Geoscience Reference
In-Depth Information
light. Sure enough, when he developed the plate, he found that it had captured the
image of the uranium crystal. But when the Sun failed to shine for a few days,
Becquerel temporarily suspended the experiment, storing the crystal atop a pho-
tographic plate inside a drawer where no light could reach it. When he later de-
veloped the plate, expecting to find at most a faint impression from the residual
phosphorescence of the uranium mineral, he found instead an image of the crystal
that was as sharp as it had been after the mineral had sat in the sunlight. Becquerel
realized that phosphorescence does not depend on sunlight but instead comes from
within the crystal itself. Like cathode-ray tubes, some minerals also emit strange
rays. Unlike the tubes, the minerals do so spontaneously.
Becquerel suggested that his student Marie Curie (1867-1934) take up the study
of the new rays, thus launching one of the most productive careers in the history of
science. For her Ph.D. thesis topic, Marie Curie set out to discover whether other
substances also emit the uranium rays, a process that she and Becquerel had named
radioactivity. Instead of a photographic plate to detect radiation, she used a sensit-
ive instrument called an electrometer, invented by her husband Pierre and his older
brother. Her work soon disproved a scientific belief that went back to the Greeks:
that the atom is the smallest particle of matter.
Marie Curie (and independently a German scientist) soon found that the element
thorium is also radioactive. She then discovered that pitchblende emitted radio-
activity at four times the rate of pure uranium. She deduced that pitchblende must
contain another element or elements more radioactive than uranium. She and Pi-
erre set out to identify those elements and soon found two: one they named one
polonium, after her native Poland; the other they named radium.
