nuclear chain reaction and realized that this process could be used to produce atomic weapons.

A nuclear chain reaction occurs when a one nuclear reaction causes one or more nuclear reactions to occur. For example when a uranium nucleus breaks down it releases neutrons which bombard other uranium nuclei and cause them to break down. In turn when these nuclei break down they release neutrons that cause more uranium nuclei to break down. This self perpetuating process produces thousands of times more energy than any chemical reaction. The rate at which this process occurs can be controlled by absorbing some of the neutrons and thus slowing the reaction. In nuclear power plants this is done by inserting control rods which absorb neutrons. In nuclear weapons the process is allowed to continue without moderation.

For her work discovering the nuclear fission and the nuclear chain reaction Lise Meitner is the Dead Scientist of the Week for the week of November 1-7, 2009

References:

Lise Meitner Wikipedia entry

Splitting of the Atom at sjmv.org

Lise Meitner: A Battle for Ultimate Truth at sdsc.edu

Nuclear Chain Reaction Wikipedia Entry