More on α, β, and γ decay
Another example of α decay
The alpha decay example involves the decay of thorium into radium. Figure E18.4 .1
shows this decay. The alpha decay involves emission of the nucleus of helium (2 protons
and 2 neutrons) by the original nucleus.
Fig. E18.4.1 The alpha decay of thorium-232 into radium-226. Note that Z decreases by 2 and A decreases
by 4. This is just what the Z and A of helium are. In this extension, we show the protons as blue and
neutrons as yellow.
Other examples of β decay
Figure E18.4.2 shows the beta-minus decay of carbon-14 to nitrogen-14. Carbon, being a
constituent of living things, is incorporated into the cells of living organisms. A small part
of the carbon is radioactive carbon-14, and it no longer is incorporated after the living
thing dies. What is there then decays.
Fig. E18.4.2 The beta decay of carbon-14 to nitrogen-14. In this case, a neutron changes into a proton. The
mass number A does not change. Z increases by 1.
Energy, Ch. 18, extension 4 More on α, β and γ decay
The age of the object that contained the carbon can be determined by seeing how much of
the carbon must have decayed. The charge on the nucleus increases by 1 because the
electron carries away a charge of -1. An antineutrino is also emitted. The nitrogen-14
moves quite slowly in recoil.
Fig. E18.4.3 The beta decay of fluorine-18 to oxygen-18. In this case, a neutron changes into a proton. The
mass number A does not change. Z decreases by 1.
In beta-plus decay (such as the decay of fluorine-18 into oxygen-18, Fig. E18.4.3), the
particle emitted is the antiparticle of the electron (the positron), and so has a charge of +1.
The new nucleus has a proton changed into a neutron because the positive charge has been
carried off by the positron. The other particle emitted is the neutrino, the antiparticle of
the antineutrino. The oxygen nucleus does move slightly in recoil, but the speed is very
small.
Other examples of γ decay
Gamma decay is the one case in which the name of the nucleus remains unchanged. What
happens is that an excited nucleus decays into a nucleus that is less excited (more stable).
In this case, dysprosium-152 becomes de-excited by emitting a particle called a photon
(Fig. E18.4.4). The photon has no charge, so none of the nucleons have to change identity
in the decay as happened in the case of beta decay.
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Energy, Ch. 18, extension 4 More on α, β and γ decay
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Fig. E18.4.4 The gamma decay of dysprosium -152.
The gamma decay of an excited state of cobalt-60 mentioned in the chapter can be
illustrated by use of what is called an energy-level diagram. Figure E18.4.5 shows the
diagram for this decay:
60m
27 Co
→
60m
28 Ni
+ β - + —. The excited nickel nucleus may decay
to an intermediate state or to the ground state, as shown on the diagram:
60m
28 Ni
→
60m'
28 Ni
+
1
or
60m
28 Ni
60
→ 28 Ni +
2
.
Fig. E18.4.5 The energy-level diagram for beta-minus decay from the excited state of cobalt-60 to one of
the excited states of nickel-60, followed by one or two gamma decays with energies 826 keV, 1332 keV,
and 2158 keV.
Energy, Ch. 18, extension 4 More on α, β and γ decay
If the nucleus ends up in the excited state, 60m', it subsequently decays to the ground
state
60m'
28 Ni
60
→ 28 Ni +
3
.
These energies correspond to photon wavelengths of 576 fm, 933 fm, and 1504 fm,
smaller than an atom, bigger than a nucleus.
There are two other processes often lumped together with α, β, and γ deacys, in that they
also involve a change in the nuclear identity. They are spontaneous fission and electron
capture.
Spontaneous fission
Nuclei such as uranium-235 and thorium-233 undergo spontaneous fission. In
spontaneous fission, a nucleus suddenly breaks apart with no outside interference (normal
fission of uranium-235 involves capture of a thermal neutron from outside the nucleus).
These special nuclei are occasionally excited somehow and begin oscillating, as discussed
in Extension 18.3, The liquid drop model. The process produces two smaller nuclei and
extra neutrons, so things have changed from the original nucleus. As with thermal-neutron
activated fission, the identity of the smaller nuclei follows a random distribution.
Electron capture
As explained in Chapter 5, electrons fill energy levels outside the nucleus of an atom.
While it is often convenient to visualize the electron as a “planet” orbiting a nuclear
“sun,” electrons or atoms cannot be located in one spot the way we usually think of
locating objects. The electrons are spread out in space somehow—they are even in the
space “occupied” by the nucleus. Under certain circumstances, a nucleus will absorb one
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Energy, Ch. 18, extension 4 More on α, β and γ decay
of its electrons, changing a proton into a neutron and emitting a neutrino. This process is
known as electron capture.
The process produces a different nucleus from what was there before: the new nucleus
will have a Z of 1 less than the parent, because one proton been lost, has but its mass
number A will be the same because a neutron is also a nucleon with almost the sdame
mass as the proton.
It is possible to shield against penetration by α, β, and γ radiation. Alpha particles are
highly ionizing and cannot penetrate far. Betas are also ionizing, but do not interact as
readily as alphas. Gammas do not interact as much as either alphas or betas, but if they
do, they can generally transfer greater amounts of energy. Figure E18.4.6 shows one way
to present this comparison.
Fig. E18.4.6 Alpha particles can be stopped by a sheet of paper, beta particles by a thin sheet of metal, but
gamma rays can penetrate thick layers of lead.
(Adapted from an LBL graphic)
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Energy, Ch. 18, extension 4 More on α, β and γ decay
A more complete discussion of these and more advanced topics is available in Ref. 6.
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