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There are many different units used to describe radiation.

Everything from information about safe dose levels for humans to Geiger counter displays use many varying units to express the radiation received and emitted from things that are radioactive.

Below are listed and explained the most common units still in use today.

Roentgen (R)

The roentgen is a unit used to measure a quantity called exposure. This can only be used to describe an amount of gamma and X-rays, and only in air. It is a measure of the ionizations of the molecules in a mass of air. The main advantage of this unit is that it is easy to measure directly, but it is limited because it is only for deposition in air, and only for gamma and x rays.

Rad (radiation absorbed dose)

The rad is a unit used to measure a quantity called absorbed dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. The unit rad can be used for any type of radiation, but it does not describe the biological effects of  radiation.

Rem (roentgen equivalent man)

The rem is a unit used to derive a quantity called equivalent dose. This 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 often expressed in terms of thousandths of a rem, or mrem. To determine equivalent dose (rem), you multiply absorbed dose (rad) by a quality factor (Q) that is unique to the type of radiation.

Curie (Ci)

The curie is a unit used to measure a radioactivity. One curie is that quantity of a radioactive material that will have 37,000,000,000 transformations (decays)  in one second. Often radioactivity is expressed in smaller units like: thousandths (mCi), one millionths (uCi) or even billionths (nCi) of a curie. The relationship between becquerels and curies is: 3.7 x 1010 Bq in one curie.

Gray (Gy)

The gray is a unit used to measure a quantity called absorbed dose. This relates to the amount of energy actually absorbed in some material, and is used for any type of radiation and any material. One gray is equal to one joule of energy deposited in one kg of a material. The unit gray can be used for any type of radiation, but it does not describe the biological effects of the different radiations. Absorbed dose is often expressed in terms of hundredths of a gray, or centi-grays. One gray is equivalent to 100 rads.

Sievert (Sv)

The sievert is a unit used to derive a quantity called equivalent dose. This 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 often expressed in terms of millionths of a sievert, or micro-sievert. To determine equivalent dose (Sv), you multiply absorbed dose (Gy) by a quality factor (Q) that is unique to the type of radiation. One sievert is equivalent to 100 rem.

Becquerel (Bq)

The Becquerel is a unit used to measure a radioactivity. One Becquerel is that quantity of a radioactive material that will have 1 transformations in one second. Often radioactivity is expressed in larger units like: thousands (kBq), one millions (MBq) or even billions (GBq) of a becquerels. As a result of having one Becquerel being equal to one transformation per second, there are 3.7 x 1010 Bq in one curie.

Note : A common measurement of the presence of radon in a structure is given in Becquerels per cubic meter (Bq/m3). Measuring becquerels per cubic meter with a Geiger counter is not possible because the counter only measures the radiation the probe encounters rather than the radiation existing in a cubic meter of air.

Some examples of radiation doses

6000 mSv  

The dose which may lead to death when received all at once

1000 mSv  

The dose which may cause symptoms of a radiation sickness (e.g. tiredness and nausea) if received within 24 hours

100 mSv  

The highest permitted dose for a radiation worker over a period of five years

4 mSv  

The average annual radiation dose for Finns caused by indoor radon, X-ray examinations, etc

2 mSv  

The annual dose of cosmic radiation received by a person working in an plane

0.1 mSv  

The radiation dose received by a patient having his/her lungs X-rayed

0.01 mSv  

The radiation dose received by a patient having his/her teeth X-rayed

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