For a fixed atomic number (Z), decay lifetimes decrease to infinitesimal values when departing from the stable isotopes region in both directions (to the left, i.e. for lower atomic masses, as well as to the right, for
larger atomic masses). It is therefore straightforward to imagine the ß stability valley as a narrow canyon, with stable isotopes distributed along the valley floor and the unstable ones along the cliffs,
at various heights basing on their lifetimes (the fastest the decay, the closer the position to the canyon summit).
The s process, represented by horizontal red arrows (each one is a neutron capture), ALWAYS occurs close to the ß stability valley.
Each time an unstable nucleus is created by a neutron capture on a stable one, it has the time to decay to its stable isobar (a stable nucleus with the same atomic mass, but a different atomic number)
before capturing another neutron. Those decays can be ß+ (protons transform into neutrons: downward arrows) or ß- (neutrons transform into protons: upward arrows).
Along the s-process path, there exist nuclei whose nuclear structure is so particularly stable (they are named “magic” nuclei) that the corresponding elements accumulate with respect to the
neighbours (red filled boxes). As a consequence, we have “peaks” in the distribution of heavy elements synthesized via the s process (at N=50,
N=82 and N=126). This is illustrated in Figure 3, which shows how
the s process is characterized by 3 prominent peaks: the first at strontium-yttrium-zirconium (Sr-Y-Zr), the second at barium-lanthanum-cerium-neodymium
(Ba-La-Ce-Nd) and the third at lead (Pb). The s process is responsible for the production of half the elements heavier than iron.
The remaining half is synthesized through the rapid neutron capture process (r process). In this case, isotopes very far from the ß stability valley can be produced via a series of
multiple neutron captures starting from a single stable isotope (horizontal arrows in Figure 9). In fact, due to the extremely large neutron flux, unstable nuclei just synthesized cannot decay and in turn are forced to capture
a neutron. Such a sequence proceeds until isotopes with very short lifetimes (milliseconds) are created.
In fact, due to the extremely high neutron flux, freshly sinthesized nuclei cannot decay and are forced
to experience multiple neutron captures. This series of neutron captures leads to the synthesis of isotopes with very short
lifetimes (milliseconds). At such conditions, the ß decay may be faster than the neutron capture and the nucleus
can decay, increasing its charge (upward blue arrows). In correspondence of neutron magic nuclei (see
above) elements accumulate (colored blue boxes). Once the neutron flux comes to an end,
those isotopes can decay to their relative stable isobars along the stability valley.
In Figure 3, as a consequence, the three peaks of the r process
appear: the first in correspondence of selenium-bromine-krypton (Se-Br-Kr), the second at tellurium-iodine-xenon (Te-I-Xe) and
the third at iridium-platinum-gold (Ir-Pt-Au). Moreover, it's worth stressing
that also long-lived terrestrial radioactive elements (as thorium and uranium)
have been created by the r process.
The two neutron processes we just described are extremely different. It's amazing that all heavy chemical
elements have been created in
so different and unique conditions (for the sake of clarity,
there are other intermediate processes, whose relevance, however, is marginal with respect to the main
components s and r). There is still one aspect to be understood:
WHERE do these process occur? This question kept busy nuclear
astrophysicists in the last 40 years...and cause them to lay awake at night!!!
The stellar environments of interest to us are two: low mass stars during their Asymptotic Giant Branch
phase (for the s process) and neutron star binary systems (for the r process).