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                     Radioactivity: Historical Figures

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   This article will focus on the efforts of four scientists: Wilhelm
   Conrad Roentgen, Antoine Henri Becquerel, Marie Sklodowska Curie, and
   Ernest Rutherford. It emphasizes their contributions to the
   elucidation of radioactivity and the "key" experiments they performed
   pertaining to their discoveries. The biographies and photographs are
   adapted from The Health Physics Society Centennial Calendar by
   permission of the Health Physics Society.


    Wilhelm Rontgen ca. 1895. Inset photo: Radiograph of Frau Rontgen's

   Wilhelm Conrad Roentgen (1845-1923)

   On November 8, 1895, at the University of Wurzburg, Wilhelm Roentgen's
   attention was drawn to a glowing fluorescent screen on a nearby table.
   Roentgen immediately determined that the fluorescence was caused by
   invisible rays originating from the partially evacuated glass
   Hittorf-Crookes tube he was using to study cathode rays (i.e.,
   electrons). Surprisingly, these mysterious rays penetrated the opaque
   black paper wrapped around the tube. Roentgen had discovered X rays, a
   momentous event that instantly revolutionized the field of physics and

   However, prior to his first formal correspondence to the University
   Physical-Medical Society, Roentgen spent two months thoroughly
   investigating the properties of X rays. Silvanus Thompson complained
   that Roentgen left "little for others to do beyond elaborating his
   work." For his discovery, Roentgen received the first Nobel Prize in
   physics in 1901. When later asked what his thoughts were at the moment
   of his discovery, he replied "I didn't think, I investigated. "It was
   the crowning achievement in a career beset by more than its share of

   As a student in Holland, Roentgen was expelled from the Utrecht
   Technical School for a prank committed by another student. Even after
   receiving a doctorate, his lack of a diploma initially prevented him
   from obtaining a position at the University of Wurzburg. He even was
   accused of having stolen the discovery of X rays by those who failed
   to observe them.

   Nevertheless, Roentgen was a brilliant experimentalist who never
   sought honors or financial profit for his research. He rejected a
   title (i.e., von Roentgen) that would have provided entry into the
   German nobility, and donated the money he received from the Nobel
   Prize to his University. Roentgen did accept the honorary degree of
   Doctor of Medicine offered to him by the medical faculty of his own
   University of Wurzburg. However, he refused to take out any patents in
   order that the world could freely benefit from his work. At the time
   of his death, Roentgen was nearly bankrupt from the inflation that
   followed World War I.



                              Henri Becquerel

   Antoine Henri Becquerel (1852-1908)

   Henri Becquerel was born into a family of scientists. His grandfather
   had made important contributions in the field of electrochemistry
   while his father had investigated the phenomena of fluorescence and
   phosphorescence. Becquerel not only inherited their interest in
   science, he also inherited the minerals and compounds studied by his
   father.And so, upon learning how Wilhelm Roentgen discovered X rays
   from the fluorescence they produced, Becquerel had a ready source of
   fluorescent materials with which to pursue his own investigations of
   these mysterious rays.

   The material Becquerel chose to work with was potassium uranyl
   sulfate, K2UO2(S)4)2, which he exposed to sunlight and placed on
   photographic plates wrapped in black paper. When developed, the plates
   revealed an image of the uranium crystals. Becquerel concluded "that
   the phosphorescent substance in question emits radiation which
   penetrates paper opaque to light." Initially he believed that the
   sun's energy was being absorbed by the uranium which then emitted X

   Further investigation, on the 26th and 27th of February, was delayed
   because the skies over Paris were overcast and the uranium-covered
   plates Becquerel intended to expose to the sun were returned to a
   drawer. On the first of March, he developed the photographic plates
   expecting only faint images to appear. To his surprise, the images
   were clear and strong. This meant that the uranium emitted radiation
   without an external source of energy such as the sun. Becquerel had
   discovered radioactivity, the spontaneous emission of radiation by a

   Later, Becquerel demonstrated that the radiation emitted by uranium
   shared certain characteristics with X rays but, unlike X rays, could
   be deflected by a magnetic field and therefore must consist of charged
   particles. For his discovery of radioactivity, Becquerel was awarded
   the 1903 Nobel Prize for physics.



   Marie Curie ca. 1920. Inset: Pierre Curie (Marie's favorite picture of
                               her husband).

