Energy and Human Ambitions on a Finite Planet, 2021a

15 Nuclear <strong>Energy</strong> 244
2. Beta-minus (β − ) decay is a manifestation of the weak nuclear
force, in which a neutron within the nucleus converts to a proton,
<strong>and</strong> in the process spits out an electron (β − particle, really just e − )
to conserve total electric charge, <strong>and</strong> a neutrino—which we will
ignore. 11 The mass number, A is unchanged, but N goes down one
<strong>and</strong> Z goes up one (gaining a proton <strong>and</strong> losing a neutron). Thus
on the chart of nuclides the motion is one left, one up. It’s like a
chess move (Figure 15.7);
3. Beta-plus (β + ) decay, like β − , is a manifestation of the weak nuclear
force, in which a proton within the nucleus converts to a neutron,
emitting a positron (β + ,ore + , or anti-electron; a form of antimatter)
again maintaining charge conservation, <strong>and</strong> an ignored neutrino.
Similar to β − decay, A is unchanged, but Z is reduced by one <strong>and</strong>
N gains one. On the chart, the move is diagonal: down one <strong>and</strong>
right one (Figure 15.7).
4. Gamma decay (γ) happens when a nucleus is in an excited energy
state, having been rattled by some other decay or bombardment,
<strong>and</strong> it emits a high-energy photon, called a gamma ray, as it settles
into a lower energy state (Figure 15.6). For γ decays, Z, N, <strong>and</strong> A
do not change, so the nucleus does not morph into another flavor,
<strong>and</strong> thus does not move on the Chart of the Nuclides.
11: Perhaps it is fair to ignore neutrinos
since they ignore us. Neutrinos interact
so infrequently with matter that a neutrino
could fly through light years of rocky
(Earth-like) material before being expected
to hit something (interact). This extreme
non-interactivity earns it the title of “ghost”
particle.
Figure 15.6: Gamma decay of an excited
nucleus.
Figure 15.7 demonstrates the motion of each of these decays on the Chart
of the Nuclides, <strong>and</strong> Table 15.2 summarizes the nucleon arithmetic.
6
5
4
C8 C9 C10 C11 C12 C13 C14 C15
2.0e-21 s 0.127 s 19.29 s 20.36 m
5715 yr 2.450 s
4p+ –
5.0e-21 s 53.3 days ~7e-17 s
1.5 Myr 13.8 s 0.0215 s v. short
2p+ e - capt. to Li7 2 – n?
B7 B8 B9 B10 B11 B12 B13 B14
3e-22 s 0.770 s 8e-19 s
0.02020 s 0.0174 s 0.013 s
3p+ p+2
Be6 Be7 Be8 Be9 Be10 Be11 Be12 Be13
3
Li4
8e-23 s
p
Li5
~3e-22 s
p+
Li6
Li7
Li8 Li9 Li10 Li11
0.840 s 0.1783 s 2e-21 s 0.0086 s
n
9
1
0
Z
H1
–
2
He3
H2
n1
10.25 m
0 1 N
He4
He5 He6 He7
7e-22 s 0.807 s 3e-21 s
n+
n
H3 H4 H5 H6
12.32 yr 8e-23 s v. short 3e-22 s
n n 3n or 4n
2 3 4 5
He8 He9 He10
0.119 s v. short 2e-21 s
n
2n
6 7 8
+ + Figure 15.7: Radioactive decays shown as
moves on the “chess board” of the Chart of
the Nuclides. The different decay types are
color-coded to match Figure 15.8, <strong>and</strong> are
only shown in a few representative squares.
Decays frequently occur in a series, one
after the other (a decay chain), as hinted
by the double-sequence starting at 12Be
12 <strong>and</strong> ending on C. Note that the square
of every unstable nuclide indicates a decay
type, even if arrows are not present.
Decay Z → N → A →
α Z − 2 N − 2 A − 4
β − Z + 1 N − 1 unchanged
β + Z − 1 N + 1 unchanged
γ unchanged unchanged unchanged
Table 15.2: Summary of decay math on nucleon
counts.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.