Kerma (physics)
Encyclopedia
Kerma is the sum of the initial kinetic energies
of all the charged particle
s liberated by uncharged ionizing radiation
(i.e., indirectly ionizing radiation such as photon
s and neutron
s) in a sample of matter
, divided by the mass
of the sample. It is defined by the quotient
.
Kerma is different from absorbed dose
, according to the energies involved. Whilst roughly equal at low energies, kerma is much higher than absorbed dose at higher energies, as some of the energy escapes from the absorbing volume in the form of bremsstrahlung
X-rays or fast moving electrons.
The unit for kerma is joule
per kilogram
(gray
(Gy)), which is the same as for absorbed dose.
The word "kerma" is an acronym for "kinetic energy released in material", "kinetic energy released in matter", or sometimes "kinetic energy released per unit mass".
The photon energy is transferred to matter in a two-step process. First, energy is transferred to the secondary charged particles through various photon interactions (e.g. photoelectric effect
, Compton scattering
, pair production
and photonuclear excitation). Next, these secondary charged particles transfer the energy to the medium through atomic excitation and ionizations.
For low energy photons, kerma is numerically approximately the same as absorbed dose
; however, for higher energy photons it starts to differ. This is because the extremely energetic electrons produced may deposit some of their energy outside the region of interest, or some may lose their energy through bremsstrahlung
. This energy would be counted in kerma, but not in absorbed dose. For low x-ray energies, this is usually a negligible distinction. This can be understood when one looks at the components of kerma.
In fact, kerma has two parts to it: Collision kerma
and radiative kerma 
. i.e. 
. Collision kerma results in the production of electrons that dissipate their energy as ionization due to the interaction between the charged particle and the atomic electrons. Radiative kerma results in the production of radiative photons due to the interaction between the charged particle and the atomic nuclei, but can also result from annihilation in flight.
Frequently, the quantity
is of interest, and is usually expressed as
,
where
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
of all the charged particle
Charged particle
In physics, a charged particle is a particle with an electric charge. It may be either a subatomic particle or an ion. A collection of charged particles, or even a gas containing a proportion of charged particles, is called a plasma, which is called the fourth state of matter because its...
s liberated by uncharged ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
(i.e., indirectly ionizing radiation such as photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s and neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s) in a sample of matter
Matter
Matter is a general term for the substance of which all physical objects consist. Typically, matter includes atoms and other particles which have mass. A common way of defining matter is as anything that has mass and occupies volume...
, divided by the mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
of the sample. It is defined by the quotient
Quotient
In mathematics, a quotient is the result of division. For example, when dividing 6 by 3, the quotient is 2, while 6 is called the dividend, and 3 the divisor. The quotient further is expressed as the number of times the divisor divides into the dividend e.g. The quotient of 6 and 2 is also 3.A...
.Kerma is different from absorbed dose
Absorbed dose
Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation per unit mass...
, according to the energies involved. Whilst roughly equal at low energies, kerma is much higher than absorbed dose at higher energies, as some of the energy escapes from the absorbing volume in the form of bremsstrahlung
Bremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
X-rays or fast moving electrons.
The unit for kerma is joule
Joule
The joule ; symbol J) is a derived unit of energy or work in the International System of Units. It is equal to the energy expended in applying a force of one newton through a distance of one metre , or in passing an electric current of one ampere through a resistance of one ohm for one second...
per kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
(gray
Gray (unit)
The gray is the SI unit of absorbed radiation dose of ionizing radiation , and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter ....
(Gy)), which is the same as for absorbed dose.
The word "kerma" is an acronym for "kinetic energy released in material", "kinetic energy released in matter", or sometimes "kinetic energy released per unit mass".
The photon energy is transferred to matter in a two-step process. First, energy is transferred to the secondary charged particles through various photon interactions (e.g. photoelectric effect
Photoelectric effect
In the photoelectric effect, electrons are emitted from matter as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as photoelectrons...
, Compton scattering
Compton scattering
In physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect...
, pair production
Pair production
Pair production refers to the creation of an elementary particle and its antiparticle, usually from a photon . For example an electron and its antiparticle, the positron, may be created...
and photonuclear excitation). Next, these secondary charged particles transfer the energy to the medium through atomic excitation and ionizations.
For low energy photons, kerma is numerically approximately the same as absorbed dose
Absorbed dose
Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation per unit mass...
; however, for higher energy photons it starts to differ. This is because the extremely energetic electrons produced may deposit some of their energy outside the region of interest, or some may lose their energy through bremsstrahlung
Bremsstrahlung
Bremsstrahlung is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon because energy is conserved. The term is...
. This energy would be counted in kerma, but not in absorbed dose. For low x-ray energies, this is usually a negligible distinction. This can be understood when one looks at the components of kerma.
In fact, kerma has two parts to it: Collision kerma
and radiative kerma . i.e. . Collision kerma results in the production of electrons that dissipate their energy as ionization due to the interaction between the charged particle and the atomic electrons. Radiative kerma results in the production of radiative photons due to the interaction between the charged particle and the atomic nuclei, but can also result from annihilation in flight.Frequently, the quantity
is of interest, and is usually expressed as,where
- g is the average fraction of energy transferred to electrons that is lost through bremsstrahlung.