Geoscience Reference
In-Depth Information
from X-rays, but nevertheless were later shown to be useful in
medicine. Their discovery changed the course of geological and geo-
chronological study. This was radioactivity, a phenomenon discov-
ered by Antoine Henri Becquerel (1852-1908) in February 1896 while
he was experimenting with compounds of uranium. In a darkened
room he had placed some potassium uranyl sulfate into a sealed box
and by chance had rested an unexposed photographic plate beside it.
When the plate was developed it was foggy and demonstrated that
some radiation had passed through the box to affect the plate. He
then discovered that the path of the emissions could be deflected by a
magnetic field, thus showing that they were composed of tiny parti-
cles. Within several years three types of rays had been described:
alpha rays (a), which were weak and were absorbed by a thin metal
foil; beta rays (b), which were more penetrative and easily deflected
in a magnetic field; and gamma rays (g), which were highly pene-
trating and were not deflectable. Today these rays are measured
by the eponymous detector invented in 1913 by Hans Geiger
(1882-1945).
The search was now on for elements that produced such emis-
sions, which Marie Curie (1867-1934) called 'radioactivity'. Obviously
uranium was one such element, an element that had been known
since its isolation from pitchblende by the German chemist Martin
Klaproth (1743-1817) in 1789. In 1898 Marie Curie and her husband
Pierre (1859-1906) isolated a new radioactive element from pitch-
blende, which they called polonium, after the country of her birth, but
they realised that another as yet unisolated element must be present
and they called this radium. Eventually in 1902 after years of refining
tonnes of pitchblende purchased from Czech mines they managed to
isolate one-tenth of a gram of radium. Marie also demonstrated that
thorium, which had been discovered by the English chemist Smithson
Tennant (1761-1815), was radioactive. In 1899 actinium was discov-
ered and named by Andr´ Louis Debierne (1874-1949), an assistant of
the Curies, and researchers realised that some forms of lead showed
radioactive properties. Some discoveries proved to be episodes of false
