Measuring radiation doses

Despite the difficulty of assesing damage done by radiation, measurements of radiation doses are useful and necessary to quantify radiation dangers. The definitions and units of doses are unfortunately complicated.

unit Equivalent unit Description
R (Roentgen) 1.0 R = 2.58 E-4 C/kg This is a measue of ionization charge per mass of the sample being radiated. If a sample of mass m = 1.0 kg absorbs radiation which ionizes 2.58 E-4 C, the dose is one Roentgen. The same ionization in a smaller sample, corresponds to a larger exposure.
Gy (gray) 1.0 Gy = 1.0 J/kg This is a measure of energy deposited in a sample. If a 1.0 kg sample absorbs 1.0 J of energy, the sample had a dose of 1.0 Gy.
rad 1.0 rad = 0.01 Gy This is a scaled down version of a Grey. Only 0.01 J of energy in a 1.0 kg sample is necessary for a dose of 1.0 rads.
rem Dose in rems = RBE times Dose in rads This unit accounts for the relative harmfulness of different kinds of radiation. The RBE factor can be found in tables and depends on the sort of radiation and the energy of the individual quanta. For instance, the RBE of alpha particle ranges from 10-20 depending on the energy of the alphas.

Measurements of ionization per unit mass (e.g. Roentgens) are called exposures while measurements of energy per mass are called doses (e.g. Greys, rads and rems).The definitions are all "per mass", thus they are different from one's usual idea of a dose which is an amount not an amount-per-mass. The RBE factors of various doses, which are used to calculate biological equivalent doses in rems are shown in the table below.

Type of Radiation RBE
200-keV X-rays 1
b particles (electrons) 1
protons 10
a particles 10-20
neutrons (slow) 2
neutrons (fast) 10

Typically, a person absorbs 0.3-0.4 rems of radiation yearly. A sudden dose of a few hundred rems is surely fatal.

Examples     Radioactivity index