Radiation Units Guide

A definitive reference for understanding radiation measurement, from Source to Body.

Nuclear units can be confusing because we measure the same event at four different stages: the source, the air, the object, and finally, the human body.

Activity

How active is the source?

Measures the rate at which a radioactive source emits radiation. It's a count of how many atoms decay per second.

Like the brightness of a light bulb (lumens).
SI Standard
Becquerel (Bq)
1 decay per second
Tiny unit. Usually seen as kBq, MBq, or GBq.
Legacy (US)
Curie (Ci)
37 billion decays per second (based on 1g of Radium)
Huge unit. Usually seen as µCi or mCi.
1 Ci = 37 GBq

Exposure

How much is in the air?

Measures the amount of ionization produced in air by X-rays or gamma rays. It specifically looks at the electric charge created.

Like the amount of light falling on a surface (lux).
SI Standard
Coulomb/kg (C/kg)
Electric charge per kilogram of air
Rarely used in practice outside of physics labs.
Legacy (US)
Roentgen (R)
Amount of ionization in a volume of air
Still very common in US portable meters.
1 R ≈ 0.000258 C/kg

Absorbed Dose

How much energy hit the object?

Measures the actual energy deposited by radiation into a material mass. This tells us how much energy was transferred.

Like the heat absorbed by an object under a lamp.
SI Standard
Gray (Gy)
1 Joule of energy absorbed per 1 kg of matter
The standard international unit.
Legacy (US)
Rad
0.01 Joules per kg
Still used in some US medical & industrial settings.
1 Gy = 100 Rad

Equivalent Dose

How dangerous is it to humans?

Takes the absorbed dose and weighs it by the biological damage potential of the specific radiation type (alpha, beta, gamma).

Like how bad a sunburn you get (UV index).
SI Standard
Sievert (Sv)
Absorbed Dose (Gy) × Radiation Weighting Factor
The gold standard for safety limits.
Legacy (US)
Rem
Absorbed Dose (Rad) × Quality Factor
Standard unit in the US nuclear industry.
1 Sv = 100 Rem