Energy and Human Ambitions on a Finite Planet, 2021a

15 Nuclear <strong>Energy</strong> 245
Example 15.2.2 What will the fate of
15.4?
8He
be, according to Figure
We can play this chess game! According to the chart, the primary
decay mechanism of He8 is β − with a half-life of about a tenth of a
8
second. It will become Li, which hangs around for about a second
before undergoing another β − 8
decay to Be. This one lasts almost no
time at all (∼ 10 −16 s) before α decay into two alpha particles (two
4
He). Such a sequence is called a decay chain.
As is evident in Figure 15.8, unstable isotopes above the stable track in
Figure 15.3 tend to undergo β + decays to drive toward stable nuclei,
while those below the track tend to experience β − decays to drive up
toward the stable track. The α decays are more common for heavy nuclei
(around uranium), which drive toward the end of the train of stable
elements in Figure 15.3, ending up around lead (Pb). We can underst<strong>and</strong>
the abundance of lead as a byproduct of heavy-element decay chains.
stable
alpha ()
beta-minus ( – )
beta-plus ( ) or e – capture
spontaneous fission
neutron emission (n)
proton emission (p)
Figure 15.8: Another view of the Chart of
the Nuclides, color coded to indicate prevailing
decay modes as a function of position
on the chart. Note that β + sometimes
captures an electron rather than emitting a
positron, but amounting to the same thing,
essentially. From U.S. DoE.
Box 15.1: The Weak Nuclear Force
An aside worth making is that having discussed beta decays, governed
by the weak nuclear force, we have now covered all four
known forces of nature: gravity, electromagnetism, the weak nuclear
force, <strong>and</strong> the strong nuclear force. That’s it: a small menu, really.
The latter three are unified into a St<strong>and</strong>ard Model of Physics, but
gravity—described by General Relativity—has defied all attempts at
“gr<strong>and</strong> unification,” or a “theory of everything” trying to unite all
four forces under a single theoretical framework. One implication is
that known physics offers no other “magic” solutions to our energy
needs. No new forces have come to light in more than half-a-century,
despite dramatic advances in tools to probe the fundamental nature
of physics.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.