Physics for Geologists, Second edition

Millions of years since U-235 formed
Atomic structure and age-dating 63
Figure 5.1 The decay of uranium-235 (half-life, 704 million years).
0 1 2 3 4
Half-lives
Figure 5.2 Radioactive decay in terms of half-lives.
as the half-life (tl12), is different for each parent nuclide. The half-lives3 of
those isotopes above are:
238~ 4.47 x lo9 years
235~ 704 x lo6 years
232~h 14.0 x lo9 years
87~b 48.8 x lo9 years
40 K 1.4 x lo9 years to 4 0~a
40 K 11.9 x lo9 years to 4 0~r
40 K 1.25 x lo9 years total.
The three isotopes of carbon have already been mentioned: ':c, ':c and
'2~. The first two are stable, nearly 99 per cent of carbon atoms being 12c.
Radiocarbon, as 14c is called, is created from nitrogen in the atmosphere by
cosmic radiation. Fast neutrons are emitted from collisions with oxygen and
nitrogen atoms, and these fast neutrons in turn collide with nuclei of '$N,
dislodging a proton and leaving 'ZC. In due course, the isotope 'ZC returns
3 Derived from the values of the decay constant, h, given by Steiger and Jager (1977) using the
relationship tip = (In O.S)/( - h).
Copyright 2002 by Richard E. Chapman