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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 hand.


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 medicine.

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 difficulties.

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(SO4)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 rays.

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 material.

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 Newton.


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