The Earth in the Solar System - Geochemistry: An Introduction - page 247

and the one going on in the outer part of our Sun right now. The helium produced is
burned at greater depth within the star and therefore at a higher temperature (10 8 K)
to form C, N, and O, which in turn are raised to higher temperatures (10 8 -10 9 K) and
produce Mg, Al, Ca, and Si. Fusion stops at iron because production of heavier nuclides
requires energy input. Lithium, Be, and B, for their part, are burned in the stellar interiors
to form heavier elements and the small quantities of these elements observed in plane-
tary material were produced in the interstellar medium upon breakup of heavy nuclei by
cosmic rays.
At the very high temperatures (several billion K) resulting from gravitational collapse
of the core of the more massive stars, explosive silicon burning produces the abundant
elements of the iron group (Fe, Ni, Cr). For the elements beyond iron, processes of a
different type are at work that involve the absorption of neutrons by nuclei. Neutrons are
in general easily absorbed (astrophysicists would refer to a large neutron absoption cross-
section). They are not affected by the Coulomb electrostatic potential of the nucleus, and
therefore can approach it to the distance at which nuclear forces become dominant. Three
essential processes are the slow s process, the rapid r process and the p process, which
stand for s low and r apid neutron absorption and for p roton absorption, respectively. Most
elements are produced by a combination of processes although some of them are essentially
pure, such as Sn, Ba, Pb ( s )orXe,Eu,Pt( r ). The relatively minor p process produces
nuclei rich in protons located above the zone of stable nuclides (gray in Fig. 12.3 )by
reaction of the nuclides with high gamma radiation emitted when supernovae explode,
and these will not be discussed here. There is some agreement that the s process must
be associated with stars that have masses a few times that of our Sun from the so-called
Asymptotic Giant Branch (AGB), which are known to lose enormous amounts of material
continuously, disheveling themselves into molecular clouds which will later be used as
the raw material for new stars. r -Process elements are formed in the violent explosion of
some particular types of supernovae. There is no consensus, however, on how the more
uncommon p -process elements form.
The s and r processes can be better understood by examining a segment of the chart
of the nuclides ( Fig. 12.3 ) . It can be seen that the neutron-rich, unstable s and r nuclides
(located to the right of the stability valley) decay by β − emission, while the rare p unstable

β + decay. This pattern of radioac-
tive decay results in the preferential formation of nuclides in and around the valley of
stability.
Neutrons are being produced permanently in stars by multiple reactions involving light
elements. The s process starts beyond the peak of the iron group, i.e. beyond the top of the
curve of the binding energy per nucleon ( Fig. 12.2 ) , with iron “seeds.” It arises from high-
temperature (300 million K) equilibrium between absorption of these neutrons by nuclei
and radioactive

nuclides (left) decompose by electron capture or

β − decay. The rate of change of the number n i per unit volume of a stable
nuclide containing i neutrons and which is bathed in a medium with a concentration of
neutrons N n agitated at the mean thermal velocity

v n is given by the equation:

d n i
d t =− σ i N n v n n i + σ i − 1 N n v n n i − 1

(12.1)