Antoine Henri Becquerel
(15
December 1852 – 25 August 1908) was a French physicist, Nobel laureate,
and the discoverer of radioactivity along with Marie Skłodowska-Curie
and Pierre Curie, for which all three won the 1903 Nobel Prize in
Physics.
Contents
Becquerel was born in Paris into a
family which produced four generations of scientists: Becquerel's
grandfather (Antoine César Becquerel), father (Alexandre-Edmond
Becquerel), and son (Jean Becquerel). He studied engineering at the
École Polytechnique and the École des Ponts et Chaussées. In 1890 he
married Louise Désirée Lorieux.
Career
In 1892, he became
the third in his family to occupy the physics chair at the Muséum
National d'Histoire Naturelle. In 1894, he became chief engineer in the
Department of Bridges and Highways.
Becquerel's
discovery of radioactivity is a famous example of serendipity, of how
chance favors the prepared mind. Becquerel had long been interested in
the phosphorescence, the emission of light of one color following a
body's exposure to light of another color. In early 1896, in the wave of
excitement following Wilhelm Conrad Röntgen's discovery of X-rays the
previous fall, Becquerel thought that phosphorescent materials, such as
some uranium salts, might emit penetrating X-ray-like radiation when
illuminated by bright sunlight. His first experiments appeared to show
this.
Describing them to the French Academy of Sciences on 24 February 1896, he said:
One wraps a Lumière photographic plate with a bromide emulsion in two
sheets of very thick black paper, such that the plate does not become
clouded upon being exposed to the sun for a day. One places on the sheet
of paper, on the outside, a slab of the phosphorescent substance, and
one exposes the whole to the sun for several hours. When one then
develops the photographic plate, one recognizes that the silhouette of
the phosphorescent substance appears in black on the negative. If one
places between the phosphorescent substance and the paper a piece of
money or a metal screen pierced with a cut-out design, one sees the
image of these objects appear on the negative ... One must conclude from
these experiments that the phosphorescent substance in question emits
rays which pass through the opaque paper and reduce silver salts.
Becquerel in the lab
But further experiments led him to doubt and then abandon this hypothesis. On 2 March 1896 he reported
I will insist particularly upon the following fact, which seems to me
quite important and beyond the phenomena which one could expect to
observe: The same crystalline crusts [of potassium uranyl sulfate],
arranged the same way with respect to the photographic plates, in the
same conditions and through the same screens, but sheltered from the
excitation of incident rays and kept in darkness, still produce the same
photographic images. Here is how I was led to make this observation:
among the preceding experiments, some had been prepared on Wednesday the
26th and Thursday the 27th of February, and since the sun was out only
intermittently on these days, I kept the apparatuses prepared and
returned the cases to the darkness of a bureau drawer, leaving in place
the crusts of the uranium salt. Since the sun did not come out in the
following days, I developed the photographic plates on the 1st of March,
expecting to find the images very weak. Instead the silhouettes
appeared with great intensity ... One hypothesis which presents itself
to the mind naturally enough would be to suppose that these rays, whose
effects have a great similarity to the effects produced by the rays
studied by M. Lenard and M. Röntgen, are invisible rays emitted by
phosphorescence and persisting infinitely longer than the duration of
the luminous rays emitted by these bodies. However, the present
experiments, without being contrary to this hypothesis, do not warrant
this conclusion. I hope that the experiments which I am pursuing at the
moment will be able to bring some clarification to this new class of
phenomena.
By May 1896, after other experiments involving
non-phosphorescent uranium salts, he arrived at the correct explanation,
namely that the penetrating radiation came from the uranium itself,
without any need of excitation by an external energy source.
There
followed a period of intense research in radioactivity, including the
discovery of additional radioactive elements thorium, polonium and
radium, the latter two by Becquerel's doctoral student Marie Curie and
her husband Pierre Curie.
In 1903, Becquerel shared the
Nobel Prize in Physics with Pierre and Marie Curie "in recognition of
the extraordinary services he has rendered by his discovery of
spontaneous radioactivity".
As often happens in
science, radioactivity came close to being discovered nearly four
decades earlier when, in 1857, Abel Niepce de Saint-Victor, who was
investigating photography under Michel Eugène Chevreul, observed that
uranium salts emitted radiation able to darken photographic emulsions.
By 1861, Niepce de Saint-Victor realized that uranium salts produce "a
radiation that is invisible to our eyes".(Note that Niepce de
Saint-Victor knew Edmond Becquerel, Henri Becquerel's father.)
https://en.wikipedia.org/wiki/Henri_Becquerel