What is Radiation Dose?

When people talk about dose, they're usually using sieverts (or rem, if in the US). Sieverts (Sv) are best used for external doses, like say the dose you receive from a giant hunk of radioactive cobalt across the room.

To understand sieverts, you should first understand grays (US equivalent: rad). 1 gray is equal to 1 Joule of energy per kilogram of matter. Energy/matter provides an idea of how much effect the radiation has, scaled for the size of whatever it's hitting. Sieverts include a quality factor in order to take into account the biological effect of the radiation (1 Sv = 1 Gy*quality factor). Alpha particles have a quality factor of 20, because they deposit all their energy in a small area. Gamma rays and beta particles have a quality factor of 1, because their energy is more spread out, meaning the tissues struck by gamma rays are more likely to recover. So, while different types of radiation have different effects, 1 Sv of alphas is equivalent to 1 Sv of gammas.

Contamination is a little different from radiation. Radiation consists of tiny invisible particles, the largest of which is the size of a helium nucleus. Contamination consists of actual atoms or molecules, and is best understood as "radioactive stuff wherever we don't want it". It is usually expressed in terms of counts per minute (cpm) or decays per minute (dpm), and can be separated into "removable" and "fixed", which are exactly what they sound like. Counts per minute are what you see on your geiger counter. Dpm is just cpm corrected for the efficiency of the detector (about 10% on your average geiger counter, should be calculated when the meter is calibrated). At the Reed Reactor, we consider something "contaminated" and in need of cleaning when it's reading 1000 dpm per 100 square centimeters.