Radiation Dose Unit Converter

What each unit actually measures

Three different questions get asked about radiation, and each has its own family of units. How much energy landed in the tissue? How much biological harm will that do? And how fast is the source decaying? Confusing the three is the usual reason a number looks alarming when it is not, or the reverse.

Absorbed Dose (energy deposited in tissue)

• Gray (Gy): the SI unit. 1 Gy = 1 joule of energy absorbed per kilogram of tissue.
• Rad: the older CGS unit. 1 Gy = 100 rad (1 rad = 0.01 Gy).

Equivalent Dose (biological effect) Different radiation types cause different amounts of biological damage even when the same energy is deposited. The equivalent dose accounts for this by multiplying absorbed dose by a radiation weighting factor (wR):

• Equivalent dose (Sv) = Absorbed dose (Gy) × wR
• Sievert (Sv): the SI unit for equivalent dose.
• Rem: the older unit. 1 Sv = 100 rem.
• Millisievert (mSv): 1 Sv = 1,000 mSv. Most practical doses are expressed in mSv.

Radiation Weighting Factors (wR)

• X-rays and gamma rays: wR = 1 (reference radiation)
• Beta particles (electrons): wR = 1
• Protons: wR = 2
• Neutrons: wR = 5 to 20, rising with energy (ICRP 103 replaced the old steps with a continuous function)
• Alpha particles: wR = 20, the most damaging per unit of energy deposited

Note the direction of the conversion. Weighting factors turn absorbed dose into equivalent dose, so they apply to Grays and rads, never to Sieverts or rems. A dose already quoted in Sieverts has had its wR applied; multiplying again is a real and easy mistake to make, and it inflates an alpha dose twentyfold.

Activity (how many decays per second)

• Becquerel (Bq): 1 Bq = 1 nuclear decay per second. The SI unit for radioactivity.
• Curie (Ci): 1 Ci = 3.7 × 10¹⁰ Bq (based on the activity of 1 gram of radium-226).
• 1 Ci = 37 GBq (gigabecquerels).

Annual Background Radiation The average person receives about 2.4 mSv a year from natural background: cosmic rays, radon seeping out of the ground, and naturally radioactive isotopes in food and building materials. The spread around that average is wide. Cosmic dose roughly doubles for every 1,500 to 2,000 metres of altitude, so Denver at 1,600 m takes about twice the sea-level cosmic component, and radon alone varies by more than a factor of ten between regions depending on the local geology. Radon is usually the largest single contributor, which is why it is the part worth testing for.