Equivalent Radiation Dose

Equivalent dose: H (Sv) = D (Gy) x w_R

The equivalent dose in sievert (Sv) takes the absorbed dose in gray (Gy) and turns it into a measure of biological damage, by multiplying it by a radiation weighting factor w_R. With X-rays, gamma and beta radiation, w_R = 1, so H and D end up with the same numerical value. Alpha particles carry w_R = 20, meaning twenty times the damage per Gy. Neutrons fall somewhere between 5 and 20, depending on their energy. To see how this plays out: 0.01 Gy of X-rays gives 0.01 Sv, or 10 mSv, while that same 0.01 Gy of alpha gives 0.20 Sv, or 200 mSv.

Reference doses

Some figures for scale: a chest X-ray runs about 0.1 mSv, a mammography about 0.4 mSv, and a chest CT roughly 7 mSv. Natural terrestrial background sits near 2.4 mSv/year, climbing in granitic terrain, and an intercontinental long-haul flight adds about 30 microSv. In Brazil the CNEN occupational limit is 50 mSv/year, with the public limit at 1 mSv/year. The same idea shows up in radiation protection (CNEN NN 3.01), medical radiology, nuclear medicine and aviation dosimetry.

FAQ

What is the difference between Gy and Sv? Gy measures the energy absorbed per kg, which is a purely physical quantity. Sv takes that energy and weights it by the type of radiation, so it reflects biological damage.

And what is the effective dose (also in Sv)? You take the equivalent dose and multiply it by a tissue weighting factor w_T, since each organ has its own sensitivity. The effective dose is the number reported for whole-body exposure.

Where do alpha doses come from? They come from internal contamination by radon, uranium, plutonium or polonium-210. A sheet of paper is enough to stop alpha radiation from outside the body, so the real danger is inhaling or ingesting the source.

Is a chest CT dangerous? A single 7 mSv CT scan adds roughly 0.04% to lifetime cancer risk. That is small against the clinical benefit, though it stops being negligible once you start stacking up repeated imaging.