Half-Value Layer
Encyclopedia
Half-value layer (also half-value thickness) is the thickness of specified material which reduces the intensity
of radiation
entering the material by half. This is also expressed in terms of the air kerma rate (AKR): the half-value layer is "the thickness of specified material which attenuates the beam of radiation to an extent such that the AKR is reduced to one-half of its original value. In this definition the contribution of all scattered radiation, other than any which might be present initially in the beam concerned, is deemed to be excluded." Rather than air kerma rate, measurements of air kerma, exposure, or exposure rate can also be used to determine half value layer, so long as it is given in the description.
Half-value layer refers to the first half-value layer, where subsequent (i.e. second) half-value layers refer to the amount of specified material that will reduce the air kerma rate by one-half after material has been inserted into the beam that is equal to the sum of all previous half-value layers.
Quarter-value layer is the amount of specified material that reduces the air kerma rate (or exposure rate, exposure, air kerma, etc...) to one fourth of the value obtained without any test filters. The quarter-value layer is equal to the sum of the first and second half-value layers.
The homogeneity factor (HF) describes the polychromatic nature of the beam and is given by:
The HF will always be less than or equal to one (it is only equal to one in the case of a monoenergetic beam). In case of a polychromatic beam, the HF is less than one because of beam hardening.
HVL is related to Mean free path
, however the mean free path is the average distance a unit of radiation can travel in the material before being absorbed, whereas HVL is the average amount of material needed to absorb 50% of all radiation (i.e., to reduce the intensity of the incident radiation by half).
Intensity (physics)
In physics, intensity is a measure of the energy flux, averaged over the period of the wave. The word "intensity" here is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech...
of radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
entering the material by half. This is also expressed in terms of the air kerma rate (AKR): the half-value layer is "the thickness of specified material which attenuates the beam of radiation to an extent such that the AKR is reduced to one-half of its original value. In this definition the contribution of all scattered radiation, other than any which might be present initially in the beam concerned, is deemed to be excluded." Rather than air kerma rate, measurements of air kerma, exposure, or exposure rate can also be used to determine half value layer, so long as it is given in the description.
Half-value layer refers to the first half-value layer, where subsequent (i.e. second) half-value layers refer to the amount of specified material that will reduce the air kerma rate by one-half after material has been inserted into the beam that is equal to the sum of all previous half-value layers.
Quarter-value layer is the amount of specified material that reduces the air kerma rate (or exposure rate, exposure, air kerma, etc...) to one fourth of the value obtained without any test filters. The quarter-value layer is equal to the sum of the first and second half-value layers.
The homogeneity factor (HF) describes the polychromatic nature of the beam and is given by:
The HF will always be less than or equal to one (it is only equal to one in the case of a monoenergetic beam). In case of a polychromatic beam, the HF is less than one because of beam hardening.
HVL is related to Mean free path
Mean free path
In physics, the mean free path is the average distance covered by a moving particle between successive impacts which modify its direction or energy or other particle properties.-Derivation:...
, however the mean free path is the average distance a unit of radiation can travel in the material before being absorbed, whereas HVL is the average amount of material needed to absorb 50% of all radiation (i.e., to reduce the intensity of the incident radiation by half).