millions of kelvins, many nuclei present have high enough speeds for reactions to be
frequent. Indeed, the reactions provide the energy that maintains these temperatures.
In the laboratory, it is easy to produce nuclear reactions on a small scale, either with
alpha particles from radionuclides or with protons or heavier nuclei accelerated in
various ways. But only one type of nuclear reaction has as yet proved to be a practical
source of energy on the earth, namely the fission of certain nuclei when struck by
neutrons.
Many nuclear reactions actually involve two separate stages. In the first, an incident
particle strikes a target nucleus and the two combine to form a new nucleus, called a
compound nucleus,whose atomic and mass numbers are respectively the sum of the
atomic numbers of the original particles and the sum of their mass numbers. This idea
was proposed by Bohr in 1936.
A compound nucleus has no memory of how it was formed, since its nucleons are
mixed together regardless of origin and the energy brought into it by the incident
particle is shared among all of them. A given compound nucleus may therefore be
formed in a variety of ways. To illustrate this, Fig. 12.15 shows six reactions whose
product is the compound nucleus^147 N*. (The asterisk signifies an excited state.
Compound nuclei are always excited by amounts equal to at least the binding energies
of the incident particles in them.) Compound nuclei have lifetimes on the order of
10 ^16 s or so. Although too short to permit actually observing such nuclei directly,
such lifetimes are long relative to the 10^21 s or so a nuclear particle with an energy
of several MeV would need to pass through a nucleus.
A given compound nucleus may decay in one or more ways, depending on its
excitation energy. Thus^147 N* with an excitation energy of, say, 12 MeV can decay in
any of the four ways shown in Fig. 12.15.^147 N* can also simply emit one or more
gamma rays whose energies total 12 MeV. However, it cannotdecay by the emission of
a triton (^31 H) or a helium-3 (^32 He) particle since it does not have enough energy to
liberate them. Usually a particular decay mode is favored by a compound nucleus in
a specific excited state.Nuclear Transformations 447
14
7 N*
11(^31) H 6 C
11
(^32) He 5 B
10
(^42) He 5 B
12
(^21) H 6 C
13
(^11) H 6 C
13
(^10) n 7 N
10
5 B^42 He
12
6 C^21 H
13
6 C^11 H
13
7 N^10 n
Figure 12.15Six nuclear reactions whose product is the compound nucleus^147 N and four ways in
which^147 N can decay if its excitation energy is 12 meV. Other decay modes are possible if the excitation
energy is greater, fewer are possible if this energy is less. In addition,^147 N* can simply lose its excita-
tion energy by emitting one or more gamma rays.
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