Geology Reference
In-Depth Information
evening on my bedside table when I went to bed, so I could see
it glowing in the dark. One night I thought to look at it under
a magnifying glass, and was amazed at what I saw. The light
from the dots was not steady but a quivering shower of sparks,
like the tiniest firework imaginable. At the time I did not under-
stand what it was I was seeing, but now I realise that each spark
was generated by an atom of helium being emitted from the
radioactive substance in the green luminous paint.
This same phenomenon was exploited when trying to deter-
mine the production rate of helium. An apparatus was arranged
so that the helium atoms being emitted from a very small but
very accurately known quantity of radium were impelled through
a specially designed chamber. The passage of each particle set
up a tiny electric current which gave a 'kick' to the needle of an
electrometer. By counting the kicks, the particles themselves
could be counted and the production rate of helium measured.
This was, of course, an early version of the Geiger counter, now
used to detect places contaminated by radioactivity.
But the decay rate was not all that Rutherford needed to know.
In 1902 he and Soddy had determined the fundamental law of
radioactivity. Written on a tablet of stone, the law said this: the
number of radioactive atoms that decay in a given time, say a
year, is dependent upon one thing, and one thing only - the
number of radioactive atoms that are present. Put simply, the
more atoms that are present, the more there are to decay. If there
were a 100 atoms of a radioactive element in a rock, which
decayed on average at 10 per cent a year, then after the first year
10 will have decayed, and 90 would be left, but after the second
year only 9 will have decayed (10 per cent of 90) and 81 will be
left. Thus the number that decay is directly proportional to the
number that is present. Conversely, the numbers of daughter
atoms increase in the same way and, provided that none of the
parent or daughter atoms escape, then the sum of parent and
daughter will always equal the original amount present. Thus
when measuring the age of a rock it is necessary to know not
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