# Gray

Physical unit
Unit name Gray
Unit symbol ${\ displaystyle \ mathrm {Gy}}$
Physical quantity (s) Energy dose , kerma
Formula symbol ${\ displaystyle D}$
dimension ${\ displaystyle {\ mathsf {L ^ {2} \; T ^ {- 2}}}}$
system International system of units
In SI units ${\ displaystyle \ mathrm {1 \, Gy = 1 \; {\ frac {J} {kg}} = 1 \; {\ frac {m ^ {2}} {s ^ {2}}}}}$
Named after Louis Harold Gray
Derived from Joules , kilograms

The gray [ ɡɹɛɪ̯ ] (unit symbol Gy ) is a quantity derived from the SI units joule and kilogram . They are caused by ionizing radiation caused energy dose and describes the per unit mass energy absorbed. The kerma is also measured in gray.

The Gray is named after the British physicist and father of radiobiology , Louis Harold Gray . It is mainly used in medicine , for example for specifying the radiation dose used in radiation therapy or nuclear medicine therapy.

## definition

The gray was defined as an SI unit in 1975:

${\ displaystyle \ mathrm {1 \, Gy = 1 \; {\ frac {J} {kg}}}}$

## Relation to other units

### Sievert

In order to be able to better compare the absorbed dose of different types of radiation with regard to the harmful effect on organisms, it is multiplied by a weighting factor, the radiation weighting factor . It takes into account the relative biological effectiveness of the radiation. Since the radiation weighting factor is a dimensionless number, the dose equivalent and the absorbed dose have the same dimensions . However, in order to emphasize the difference between the dose sizes for practical use and since the numerical value of the dose size can also change due to the radiation weighting factor, Gray is used for energy doses and Sievert for weighted dose sizes . The term J / kg should not be used.

### Wheel (obsolete unit)

Until December 31, 1985, the official unit of absorbed dose was the rad (unit symbol rd ).

${\ displaystyle \ mathrm {1 \, rd = 0 {,} 01 \, Gy = 0 {,} 01 \, {\ frac {m ^ {2}} {s ^ {2}}} = 0 {,} 01 \, {\ frac {J} {kg}}}}$