Download Benchmarks: Henri Becquerel discovers radioactivity on February

Benchmarks: Henri Becquerel discovers radioactivity on
February 26, 1896
February 26, 1896, was an overcast day in Paris — and that presented a problem for French physicist
Antoine Henri Becquerel. Becquerel was hoping to demonstrate a link between minerals that glow when
exposed to strong light and a new type of electromagnetic radiation called X-rays. The weather thwarted
this experiment — but that failure inadvertently produced an entirely new discovery: natural radioactivity.
Becquerel was interested in the phenomenon of fluorescence, in which some materials glow when
exposed to sunlight. Physicist Wilhelm Röntgen had recently discovered X-rays; Becquerel thought the
two phenomena might be connected, and had designed an experiment of his own. He planned to expose a
fluorescing material to the sun, and then place it and a metal object over an unexposed photographic plate.
If the developed plate showed the image of the object, he concluded, that would suggest that fluorescing
materials are actually emitting X-rays.
But the next day was cloudy as well, and Becquerel was forced to
postpone his experiment. He wrapped his fluorescing crystals — a
uranium compound called potassium uranyl sulfate — in a black cloth,
along with the photographic plate and a copper Maltese cross, and
waited for a sunnier day.
Several days later, when Becquerel finally removed the plate from the
drawer, he discovered to his surprise that a distinct image of the cross
appeared on the plate — although it had never been exposed to sunlight.
The only conclusion was that the crystals themselves were emitting
radiation. Excited by this prospect, Becquerel decided to repeat the conditions of his unintentional
experiment: He again placed a crystal of uranium salt on a photographic plate; he also experimented with
putting a crystal on a photographic plate with a sheet of aluminum between, and with a sheet of glass.
After being placed in the dark for several hours, all three plates were blackened by radiation (the crystal
in direct contact with the plate showed the strongest blackening). “I am now convinced that uranium salts
produce invisible radiation, even when they have been kept in the dark,” he wrote in his diary of his
experiments.
This discovery of spontaneous “radioactivity” (a term coined by Becquerel’s doctoral student, Marie
Curie) eventually earned Becquerel a Nobel Prize for Physics in 1903, which he shared with Marie Curie
and her husband Pierre Curie.
Becquerel came from a family of scientists: His grandfather, Antoine César Becquerel, had discovered
piezoelectricity (the electrical charge that accumulates in crystals and other materials as a result of applied
mechanical strain). His father, Alexandre-Edmond Becquerel, had invented the phosphoroscope, a device
that measures how long a phosphorescent material continues to glow after removing the source of light.
Becquerel spent a lot of time in his father’s laboratory, and he was initially interested almost exclusively
in optics. When he became a research physicist, he embarked on his own study of the radiation of light:
He explored how magnetic fields polarized light, how infrared light produced phosphorescence in some
materials and how crystals absorb light. Upon his father’s death in 1891, Becquerel succeeded to his
father’s two chairs, one a Chair of Physics at the Conservatoire National des Arts et Métiers and the other
a Chair of Physics at the Muséum National d’Histoire Naturelle, both in Paris.
His research took a new turn when he attended a lecture on X-rays at the Académie des Sciences in Paris.
In January 1896, the French mathematician Jules-Henri Poincaré had received a letter from Röntgen,
which contained several surprising photographs that showed the outline of bones within a hand. In the
letter, Röntgen explained that the images had been taken with a new discovery, the X-ray. Poincaré was
astonished, and reproduced the images himself. Poincaré presented his own images at the Académie two
weeks later, to enthusiastic response.
Becquerel was in the audience that day, and wondered whether there was any connection between the
ghostly X-ray images and the phenomena of fluorescence and phosphorescence that he and his father had
studied. Becquerel had already studied the phosphorescence of uranium salts in particular and was
familiar with photography, so he decided to undertake his own experiments on the subject of X-rays.
On Feb. 24, 1896, Becquerel presented his initial results to the Académie des Sciences: His
phosphorescing uranium salts, after exposure to sunlight, had left faint images on several photographic
plates. But the smudgy images were far less intriguing than the sharp X-ray images shown a few weeks
earlier, and Becquerel resolved to try again. He prepared new arrays of crystals and photographic plates,
and decided that he needed very strong sunlight to produce the best images.
But nature didn’t cooperate; Becquerel didn’t get his sunny day. Still eager to show something to the
Académie, he took the plates and the crystals out of his drawer. He expected to see more of the same faint
images, but was startled to find instead crisp silhouettes of his metal objects, including the Maltese cross.
Stimulation of the crystals by sunlight before or during the experiment, it seemed, was not necessary to
produce the images — suggesting that the crystals themselves were emitting radiation, without external
stimulation. On March 1, 1896, Becquerel presented the discovery of spontaneous radioactivity to the
Académie.
The discovery of spontaneous radioactivity spread rapidly and engendered a flurry of new research on the
phenomenon, much of it by Marie and Pierre Curie. Becquerel also continued to study the phenomenon:
In 1899, he discovered that X-rays could be deflected by a magnetic field, suggesting that the radiation
contained electrically charged particles. The international unit of radioactivity, the becquerel (defined as
one nucleus decay per second), was named for him. But Becquerel was still fascinated by the interaction
between crystals and light, and he eventually returned to this research, studying how crystals absorb and
polarize light.
Meanwhile, Marie Curie took on the study of uranium rays for her thesis research. While studying the
uranium-bearing minerals pitchblende and chalcolite, she discovered that in addition to uranium, other
elements emitted the “Becquerel rays”: thorium and a powerfully radioactive element that Curie dubbed
“radium.”
The discovery of radioactivity had profound impacts on chemistry and physics at the time. The powerful
radiation, including heat, spontaneously emitted by radium seemed to contradict the law of conservation
of energy: What was the source of that energy? Physicists began to reconsider the structure of the atom,
and ponder whether some change in the atom itself could be responsible.
In 1899, physicist Ernest Rutherford discovered that these materials actually emit different types of
radiation (alpha, beta and gamma rays), defined by their penetrating power. A decade later, Rutherford
proposed a model of the atom in which a small, dense nucleus of protons was surrounded by orbiting
electrons — and later demonstrated that the source of the radioactivity was the spontaneous disintegration
of this atom, thereby “transmuting” the element into another element. In 1919, Rutherford — now known
as the father of nuclear physics — published a paper that detailed “splitting an atom”; he had succeeded in
forcing protons out of the nucleus, the first step to the 1938 discovery of nuclear fission.
Becquerel died only 12 years after his discovery of radioactivity, at age 54. Although his cause of death
was unspecified, he had developed serious burns on his skin, likely from the handling of radioactive
materials. A few decades later, Marie Curie died of aplastic anemia, likely from exposure to radiation
without proper safety measures. The damaging effects of ionizing radiation were still unknown at the
time.