   Pierre Curie (1859-1906)
   Marie Curie (1867-1934)

   By the time he met Marie Sklodowska, Pierre Curie had already
   established an impressive reputation. In 1880, he and his brother
   Jacques had discovered piezoelectricity whereby physical pressure
   applied to a crystal resulted in the creation of an electric
   potential. He also had made important investigations into the
   phenomenon of magnetism including the identification of a temperature,
   the curie point, above which a material's magnetic properties
   disappear. However, shortly after his marriage to Marie in 1895,
   Pierre subjugated his research to her interests.

   Together, they began investigating the phenomenon of radioactivity
   recently discovered in uranium ore. Although the phenomenon was
   discovered by Henri Becquerel, the term radioactivity was coined by
   Marie. After chemical extraction of uranium from the ore, Marie noted
   the residual material to be more "active" than the pure uranium. She
   concluded that the ore contained, in addition to uranium, new elements
   that were also radioactive. This led to their discoveries of the
   elements of polonium and radium, but it took four more years of
   processing tons of ore under oppressive conditions to isolate enough
   of each element to determine its chemical properties.

   For their work on radioactivity, the Curies were awarded the 1903
   Nobel Prize in physics. Tragically, Pierre was killed three years
   later in an accident while crossing a street in a rainstorm. Pierre's
   teaching position at the Sorbonne was given to Marie. Never before had
   a woman taught there in its 650 year history! Her first lecture began
   with the very sentence her husband had used to finish his last. In his
   honor, the 1910 Radiology Congress chose the curie as the basic unit
   of radioactivity: the quantity of radon in equilibrium with one gram
   of radium (current definition: 1 Ci = 3.7x1010 dps). A year later,
   Marie was awarded the Nobel Prize in chemistry for her discoveries of
   radium and polonium, thus becoming the first person to receive two
   Nobel Prizes. For the remainder of her life she tirelessly
   investigated and promoted the use if radium as a treatment for cancer.
   Marie Curie died July 4, 1934, overtaken by pernicious anemia no doubt
   caused by years of overwork and radiation exposure.



    Ernest Rutherford in his Laboratory at McGill University ca. 1903.

   Ernest Rutherford (1871-1937)

   Ernest Rutherford is considered the father of nuclear physics. Indeed,
   it could be said that Rutherford invented the very language to
   describe the theoretical concepts of the atom and the phenomenon of
   radioactivity. Particles named and characterized by him include the
   alpha particle, beta particle and proton.

   Even the neutron, discovered by James Chadwick, owes its name to
   Rutherford. The exponential equation used to calculate the decay of
   radioactive substances was first employed for that purpose by
   Rutherford and he was the first to elucidate the related concepts of
   the half-life and decay constant. With Frederick Soddy at McGill
   University, Rutherford showed that elements such as uranium and
   thorium became different elements (i.e., transmuted) through the
   process of radioactive decay. At the time, such an incredible idea was
   not to be mentioned in polite company: it belonged to the realm of
   alchemy, not science.

   For this work, Rutherford won the 1908 Nobel Prize in chemistry. In
   1909, now at the University of Manchester, Rutherford was bombarding a
   thin gold foil with alpha particles when he noticed that although
   almost all of them went through the gold, one in eight thousand would
   "bounce" (i.e., scatter) back. The amazed Rutherford commented that it
   was "as if you fired a 15-inch naval shell at a piece of tissue paper
   and the shell came right back and hit you."

   From this simple observation, Rutherford concluded that the atom's
   mass must be concentrated in a small positively-charged nucleus while
   the electrons inhabit the farthest reaches of the atom. Although this
   planetary model of the atom has been greatly refined over the years,
   it remains as valid today as when it was originally formulated by
   Rutherford. In 1919, Rutherford returned to Cambridge to become
   director of the Cavendish laboratory where he had previously done his
   graduate work under J.J. Thomson. It was here that he made his final
   major achievement, the artificial alteration of nuclear and atomic
   structure. By bombarding nitrogen with alpha particles, Rutherford
   demonstrated the production of a different element, oxygen. "Playing
   with marbles" is what he called; the newspapers reported that
   Rutherford had "split the atom." After his death in 1937, Rutherford's
   remains were buried in Westminster Abbey near those of Sir Isaac