Operational Quantities
For all types of external radiation, the operational quantities for area monitoring are defined based on a dose equivalent value at a point in the ICRU sphere. This sphere is made of tissue-equivalent material (30 cm in diameter, with a density of 1 g/cm³), and has a mass composition of: 76.2% oxygen, 11.1% carbon, 10.1% hydrogen, and 2.6% nitrogen. For radiation monitoring, this sphere adequately approximates the human body regarding the scattering and attenuation of radiation fields under consideration.
The operational quantities for area monitoring defined in the ICRU sphere retain their character as a point quantity. This is achieved by introducing the concepts of expanded and aligned radiation fields:
- Expanded radiation field: A hypothetical field where the spectral and angular fluence values are the same at all points within a sufficiently large volume, equal to the values at the point of interest in the actual field. This ensures the ICRU sphere is exposed to a homogeneous radiation field with consistent fluence, energy distribution, and direction distribution as in the real field.
- Aligned radiation field: In the expanded field, all radiation is aligned so that it is directed opposite to a radius vector Ω specified for the ICRU sphere. In this field, the ICRU sphere is homogeneously irradiated from one direction, and the fluence is the integral of the angular differential fluence at the point of interest, summed over all directions.
For area monitoring, the operational quantities are as follows:
Ambient Dose Equivalent (H*(10))
The ambient dose equivalent at a point in a radiation field is the dose equivalent that would be produced by the corresponding expanded and aligned field in the ICRU sphere at a depth of 10 mm along the radius vector opposing the direction of the aligned field.
Directional Dose Equivalent (H'(0.07, Ω))
The directional dose equivalent at a point in a radiation field is the dose equivalent that would be produced by the corresponding expanded field in the ICRU sphere at a depth of 0.07 mm in a specified direction Ω.
Personal Dose Equivalent (Hp(d))
The personal dose equivalent is the equivalent dose at a specified depth d in soft tissue below a point on the body.
- d = 10 mm: For penetrating radiations (photon energies above 15 keV),
- d = 0.07 mm: For weakly penetrating radiations (photon energies below 15 keV) in the skin,
- d = 3 mm: For weakly penetrating radiations (photon energies below 15 keV) in the lens of the eye.
The operational quantities for area monitoring are essential in radiation protection as they allow for a consistent assessment of radiation exposure across different fields and conditions. By using the ICRU sphere as a reference, it provides a standardized approach to measuring and monitoring radiation dose equivalents.
Summary of Radiation Protection Quantities and Units
| Quantity | Unit | Description |
|---|---|---|
| Mean Organ Dose (DT) | Gray (Gy) = J/kg | Measures the energy imparted to a specific organ or tissue from radiation. |
| Equivalent Dose (HT) | Sievert (Sv) | Accounts for the biological effectiveness of different types of radiation. |
| Effective Dose (E) | Sievert (Sv) | Summation of the equivalent doses to various organs, weighted by tissue sensitivity. |
| Ambient Dose Equivalent (HA) | Sievert (Sv) | Measures the radiation dose in the environment. |
| Personal Dose Equivalent (HP) | Sievert (Sv) | Monitors the dose received by an individual from external radiation sources. |