Wilhelm Röntgen had already found that X-rays darkened
a photographic plate. Henri Becquerel initially thought that
fluorescence produced X-rays, so, in 1896, he put a crust of
phosphorescent uranium salt over an unexposed photographic plate
protected by “two sheets of very thick black paper”;
he expected that the plate would darken under the sample when he
put it in sunlight, and it did. But Becquerel became curious when
the plate darkened on a cloudy day when left in a drawer, so he
tested a non-phosphorescent uranium salt and the plate still
Later, Becquerel showed that uranium did not emit X-rays
because X-rays can be deflected by a magnetic field, and the
radiation from uranium could not.
Marie Curie was Becquerel’s doctoral student.
Subsequently, she and her husband, Pierre, isolated polonium and
radium from uranium ore.
Ernest Rutherford named the three types of particles emitted
during radioactive decay—alpha, beta, and gamma
particles—based on their ability to penetrate other
materials. Rutherford showed that alpha particles were helium
nuclei (now known as protons); he helped demonstrate the nuclear
nature of atoms, and he proposed that nuclei were composed of
equal numbers of protons and neutrons (then unobserved).
Rutherford was the first to notice that radioactive elements have
a constant half-life, and used this to determine the age of the
earth. A speech by Rutherford inspired Leó Szilárd
to propose controlled energy-producing nuclear chain-reactions.
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