There are several aspects of radiation that are quantified with units, including:
- Absorbed dose
- Equivalent dose
- Effective dose
- Committed dose
- Collective Effective dose
| ||Radioactivity|| Absorbed|
|Common Units||curie (Ci)||rad||rem||roentgen (R)|
|SI Units||becquerel (Bq)||gray (Gy)||sievert (Sv)||coulomb/kilogram (C/kg)|
Activity is the transformation (disintegration) rate of a radioactive substance. In SI units, the activity of a radioactive source is measured in becquerels (symbol Bq), where one becquerel is equal to one nuclear disintegration per second (an older unit is the curie, where one curie is 37 billion Bq)
1 Bq = 1 disintegration per second (dps)
1 Ci = 3.7 x 1010 dps
1 Ci = 3.7 x 1010 Bq
Since the Bq represents such a small amount, you are likely to see a prefix used with Bq, as shown below:
- 1 MBq (27 microcuries)
- 1 GBq (27 millicuries)
- 37 GBq (1 curie)
- 1 TBq (27 curies)
Materials vary greatly in their radioactivity. A large amount of material can have a very small amount of radioactivity; a very small amount of material can have a lot of radioactivity. For example, uranium-238 has 0.00015 curies of radioactivity per pound (0.15 millicuries), while cobalt-60 has nearly 518,000 curies per pound.
The device used for measurement is often the familiar Geiger counter. If you put a Geiger counter over a gram of substance and count 3 clicks per second, the radioactivity of that substance would be 3 Becquerel.
Intensity of Radiation
The intensity of radiation is defined as the rate of emitted energy from unit surface area through unit solid angle. The röntgen or roentgen (R) is a measure of radiation intensity of xrays or gamma rays. It is formally defined as the radiation intensity required to produce and ionization charge of 0.000258 coulombs per kilogram of air. It is one of the standard units for radiation dosimitry, but is not applicable to alpha, beta, or other particle emission and does not accurately predict the tissue effects of gamma rays of extremely high energies. The röntgen has mainly been used for calibration of xray machines.
The röntgen was occasionally used to measure exposure to radiation in other forms than X-rays or gamma rays. To adjust for the different impact of different forms of radiation on biological matter, the "röntgen equivalent man" or rem came into use. Exposure in rems is equal to the exposure in röntgens multiplied by the Q value, a constant describing the type of radiation. The rem is now superseded by the sievert.
Radiation exposure is expressed in several ways to account for the different levels of harm caused by different forms of radiation and the different sensitivity of body tissues.
Absorbed dose of radiation
Absorbed dose is the amount of energy of an ionizing radiation absorbed per unit mass of material. The rad is a unit of absorbed radiation dose in terms of the energy actually deposited in the tissue. The rad is defined as an absorbed dose of 0.01 joules of energy per kilogram of tissue. The more recent SI unit is the gray, which is defined as 1 joule of deposited energy per kilogram of tissue. To assess the risk of radiation, the absorbed dose is multiplied by the relative biological effectiveness of the radiation to get the biological dose equivalent in rems or sieverts.
The gray is a large unit and for normal radiation protection levels a series of prefixes are used:
- nanogray (nGy) is one thousand millionth of a gray (1/1,000,000,000)
- microgray (µGy) is one millionth of a gray (1/1,000,000)
- milligray (mGy) is one thousandth of a gray (1/1,000)
The absorbed dose is not a good indicator of the likely biological effect of radiation. For example, 1 Gy of alpha radiation would be much more biologically damaging than 1 Gy of photon radiation for example. Appropriate weighting factors can be applied reflecting the different relative biological effects to find the biologically effective dose (see below).
Equivalent dose relates the absorbed dose in human tissue to the effective biological damage of the radiation. Not all radiation has the same biological effect, even for the same amount of absorbed dose. Equivalent dose is measured in an SI unit called the Sievert (Sv). Like the gray, the sievert is a large unit and for normal radiation protection levels a series of prefixes are used:
- nanoSievert (nSv) is one thousand millionth of a Sievert (1/1,000,000,000)
- microSievert (µSv) is one millionth of a Sievert (1/1,000,000)
- milliSievert (mSv) is one thousandth of a Sievert (1/1,000)
To determine equivalent dose (Sv), you multiply absorbed dose (Gy) by a radiation weighting factor that is unique to the type of radiation. The radiation weighting factor WR takes into account that some kinds of radiation are inherently more dangerous to biological tissue, even if their "energy deposition" levels are the same. (An older weighting factor is the quality factor).
For x-rays and gamma rays and electrons absorbed by human tissue, WR is 1. For alpha particles it is 20. To compute Sieverts from Grays, simply multiply by WR. This is obviously a simplification. The radiation weighting factor WR approximates what otherwise would be very complicated computations. The values for WR change periodically as new research refines the approximations.
Exposure occurs over time, of course. The more Sieverts absorbed in a unit of time, the more intense the exposure. And so we express actual exposure as an amount over a specific time period, such as 5 millisieverts per year. This is called the "dosage rate". In Australia, for example, the dosage rate from background radiation, the sum of all natural radiation, is about 2 millisieverts per year.
|Tissue Weighting Factors (WT) for Individual Tissues and Organs|
|Tissue or Organ||Tissue Weighting Factor (WT)|
|Gonads (testes or ovaries)||0.20|
|Red bone marrow||0.12|
|** The remainder is composed of the following additional tissues and organs: adrenal, brain, upper large intestine, small intestine, kidney, muscle, pancreas, spleen, thymus and uterus.|
Effective dose is a measure of dose in which the type of radiation and the sensitivity of tissues and organs to that radiation is taken into account. The probability of a harmful effect from radiation exposure depends on what part or parts of the body are exposed. Some organs are more sensitive to radiation than others. A tissue weighting factor (wT) is used to take this into account. When an equivalent dose to an organ is multiplied by the (wT) for that organ, the result is the effective dose to that organ:
Effective dose = sum of [organ doses x tissue weighting factor]
The unit of effective dose is the sievert (Sv).
When a radioactive material is gets in the body by inhalation or ingestion, the radiation dose constantly accumulates in an organ or a tissue. The total dose accumulated during the 50 years following the intake is called the committed dose. The quantity of committed dose depends on the amount of ingested radioactive material and the time it stays inside the body.
Collective effective dose
Collective effective dose is the product of the mean effective dose in a group and the number of individuals in that group. With some reservations, it can be thought of as representing the total consequences of the exposure of a population or group.