31.4 Nuclear decay and conservation laws

Learning objectives

By the end of this section, you will be able to:

• Define and discuss nuclear decay.
• State the conservation laws.
• Explain parent and daughter nucleus.
• Calculate the energy emitted during nuclear decay.

The information presented in this section supports the following AP® learning objectives and science practices:

• 5.B.8.1 The student is able to describe emission or absorption spectra associated with electronic or nuclear transitions as transitions between allowed energy states of the atom in terms of the principle of energy conservation, including characterization of the frequency of radiation emitted or absorbed. (S.P. 1.2, 7.2)
• 5.C.1.1 The student is able to analyze electric charge conservation for nuclear and elementary particle reactions and make predictions related to such reactions based upon conservation of charge. (S.P. 6.4, 7.2)
• 5.C.2.1 The student is able to predict electric charges on objects within a system by application of the principle of charge conservation within a system. (S.P. 6.4)
• 5.G.1.1 The student is able to apply conservation of nucleon number and conservation of electric charge to make predictions about nuclear reactions and decays such as fission, fusion, alpha decay, beta decay, or gamma decay. (S.P. 6.4)

Nuclear decay    has provided an amazing window into the realm of the very small. Nuclear decay gave the first indication of the connection between mass and energy, and it revealed the existence of two of the four basic forces in nature. In this section, we explore the major modes of nuclear decay; and, like those who first explored them, we will discover evidence of previously unknown particles and conservation laws.

Some nuclides are stable, apparently living forever. Unstable nuclides decay (that is, they are radioactive), eventually producing a stable nuclide after many decays. We call the original nuclide the parent    and its decay products the daughters . Some radioactive nuclides decay in a single step to a stable nucleus. For example, ${}^{\text{60}}\text{Co}$ is unstable and decays directly to ${}^{\text{60}}\text{Ni}$ , which is stable. Others, such as ${}^{\text{238}}\text{U}$ , decay to another unstable nuclide, resulting in a decay series    in which each subsequent nuclide decays until a stable nuclide is finally produced. The decay series that starts from ${}^{\text{238}}\text{U}$ is of particular interest, since it produces the radioactive isotopes ${}^{\text{226}}\text{Ra}$ and ${}^{\text{210}}\text{Po}$ , which the Curies first discovered (see [link] ). Radon gas is also produced ( ${}^{\text{222}}\text{Rn}$ in the series), an increasingly recognized naturally occurring hazard. Since radon is a noble gas, it emanates from materials, such as soil, containing even trace amounts of ${}^{\text{238}}\text{U}$ and can be inhaled. The decay of radon and its daughters produces internal damage. The ${}^{\text{238}}\text{U}$ decay series ends with ${}^{\text{206}}\text{Pb}$ , a stable isotope of lead.