Geoscience Reference
In-Depth Information
concept of the 'half-life' or 'half-transformation' as he put it. This is
the time period in which the activity of a radioactive element drops to
half its original value. For example, in the first step of the uranium
decay sequence between uranium and uranium X 1, the next element
in the line of descent, the half-life is 4.51
10
9
years (a very long time)
and it is always this long. In some other radioactive atoms the half-
life between two adjacent steps of the series may be as low as 0.019
seconds.
By 1908 some additional steps had been inserted into the decay
sequences (Figure
13.2
), and later still more were added as further
research was carried out. In 1910 Soddy wondered if there existed
varieties of the radioactive elements, varieties that in 1913 he called
'isotopes'. An isotope is a species of an atom that has the same atomic
number but different number of neutrons in its nucleus. All nucleii
contain protons, which have a positive charge, and neutrons, which
have no charge. The atomic number of an atom is the number of
positive units in its nucleus. Soon after Soddy's speculations it was
discovered that there were a whole host of radioactive isotopes.
Uranium alone has several isotopes:
234
uranium (
234
U),
235
U and
238
U while lead (Pb) has
204
Pb,
206
Pb,
207
Pb and
208
Pb.
What is remarkable is that within a few years of the discovery
of radioactivity so much was known about it. In 1903 a journal
Le Radium was established in Paris and published by Buffon's old
publisher Masson of Boulevard Saint-Germain. It was supported by a
Comit ยด Scientifique which numbered Becquerel, Madame Curie,
Debierne and Rutherford among its members. It is therefore hardly
surprising that it became the premier journal for students of radio-
activity. The journal also carried advertisements, one of which offered
one milligram of pure radium bromide for 400 francs and one gram of
uranium salt for 1 franc (Figure
13.3
). When we compare what was
known of the decay sequences by the beginning of the First WorldWar
to what was known sixty years later, there is remarkable similarity.
Today the decay series are slightly longer thanks to the insertion of
some additional steps into the pioneers' schemes.
