Quantifying Radiation

Unit (Symbol)DefinitionUnits
Exposure (X)
Amount of ionization per mass of air due to X-rays and gamma rays.Roentgen (R)
Absorbed Dose (D)
Amount of energy imparted by radiation per unit mass.SI: Gray (Gy)

Traditional: rad
1 rad = 0.01Gy
Kerma (K)
Kinetic Energy Released per unit MAss
Kinetic energy transferred to charged particles per unit mass.
SI: Gray (Gy)

Traditional: rad
1 rad = 0.01Gy
Equivalent Dose (HT)
A measure of absorbed dose weighted for the biological effectiveness
of the types(s) of radiation (relative to the low LET photons and electrons)
to produce stochastic health effects in humans.
SI: Sievert (Sv)

Traditional: rem
1 rem = 0.01Sv
Dose Equivalent (H)
A measure of absorbed dose weighted for the biological effectiveness of
the types of radiation (relative to low LET photons and electrons) to
produce stochastic health effects in humans.
Sievert (Sv)

Traditional: rem
1 rem = 0.01Sv
Effective Dose (E)
A measure of equivalent dose, weighted for the biological sensitivity of
the exposed tissues and organs (relative to whole body exposure) to
stochastic health effects in humans.
Sievert (Sv)

Traditional: rem
1 rem = 0.01Sv
Effective Dose
Equivalent (HE)

A measure of equivalent dose, weighted for the biological sensitivity of
the exposed tissues and organs (relative to whole body exposure) to
stochastic health effects in humans.
Sievert (Sv)

Traditional: rem
1 rem = 0.01Sv

Exposure

Exposure (X) is defined by ICRU as the quotient of dQ by dm where dQ is the absolute value of the total charge of the ions of one sign produced in air when all electrons liberated by photons in air of mass dm are completely stopped in air. Exposure is only defined up to about 3MeV because of the size of a free air ionization chamber required for higher energy measurements.

Fluence/Flux

Fluence (Φ) is a measure of the total number of quanta which enter an area.

Flux (φ) is a measure of the rate at which quanta enter an area per unit time. That means that flux is the time derivative of fluence.

Linear Attenuation Coefficient

Linear attenuation coefficient (μ) is the fraction of a photon beam which is attenuated per unit path length (units: cm-1).

  • I(x) is the intensity exciting the material
  • I(0) is the incident intensity

Absorbed Dose (D)

Absorbed dose is the energy deposited per unit mass to a medium by radiation.

Dose to matter

Dose to air (Dair) may be found directly from exposure (X) and the roentgen-to-cGy conversion factor fair as in the below equation.

Dose to medium (Dmed) may be found directly using the below equation where fmed is the f-factor and A is a transmission factor. In reality, dose is often measured in a gas filled ionization chamber which requires Cavity Theory to convert Dair to Dmed.

Key Point: Ion chambers placed in medium collect charge in air which is then converted, through application of Cavity Theory, to the dose to the medium.

KERMA (K)

KERMA, which stands for Kinetic Energy Released per unit MAss, is the quotient of the initial kinetic energies of all charge ionizing particles liberated by uncharged particles (photons) per mass of material.

KERMA is divided into two components:

  • Collision KERMA (Kcoll) is the amount of energy released in collision type interactions.
    • Under conditions of Charged Particle Equilibrium (CPE), KColl = Absorbed Dose.
    • In practice, because of scattering and attenuation, CPE is never established but Transient Charged Particle Equilibrium is established.
    • Under TCPE: KColl ∝ Absorbed Dose
  • Radiative KERMA (Krad) is the amount of energy released in radiative type interactions.

TERMA

Total Energy Released per unit MAss, TEMRA, is similar to KERMA but includes energy losses due to Coherent (Rayleigh) scattering. For MV beams, Coherent scattering is negligible, so KERMA = TERMA.

Equivalent Dose (H)

Equivalent dose represents the stochastic health effects of low levels of ionizing radiation to the human body.

Radiation Weighting Factors

Radiation TypeWR
X-rays, gamma rays, electrons and muons1
Neutrons
  • <10keV: 5
  • 10-100keV: 10
  • 100keV - 2MeV: 20
  • 2MeV-20MeV: 10
  • >20Mev: 5
Protons > 2MeV2
Alpha particles, fission fragments and non-relativistic heavy nuclei20
Source: ICRP 60

Key Point: Although neutrons are indirectly ionizing, they have a high radiation weighting factor. This is because they transfer their energy to protons which have a high LET due to their charge and large mass.

Effective Dose (E)

Effective dose uses tissue weighting factors to compare the risk of stochastic effects from a specific distribution of doses to organs with the risk of stochastic effects of a uniform whole body dose. This is achieved by multiplying equivalent dose (HT) to a tissue with a tissue weighting factor (wTand summing for all tissues.

Because the tissue weighting factors (wT) were developed based on stochastic risks for a generic population of diversified age and gender, values of effective dose represent stochastic risk to a generic population rather than to any individual. Further, because these values are somewhat subjective, tissue weighting factors vary between the reports of different agencies.

Tissue Weighting Factors (WT)

OrganWT
Gonads0.20
Red bone marrow
Colon
Lung
Stomach
0.12
Bladder
Breast
Liver
Esophagus
Thyroid
0.05
Skin
Bone surface
0.1
Remainder0.05
Total1.0
Data from Table 1.1 of NCRP 116

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