The concept of activity is used as the measure of radioactivity. The activity is the average number of radioactive transformations of definite nuclei per unit time. The measurement unit of the activity is becquerel (Bq); the activity of 1 Bq is correspondent to one transformation of a radionuclide in one second. There is also off-system unit of activity named curie (Ci); 1 Ci = 3,7×10¹º Bq.
To measure the emittance of contaminated surfaces one uses the concept of flux density. This is the number of particles that cross the unit area normally to it per unit time. The flux density unit - 1/(s×m²), that is such flux density when one particle traverses during 1 second the area of 1 m².
To measure the impact of ionising radiation the absorbed dose concept is used. This is the average energy imparted to matter. The absorbed dose unit is gray (Gy); 1 Gy is referred to absorption o 1 J of ionising radiation energy by 1 kg of matter.
Absorbed dose rate specifies the intensity of the radiation impact on matter; the rate of 1 Gy/s is correspondent to absorption of the radiation energy of 1 J by 1 kg of matter in 1 second.
In the band of absorbed doses values, when the radiation causes only stochastic effects, the differences between radiation impact on various human organs and tissues in respect to types of ionising radiation and a tissue radiation sensitivity can be taken into account only onto the base of expert estimates and evaluations. It impossible to make direct experimental measurements of them. That is why the so called radiation protection quantities are introduced in the field of stochastic effects. They are briefed below.
Accounting a contribution of the type of radiation
All organs and tissues demonstrate different biological respond on different type of radiation. To take account this difference the radiation weighting factor wR is introduced. The equivalent dose in an organ, or tissue (T) is defined as the product
HT = wR x DT,
of the average absorbed dose DT in the organ or tissue (organ dose), formed by the type of radiation (R), and wR is weighting factor for this type of radiation.
Tissue radiosensitivity accounting
Human organs and tissues have different sensitivity to radiation. To take account of this difference the quantity named effective dose is introduced. The effective dose is equal to the sum over the all human organs and tissues of products of the equivalent dose for each organ or tissue and corresponding tissue weighting factor wT:
Е = ΣТ (wT x HT)
The values of weighting factors wR and wT, established in Publication 103 of the International Commission on Radiological Protection (ICRP) are of conservative kind and are intended to provide the radiation protection.
To improve the conservative estimate of the equivalent dose in cases of radiation workers individual dose and working place monitoring the simulating, so called operational quantity named as ambient dose equivalent is introduced. It is specially defined by the International Commission on Radiation Units and Measurements (ICRU), and this definition should be taken into account in measurement data processing in contemporary dosimeters.
In accordance to Publication 103 of the International Commission on Radiological Protection issued in 2007, the following annual dose limits (radiation from man-made sources additional to the natural background) are stated:
|Type of Limit||Occupational||Public|
|Effective dose||20 mSv per year, averaged over defined periods of 5 yearse||1 mSv in a yearf|
|Annual equivalent dose in:|
|Lens of the eyeb||150 mSv||15 mSv|
|Skinc,d||500 mSv||50 mSv|
|Hands and feet||500 mSv||-|
|a Limits on effective dose are for the sum of the relevant effective doses from external exposure in the specified time period and the committed effective dose is computed for a 50-year period after intake, whereas for children it is computed fo the period up to age 70 years.|
|b This limit is currently being reviewed by an ICRP Task Group.|
|c The limitation on effective dose provides sufficient protection for the skin against stochastic effects.|
|d Averaged over 1 cm2 area of skin regardless of the area exposed.|
|eWith the further provision that the effective dose should not exceed 50 mSv in any single year. Additional restrictions apply to the occupational exposure of pregnant woman.|
|f In special circumstances, a higher value of effective dose could be allowed in a single year, provided that the average over 5 years does not exceed 1 mSv per year.|
Doses at Radiation Accident Exposure
- When individual dose exceeds 100 Gy the immediate death (in first hours) takes place because of the permanent damage of nervous cells (cerebral syndrome).
- The doses of 50 – 100 Gy leads to death in 5-6 days after exposure.
- Intestinal radiation damage (gastrointestinal syndrome) is observed when the absorbed radiation dose varies in the interval of 10 – 50 Gy and leads to death on the 10th – 14th day.
The exposed patients are dwelt in sterile conditions, they are exposed to blood transfusion, their bone marrow transplantation is carried out to revive the blood production system. All these curing measures are entertained together with administration of strengthening and anti-inflammatory remedies.
The shares of different components of annual exposure from natural background at average total annual dose of 3 mSv (external + internal radiation) are shown.