32.2

Induced Nuclear Reactions

Section 31.4 discusses how a radioactive parent nucleus disintegrates spontaneously into a daughter nucleus. It is also possible to bring about, or induce, the disintegration of a stable nucleus by striking it with another nucleus, an atomic or subatomic particle, or a -ray photon. A nuclear reaction is said to occur whenever the incident nucleus, particle, or photon causes a change to occur in a target nucleus.
In 1919, Ernest Rutherford observed that when an particle strikes a nitrogen nucleus, an oxygen nucleus and a proton are produced. This nuclear reaction is written as

Because the incident particle induces the transmutation of nitrogen into oxygen, this reaction is an example of an induced nuclear transmutation.
Nuclear reactions are often written in a shorthand form. For example, the reaction above is designated by (, p) . The first and last symbols represent the initial and final nuclei, respectively. The symbols within the parentheses denote the incident particle (on the left) and the small emitted particle or proton p (on the right). Some other induced nuclear transmutations are listed below, together with the equivalent shorthand notations:

Nuclear Reaction | Notation

CONCEPTS AT A GLANCE Induced nuclear reactions, like the radioactive decay process discussed in Section 31.4, obey the conservation laws of physics. We have discussed these laws in previous chapters, as the Concepts-at-a-Glance chart in Figure 32-2 indicates. This chart is similar to that shown earlier in Figure 31-6 and emphasizes the important role that the conservation laws play in all nuclear processes. In particular, both the total electric charge of the nucleons and the total number of nucleons are conserved during an induced nuclear reaction. The fact that these quantities are conserved makes it possible to identify the nucleus produced in a reaction, as the next example illustrates.

Figure 32-2 CONCEPTS AT A GLANCE In any nuclear process, such as radioactive decay or an induced nuclear reaction, the conservation laws of physics are obeyed. Such processes provide the basis for the commercial generation of power, as in the nuclear power plant shown in the photograph. (Steve Allen/Agefotostock)

| Example 3 | | | An Induced Nuclear Transmutation

An particle strikes an aluminum nucleus. As a result, an unknown nucleus and a neutron are produced:

Identify the nucleus produced, including its atomic number Z (the number of protons) and its atomic mass number A (the number of nucleons).
Reasoning The total electric charge of the nucleons is conserved, so that we can set the total number of protons before the reaction equal to the total number after the reaction. The total number of nucleons is also conserved, so that we can set the total number before the reaction equal to the total number after the reaction. These two conserved quantities will allow us to identify the nucleus .
Solution The conservation of total electric charge and total number of nucleons leads to the equations listed below:

Conserved Quantity | Before Reaction | After Reaction
Total electric charge (number of protons) | 2 + 13 = Z + 0
Total number of nucleous | 4 + 27 = A + 1

Solving these equations for Z and A gives Z = 15 and A = 30. Since Z = 15 identifies the element as phosphorus (P), the nucleus produced is .

Induced nuclear transmutations can be used to produce isotopes that are not found naturally. In 1934, Enrico Fermi suggested a method for producing elements with a higher atomic number than uranium (Z = 92). These elements—neptunium (Z = 93), plutonium (Z = 94), americium (Z = 95), and so on—are known as transuranium elements, and none occurs naturally. They are created in a nuclear reaction between a suitably chosen lighter element and a small incident particle, usually a neutron or an particle. For example, Figure 32-3 shows a reaction that produces plutonium from uranium. A neutron is captured by a uranium nucleus, producing and a ray. The nucleus is radioactive and decays with a half-life of 23.5 min into neptunium . Neptunium is also radioactive and disintegrates with a half-life of 2.4 days into plutonium . Plutonium is the final product and has a half-life of 24 100 yr.

Figure 32-3 An induced nuclear reaction is shown in which is transmuted into the transuranium element plutonium .

The neutrons that participate in nuclear reactions can have kinetic energies that cover a wide range. In particular, those that have a kinetic energy of about 0.04 eV or less are called thermal neutrons. The name derives from the fact that such a relatively small kinetic energy is comparable to the average translational kinetic energy of a molecule at room temperature.

Check Your Understanding 2

Which one or more of the following four nuclear reactions could possibly occur? (a) (α, γ) , (b) (p, n) , (c) (p, γ) , (d) (n, p) [The answer(s) is (are) given at the end of the book.] Background: Any nuclear reaction that occurs must obey the conservation laws of physics.
For similar questions (including calculational counterparts), consult Self-Assessment Test 32.1, which is described at the end of Section 32.7.

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