10.4 Nuclear reactions  (Page 4/19)

To illustrate the concept of a decay series, consider the decay of Th-232 series ( [link] ). The neutron number, N , is plotted on the vertical y -axis, and the atomic number, Z , is plotted on the horizontal x -axis, so Th-232 is found at the coordinates $\left(N,Z\right)=\left(142,90\right).$ Th-232 decays by $\alpha$ emission with a half-life of $1.39\times {10}^{10}$ years. Alpha decay decreases the atomic number by 2 and the mass number by 4, so we have

The neutron number for Radium-228 is 140, so it is found in the diagram at the coordinates $\left(N,Z\right)=\left(140,88\right).$ Radium-228 is also unstable and decays by ${\beta }^{-}$ emission with a half-life of 5.76 years to Actinum-228. The atomic number increases by 1, the mass number remains the same, and the neutron number decreases by 1. Notice that in the graph, $\alpha$ emission appears as a line sloping downward to the left, with both N and Z decreasing by 2. Beta emission, on the other hand, appears as a line sloping downward to the right with N decreasing by 1, and Z increasing by 1. After several additional alpha and beta decays, the series ends with the stable nucleus Pb-208.

The relative frequency of different types of radioactive decays (alpha, beta, and gamma) depends on many factors, including the strength of the forces involved and the number of ways a given reaction can occur without violating the conservation of energy and momentum. How often a radioactive decay occurs often depends on a sensitive balance of the strong and electromagnetic forces. These forces are discussed in Particle Physics and Cosmology .

As another example, consider the U-238 decay series shown in [link] . After numerous alpha and beta decays, the series ends with the stable nucleus Pb-206. An example of a decay whose parent nucleus no longer exists naturally is shown in [link] . It starts with Neptunium-237 and ends in the stable nucleus Bismuth-209. Neptunium is called a transuranic element    because it lies beyond uranium in the periodic table. Uranium has the highest atomic number $\left(Z=92\right)$ of any element found in nature. Elements with $Z>92$ can be produced only in the laboratory. They most probably also existed in nature at the time of the formation of Earth, but because of their relatively short lifetimes, they have completely decayed. There is nothing fundamentally different between naturally occurring and artificial elements.

Notice that for Bi (21), the decay may proceed through either alpha or beta decay.