The discovery of radioactivity
has truly changed the world as many useful applications have been developed to
take advantage of this property of matter. Radioactivity is being used in
nuclear power plants to produce electricity, in the field of medicine to sterilize
medical instruments, and as tracers within the human body to help doctors
diagnose and treat patients.
The discovery of X-rays by the
German physicist Wilhelm Roentgen in November 1895 led to much activity in the
scientific community as this new form of energy was capable of piercing through
matter to see what was inside. X-rays could “look” inside the human body and
see the inner workings of the skeleton and organs. The French physicist Antoine
Becquerel was one of the scientists caught up in the frenzy to investigate the
properties of the new discovered X-rays. Born in 1852, Becquerel was a third-generation
physicist and had been investigating the nature of florescence and
phosphorescence. Phosphorescent minerals have the property that they absorb
light and re-emit light some time after the exciting light has been switched
off – they appear to glow in the dark!
Becquerel’s Discovery
Becquerel heard about Roentgen’s
discovery in January 1896 and began wondering if there was a connection between
the phosphorescence he had already been investigating with the newly discovered
X-rays. He wondered if any florescent material might be emitting X-rays. This
was reasonable since Roentgen discovered X-rays by the fluorescence they produced.
He began to follow up on Roentgen’s work on X-rays and started testing various
fluorescing materials to see if they also emitted X-rays. He tested various
materials for ten days without success and then decided to try a uranium salt,
uranyl potassium sulfate. This time his experiment succeeded, finding that
uranium salt emitted radiation and appeared to glow or fluoresce. He had sealed
a photographic plate in black paper, sprinkled a layer of uranium salt onto the
paper and “exposed the whole thing to sun for several hours.” When he developed
the photographic plate “I saw the silhouette of the phosphorescent substance in
black on the negative.” He mistakenly interpreted the result and thought that
sunlight activated the uranium salt to emit X-rays, much like in Roentgen’s
experiment where the X-rays flogged a photographic plate that was shielded from
light by a dark paper. Becquerel took his experiment as evidence that he was
correct and the phosphorescent uranium salts absorbed sunlight and emitted a
penetrating radiation similar to x-rays. Confident in his discovery, he
reported the results at the February 24, 1896 meeting of the French Academy of
Science.
Now serendipity steps into the
story for Paris was cloudy and overcast for the next two days which did not
allow Becquerel to repeat his experiment. Being forced to wait for a sunny day,
he put the covered photographic plate away in a dark drawer with the uranium
salt next to the plate. A few days later he decided to go ahead and develop the
photographic plate, “expecting to find the images very feeble. On the contrary,
the silhouettes appeared with great intensity. I though at ounce that the
action might be able to go on in the dark.” Apparently energetic rays were
being emitted from the uranium salt that did not require sunlight to stimulate
the emission of the energy. This was very usual as normally; a fluorescent
material requires a strong source of light such as sunlight to “charge” up the
material so that it then again fluoresce and emit the light once the sunlight
was removed. The next day, March 1, 1896, Becquerel reported at the Academy of
Sciences that the uranium salts emitted radiation without any stimulation from
sunlight. The strange new rays became known as Becquerel or uranium rays.
Figure - Image of Becquerel's photographic plate which has been fogged by exposure to radiation from a uranium salt. The shadow of a metal Cross placed between the plate and the uranium salt is clearly visible.
Becquerel began to study the
radiation emitted from the uranium salt and found it quite like X-rays, since
it penetrated matter and ionized air. The radiation came from the salt in an unending
stream and seemed to come from all directions within the material. Marie Curie
named this new phenomenon “radioactivity” – a name that would stick for
generations to come.
Figure - Henri Becquerel, Pierre Curie, and Marie Curie
The Curies and Rutherford’s Work on Radioactivity
Becquerel initially though his
new rays were similar to X-rays, but with further experimentation he was able
to show that his rays could be bent by a magnet, whereas, X-rays were not
affected by a magnet. The young graduate student in physics, Marie Curie, read
Becquerel’s paper on these new rays and decided that she would study the rays
as part of her Ph.D. thesis. She began her research immediately and with the
help of her husband, Pierre, undertook a systematic study of the strange
uranium rays. The couple devised a sensitive apparatus to measure the intensity
of the radioactivity, and soon discovered other radioactive elements: polonium,
thorium and radium.
In 1899 the British physicist
Ernest Rutherford identified two types of radiation: alpha radiation, which
produces significant ionization in the surrounding air (ionization is the
removal of one, or more, electrons from an atom, leaving it positively
charged), but can be absorbed by a single sheet of paper; and the second type
known as beta radiation produces less ionization but is capable of penetrating
a sheet of metal a few milli-meters thick. In 1900 a third type of radiation,
called gamma radiation, was discovered by Paul Villard who found that Radium
emitted an extremely penetrating electromagnetic radiation much like X-rays.
Figure - Three types of radiation emitted from a radioactive atom.
Nobel Prizes
In recognition of the work of the
Curies and Becquerel the Nobel Prize in Physics for 1903 was divided, one half
awarded to Antoine Henri Becquerel "in recognition of the extraordinary
services he has rendered by his discovery of spontaneous radioactivity",
the other half jointly to Pierre and Marie Curie, "in recognition of the
extraordinary services they have rendered by their joint researches on the
radiation phenomena discovered by Professor Henri Becquerel." Ernest
Rutherford would continue his research on radioactivity and win the 1908 Nobel Prize
in Chemistry for "for his investigations into the disintegration of the
elements, and the chemistry of radioactive substances."
References
Asimov, Isaac. Asimov’s Biographical Encyclopedia of
Science and Technology. 2nd edition. Doubleday & Company,
Inc. 1982.
Brain, Denis. The Curies: A Biography of the MostControversial Family in Science. John Wiley & Sons, Inc. 2005.
“March 1, 1896: Henri Becquerel
Discovers Radioactivity.” APS Physics. https://www.aps.org/publications/apsnews/200803/physicshistory.cfm
Accessed November 11,
2018.
“The Nobel Prize in Physics 1903.”
https://www.nobelprize.org/prizes/physics/1903/summary/ Accessed November 1, 2018.
