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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.
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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
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.
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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.
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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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