# Gamma decay

Gamma decay is one type of radioactive decay that a nucleus can undergo. What separates this type of decay process from alpha or beta decay is that no particles are ejected from the nucleus when it undergoes this type of decay. Instead, a high energy form of electromagnetic radiation - a gamma ray photon - is released. Gamma rays are simply photons that have extremely high energies which are highly ionizing.[1] As well, gamma radiation is unique in the sense that undergoing gamma decay does not change the structure or composition of the atom. Instead, it only changes the energy of the atom since the gamma ray carries no charge nor does it have an associated mass.

Figure 1. Different penetration levels of different products of decay, with gamma being one of the most highly penetrating.[2][3]

In order for a nucleus to undergo gamma decay, it must be in some sort of excited energetic state. Experiments have shown that protons and neutrons are located in discrete energy states within the nucleus, not too different from the excited states that electrons can occupy in atoms.[4] Thus if a proton or a neutron inside of the nucleus jumps up to an excited state - generally following an alpha or beta decay - the new daughter nucleus must somehow release energy to allow the proton or neutron to relax back down to ground state. When the nucleon makes this transition from a high to a low energy state, a gamma photon is emitted. The general equation that represents this process is:

$A^* \rightarrow A + \gamma$

where:

• $A^*$ is the excited atom
• $A$ is a relaxed state of the initial excited atom
• $\gamma$ is the released gamma ray photon
Figure 2. Diagram showing gamma decay of a nucleus.[3]

Knowing that an atom undergoes gamma radiation is important, but it is also possible to determine the frequency of the released gamma radiation if the initial and final states of the nucleon inside the nucleus are known. The equation representing the frequency of the gamma radiation is:[4]

$E_i - E_f = hf$

where:

• $E_i$ is the initial, higher energy state of the nucleon
• $E_f$ is the final, lower energy state of the nucleon
• $h$ is Planck's constant
• $f$ is the frequency of the emitted radiation

In addition to radioactive nuclei, there are many objects in space that emit high levels of gamma radiation.

## Applications and Importance

Gamma rays can at times be harmful due to the fact that they are generally very high energy and therefore penetrate matter very easily. Since it penetrates so easily, it is some of the most useful radiation for medical purposes.[5]

Some of the most widely used gamma emitters are cobalt-60, cesium-137, and technetium-99m. Cesium is used widely in radiotherapy - the treatment of cancer using gamma rays - as well as being used to measure soil density at construction sites and to investigate the subterranean layers of the Earth in oil wells. Cobalt is used to sterilize medical equipment and irradiate food, killing bacteria and pasteurizing the food. Technetium-99m (which has a shorter half-life than technetium-99) is the most widely used for diagnostic medical tests to investigate the brain, bone, and internal organs. As well, exposure to gamma radiation can improve the durability of wood and plastics, and is thus used to toughen flooring in high-traffic areas.[1]

In addition, uranium-238 and uranium-235 - used in fuel for nuclear power plants - undergo both alpha and gamma decays when used. Immediately following the fission process, gamma rays are released, resulting in high levels of radiation present around the reactor. However, safety precautions are in line to ensure that workers do not get close enough to this radioactive area to be harmed.[6]

## References

1. US EPA. (May 14, 2015). Gamma Rays [Online]. Available: http://www.epa.gov/radiation/understand/gamma.html#use