31.4 Nuclear decay and conservation laws  (Page 6/25)

Gamma decay

Gamma decay is the simplest form of nuclear decay—it is the emission of energetic photons by nuclei left in an excited state by some earlier process. Protons and neutrons in an excited nucleus are in higher orbitals, and they fall to lower levels by photon emission (analogous to electrons in excited atoms). Nuclear excited states have lifetimes typically of only about ${\text{10}}^{-\text{14}}$ s, an indication of the great strength of the forces pulling the nucleons to lower states. The $\gamma$ decay equation is simply

where the asterisk indicates the nucleus is in an excited state. There may be one or more $\gamma$ s emitted, depending on how the nuclide de-excites. In radioactive decay, $\gamma$ emission is common and is preceded by $\gamma$ or $\beta$ decay. For example, when ${}^{\text{60}}\text{Co}$ ${\beta }^{-}$ decays, it most often leaves the daughter nucleus in an excited state, written ${}^{\text{60}}\text{Ni*}$ . Then the nickel nucleus quickly $\gamma$ decays by the emission of two penetrating $\gamma$ s:

These are called cobalt $\gamma$ rays, although they come from nickel—they are used for cancer therapy, for example. It is again constructive to verify the conservation laws for gamma decay. Finally, since $\gamma$ decay does not change the nuclide to another species, it is not prominently featured in charts of decay series, such as that in [link] .

There are other types of nuclear decay, but they occur less commonly than $\alpha$ , $\beta$ , and $\gamma$ decay. Spontaneous fission is the most important of the other forms of nuclear decay because of its applications in nuclear power and weapons. It is covered in the next chapter.

Section summary

• When a parent nucleus decays, it produces a daughter nucleus following rules and conservation laws. There are three major types of nuclear decay, called alpha $\left(\alpha \right),$ beta $\left(\beta \right),$ and gamma $\left(\gamma \right)$ . The $\alpha$ decay equation is
  ${}_Z^A{\mathrm{X}}_N\to {}_{Z-2}^{A-4}{\text{Y}}_{N-2}+{}_2^4{\text{He}}_2.$
• Nuclear decay releases an amount of energy $E$ related to the mass destroyed $\mathrm{\Delta }m$ by
  $E=(\mathrm{\Delta }m)c^2.$
• There are three forms of beta decay. The ${\beta }^{-}$ decay equation is
  ${}_Z^A{\mathrm{X}}_N\to {}_{Z+1}^A{\text{Y}}_{N-1}+{\beta }^{-}+{\overline{\nu }}_e.$
• The ${\beta }^{+}$ decay equation is
  ${}_Z^A{\mathrm{X}}_N\to {}_{Z-1}^A{\text{Y}}_{N+1}+{\beta }^{+}+{\nu }_e.$
• The electron capture equation is
  ${}_Z^A{\mathrm{X}}_N+e^{-}\to {}_{Z-1}^A{\text{Y}}_{N+1}+{\nu }_e.$
• ${\beta }^{-}$ is an electron, ${\beta }^{+}$ is an antielectron or positron, ${\nu }_e$ represents an electron’s neutrino, and ${\overline{\nu }}_e$ is an electron’s antineutrino. In addition to all previously known conservation laws, two new ones arise— conservation of electron family number and conservation of the total number of nucleons. The $\gamma$ decay equation is
  ${}_Z{}^A{\text{X}}_N^{*}\to {}_Z{}^A{\text{X}}_N+{\gamma }_1+{\gamma }_2+\cdots$
  $\gamma$ is a high-energy photon originating in a nucleus.

Conceptual questions