In 1896
Henri Becquerel discovered radioactivity. It happened by accident
when he was investigating phosphorescence. Becquerel was aware of
Roentgen’s discovery of x-rays in 1895 and thought there might be a
relationship between phosphorescence and x-radiation.
In February 1896, Becquerel covered a photographic plate with black
paper and coated the paper with a uranium salt. Following several hours
exposure to sunlight, he removed the black paper and found that the
photographic plate was clouded.
Two days later he prepared another photographic plate in the same way
but the weather was cloudy and he put everything away in the one draw.
On 1st March, Becquerel developed this second plate and found it to be
cloudy. Whatever it was that had come from the uranium salt, penetrated
the black paper and clouded the photographic plate, had nothing to do
with sunlight or phosphorescence.
Becquerel had
discovered some new kind of radiation that had come from the uranium
salt spontaneously and without outside influence. This contrasted with
x-rays where large voltages were needed to release the radiation.
Pierre and Marie Curie
Many scientists began investigating the new phenomenon.
Marie and Pierre Curie also working in Paris did much of the
pioneering work in this new branch of physics.
Becquerel had already noted that the radiation emitted by uranium
ionised the surrounding air. Using this characterisation of ionisation
Pierre and Marie Curie began searching for other materials that could
emit radiation. When they tested an ore of uranium, called pitchblende,
for its ability to ionise air. The Curies found that the pitchblende
produced a current 300 times stronger than that produced by pure
uranium.
The Curies believed that a very active unknown substance must exist
within the pitchblende. After a lengthy and gruelling search the Curies
identified two new elements present in pitchblende in tiny quantities
both of which were capable of emitting penetrating radiation. In
reporting on their findings, they introduced the new term:
"radio-active." The two new elements they called Polonium (after Marie’s
native country) and Radium.
More than one type of radiation
Ernest Rutherford, discovered in 1898 that two different types of
radiation were produced by radioactive materials. They called them alpha
and beta rays. Later a third type of radiation was identified by and
called gamma rays.
More Information
Work of the Curies
Work of Rutherford
History of radioactivity