The rate at which nuclear transformations (disintegration) occur in a radioactive material. The SI unit for activity is reciprocal second (s-1), is called the Becquerel (Bq). 1 Bq = 1 disintegration per second (dps). The classical unit to express the amount of radioactivity is Curie. One Curie = 3.7 x 1010Bq.

I milliCi (mCi) = 3.7 x 107Bq; 1microCurie (µCi) = 3.7 x 104Bq; 1 MegaBq (MBq) = 106 Bq; 1 GigaBq (GBq) = 109 Bq, 1 TeraBq (TBq) = 1012 Bq

Specific activity of a radioisotope (called as radionuclide) is the activity per unit mass or volume of the at nuclide. The specific activity of a material is the activity per unit mass or volume of the material in which the radionuclides are uniformly distributed.

Dose

A measure of the energy deposited by radiation in a absorber.

Absorbed dose (D)

Energy absorbed by a unit mass of a substance from the radiation to which it is exposed is called Absorbed dose. The unit is the joule per kilogram, called the Gray, Gy. Old unit for absorbed dose is Radiation Absorbed Dose (rad) (1 rad = absorption of 100 erg / g of the material). Conversion factor for converting old unit to new unit is 1 Gy = 100 rad.

Equivalent dose (H)

Equivalent dose is a measure of the dose to a tissue or organ designed to reflect the amount of harm caused to the tissue or organ.

Equivalent dose is used for radiation protection purposes to account for the differences in the effectiveness in damaging human tissue for different types of ionizing radiation. The absorbed dose (in Gray) multiplied by a Radiation Weighting Factor (WR) gives equivalent dose (in Sievert). Higher the weighting factor, the radiation is more biologically harmful. H = D x WR

(WR)
(ICRP-103, 2007)
Photons 1
Electrons and muons 1
Protons and charged pions 2
Alpha particles,
fission fragments, heavy ions
20
Neutrons* A continuous curve as a
function of neutron energy

Thus, X-rays, beta radiation and gamma rays have a weighting factor of 1, while alpha rays have a weighting factor of 20. Alpha rays are 20 times more biologically harmful than X-rays and gamma rays.

Effective dose (E)

The quantity E, defined as a summation of the tissue or organ equivalent doses, each multiplied by the appropriate tissue weighting factor: The tissue weighting factor (WT) takes into account the differences in the potentials for causing stochastic effect due to the equivalent dose to the tissues or organs (T). The sum total of such weighted equivalent doses for all exposed tissues in an individual is called the effective dose.

The unit of equivalent dose and effective dose is the same as the unit of absorbed dose, namely joule per kilogram, but a special name, Sievert (Sv) is used for these units to avoid confusion with the unit of absorbed dose.

E = H x ∑WT

Committed equivalent/effective dose

When a radionuclide goes inside the body, the dose is received throughout the period of time during which the activity remains in the body. The Committed dose is the total dose delivered to tissues/organs during this period of time, which can be defined as the specified time integral of the rate of receipt of the equivalent /effective dose.

Intake of radionuclide in a year, which gives a committed effective dose (CED) of 20 mSv in 50 years post-intake, is called Annual Limit on Intake (ALI). This part of the internal dose to be added to the external dose to check dose limit compliance of the workers.

Tissue Weighting Factors, WT (ICRP-103, 2007)

Organ/tissue Tissue weighting factor
(WT)
ICRP-103, 2007
Redbone marrow 0.12
Colon 0.12
Lung 0.12
Stomach 0.12
Breast 0.12
Liver 0.04
Oesophagus 0.04
Thyroid 0.04
Skin 0.01
Bone surfaces 0.01
Salivary glands 0.01
Brain 0.01
Remainder* 0.12

∑WT  = 1.00.
*Remainder consists of 13 other tissues/organs

Collective dose (S)

It is sometimes useful to have measure of the total dose received by the groups of people or to whole population. The total dose depends on the number of individuals exposed and on the average dose from all sources of radiation received by the exposed group of population. The collective dose is defined as the summation of the products of the mean dose and the number of individuals in the each group. The unit of collective dose is man-Sievert (man.Sv) or person.Sv.