**Absorbed dose** is defined as the amount of energy deposited by ionizing radiation in a substance. **Absorbed dose** is given the symbol **D**. The** rad** (an abbreviation for **R**adiation **A**bsorbed **D**ose) is the **non-SI unit** of the absorbed dose. The absorbed dose is also measured in a unit called the gray (Gy), which is derived from the SI system. The non-SI unit **rad** is used predominantly in the USA.

Units of Absorbed Dose:

- Gray. A dose of one gray is equivalent to a unit of energy (joule) deposited in a kilogram of a substance.
- RAD. A dose of one rad is equivalent to the deposition of one hundred ergs of energy in one gram of any material.

## RAD – Unit of Absorbed Dose

A dose of **one rad** is equivalent to the deposition of **one hundred ergs** of energy in **one gram** of any material. Note that, the erg is a unit of energy and work equal to 10^{−7} joules. A related unit, the roentgen, is used to quantify the radiation exposure. The F-factor can be used to convert between rads and roentgens.

One rad is significantly lower dose than one gray, which is a large amount of absorbed dose. A person who has absorbed a whole body dose of 100 rad has absorbed one joule of energy in each kg of body tissue (i.e. 1 Gy). Absorbed doses measured in industry (except nuclear medicine) often have comparable doses to one rad and the following multiples are often used:

**1 mrad (millirad) = 1E-3 rad**

**1 krad (kilorad) = 1E3 rad**

Conversions from the SI units to other units are as follows:

- 1 Gy = 100 rad
- 1 mGy = 100 mrad

The gray and rad are physical units. They describe the physical effect of the incident radiation (i.e., the amount of energy deposited per kg), but it tells us nothing about the biological consequences of such energy deposition in living tissue.

## Gray – Unit of Absorbed Dose

A dose of **one gray** is equivalent to a unit of energy (joule) deposited in a kilogram of a substance. This unit was named in honour of **Louis Harold Gray**, who was one of the great pioneers in radiation biology. One gray is a large amount of absorbed dose. A person who has absorbed a whole body dose of 1 Gy has absorbed one joule of energy in each kg of body tissue.

**Absorbed doses** measured in industry (except nuclear medicine) often have usually lower doses than one gray, and the following multiples are often used:

**1 mGy (milligray) = 1E-3 Gy**

**1 µGy (microgray) = 1E-6 Gy**

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