Characteristics of Gamma Rays
Total photon cross sections.
Source: Wikimedia Commons
Key features of gamma rays are summarized in following few points:
- Gamma rays are high-energy photons (about 10 000 times as much energy as the visible photons), the same photons as the photons forming the visible range of the electromagnetic spectrum – light.
- Photons have no mass and no electrical charge, therefore they cannot directly ionize matter, neither gamma rays.
- Despite this fact, gamma rays ionize matter via indirect ionization.
- Although a large number of possible interactions are known, there are three key interaction mechanisms with matter.
- Gamma rays travel at the speed of light and they can travel thousands of meters in air before spending their energy.
- Since the gamma radiation is very penetrating matter, it must be shielded by very dense materials, such as lead or uranium.
- The distinction between X-rays and gamma rays is not so simple and has changed in recent decades. According to the currently valid definition, X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.
- Gamma rays frequently accompany the emission of alpha and beta radiation.
Shielding of Gamma Radiation
In short, effective shielding of gamma radiation is in most cases based on use of materials with two following material properties:
- high-density of material.
- high atomic number of material (high Z materials)
However, low-density materials and low Z materials can be compensated with increased thickness, which is as significant as density and atomic number in shielding applications.
A lead is widely used as a gamma shield. Major advantage of lead shield is in its compactness due to its higher density. On the other hand depleted uranium is much more effective due to its higher Z. Depleted uranium is used for shielding in portable gamma ray sources.
In nuclear power plants shielding of a reactor core can be provided by materials of reactor pressure vessel, reactor internals (neutron reflector). Also heavy concrete is usually used to shield both neutrons and gamma radiation.
Although water is neither high density nor high Z material, it is commonly used as gamma shields. Water provides a radiation shielding of fuel assemblies in a spent fuel pool during storage or during transports from and into the reactor core.
In general, the gamma radiation shielding is more complex and difficult than the alpha or beta radiation shielding. In order to understand comprehensively the way how a gamma ray loses its initial energy, how can be attenuated and how can be shielded we must have detailed knowledge of the its interaction mechanisms.
See also: Shielding of Gamma Radiation
See also more theory: Interaction of Gamma Radiation with Matter
See also calculator: Gamma activity to dose rate (with/without shield)
See also XCOM – photon cross-section DB: XCOM: Photon Cross Sections Database