# Sievert (unit)

Physical unit
Unit name Sievert
Unit symbol ${\ displaystyle \ mathrm {Sv}}$
Physical quantity (s) Equivalent dose
Formula symbol ${\ displaystyle H \,}$, ${\ displaystyle E \,}$
dimension ${\ displaystyle {\ mathsf {L ^ {2} \; T ^ {- 2}}}}$
system International system of units
In SI units ${\ displaystyle \ mathrm {1 \, Sv = 1 \; {\ frac {J} {kg}} = 1 \; {\ frac {m ^ {2}} {s ^ {2}}}}}$
Named after Rolf Sievert
Derived from Joules , kilograms

The Sievert (unit symbol: “Sv”), named after the Swedish physician and physicist Rolf Sievert , is a unit of measurement that is used in radiation protection against ionizing radiation . It is used as a unit of measurement for the equivalent dose and thus serves to quantify radiation exposure with regard to stochastic risks (cancer and inheritable defects).

Since a dose of 1 Sv is a very large value, the values ​​that usually occur are specified in millisievert (mSv) or microsievert (μSv) using a prefix for units of measurement (SI prefix) .

## definition

The unit of measure Sievert is defined as

${\ displaystyle \ mathrm {1 \, Sv = 1 \, {\ frac {J} {kg}}} \ ,.}$

It thus has the same definition as the unit of measurement Gray (1 Gy = 1 J / kg), the unit of energy dose .

The stochastic risks of ionizing radiation depend not only on the absorbed dose it transfers to the exposed biological tissue, but also on the properties of the types of radiation involved and on the sensitivity of the tissue concerned. The dose equivalent takes all these dependencies into account by multiplying the absorbed dose with one or more dimensionless ratios (weighting factors). The name “Sievert” is therefore used to distinguish it, indicating that the weighted dose size is meant. The term J / kg should not be used, neither for energy nor for dose equivalent.

## scope of application

The Sievert is used for the following dose sizes (for details see the article " Equivalent dose "):

• Equivalent dose as dose measure,
• Organ equivalent dose,
• Effective dose,
• Follow-up organ equivalent dose,
• Effective follow-up dose.

Dose information in Sievert is used in radiation protection in a dose range of up to a few 100 mSv, where stochastic effects are known to occur or (at low doses) are suspected and where deterministic effects are not yet decisive. In the case of significantly higher doses with the then decisive deterministic effects, radiation doses are specified solely in the form of the absorbed dose with its unit of measurement Gray. A typical area of ​​application for this is patient doses in the context of radiation therapy.

When different types of radiation act on a tissue, the dose information used in Sievert has the advantage that they can be compared directly with one another with regard to the stochastic risk. They can also be added to each other, and the sum expresses the total risk associated with the impact.

## history

The introduction of the Sievert was decided in 1978 at the 16th General Conference on Weights and Measures . It was expressly noted that the introduction of new unit names should be handled very restrictively, but in this special case it should be justified because confusing the energy dose and the equivalent dose could have fatal consequences.

The Sievert replaced the unit Rem (rem) (1 Sv = 100 rem). The Rem is no longer a legal entity in the EU and Switzerland .