Geoscience Reference
In-Depth Information
100
Heavy
elements
decay
β
−
α
90
80
s
process
α
r
process
70
β
−
s
process
60
N
=
50
50
β
−
r
process
N
=
126
40
30
r
process
N
=
82
20
40
60
80
100
120
140
160
180
N
(neutrons)
Figure 12.5
The origin of natural nuclides. The
s
process stops when
decay destroys the newly formed
nuclei. When neutron absorption is too fast to allow loss of electrons (
r
process), it progresses to
very stable nuclei (magic numbers) where it must wait for electron re-emission before
it can go further (blown-up sawtooth). When the nuclei become too heavy, they are destroyed by
fission.
α
less abundant than even nuclides. The abundance peaks (magic numbers) reported earlier
correspond to extremely stable optimal filling of nuclear energy levels making the nuclei
immune to neutron absorption. The
s
process stops at
209
Pb, when
decay becomes
predominant and decomposes nuclei formed by the slow absorption of neutrons.
Other processes are required to explain many heavy nuclides, in particular those whose
mass is greater than that of lead, such as U and Th. More highly energetic neutrons
are added to nuclei when supernovae explode, faster than they are lost by
α
β
−
emis-
sion. The
r
process of rapid absorption of neutrons by nuclei follows an
r
pathway quite
remote from the valley of stability to which nuclides eventually return by
β
−
emission
(
Fig. 12.5
). This fast track is blocked at the level of particularly stable nuclei unamenable
to the addition of neutrons (magic numbers 50, 82, 126). Further neutron absorption
must wait at each notch for an electron to be lost by
β
−
emission before a new neu-
tron can be absorbed and the nucleus can ratchet up the sawtooth of magic numbers.
The process is stopped at the heavy masses when fission destroys nuclei faster than they
can form.
The abundances of
p
-process nuclides is remarkably well predicted by stellar models
and experimentally determined cross-sections and decay constants. The abundances of
r
-process neutron-rich nuclides is deduced by subtracting the contribution of the
s
process
from the observed distribution of the elements in the universe. The names of Burbidge,
Burbidge, Fowler, and Hoyle (usually referred to as B
2
FH) is attached since 1957 to one of
the most fascinating successes in the history of science. Overall, astrophysics has been very
successful at predicting the abundances of elements and isotopes in the universe (
Fig. 12.6
).
It has been less successful at identifying the stellar sites at which this complex alchemy
takes place.
