- the roentgen (pronounced rent'-gen),
- the rad, and
- the rem.
You may recall that the different types of radiation were either particles (alpha and beta rays, protons, neutrons) or high-energy photons similar to light (X- and gamma rays). Except for beta rays - which are electrons having some 1/1836 the mass of protons or neutrons - the particles, because of their large masses, have a more catastrophic effect when colliding with a cell in the body. For this reason the quality factor - usually designated as Q - is used to adjust the absorbed dose to its biological counterpart, the rem.
Mathematically, rads x Q = rems.
Fortunately, most of the exposures we will be referring to in the study of hormesis are gamma and X-rays where Q is equal to one, allowing rads and rems to be used interchangeably. (Your radiologist, dental hygienist, and others working with X-rays will usually talk in terms of rads or millirads - but these are the same as rems and millirems, because it is the X-ray source that produces the radiation.) There is one other term with which you should have at least a vague familiarity - Linear Energy Transfer or LET. Beta, famma and X-rays are considered low LET radiation, which means they have a Q of one. High LET particles can have Qs up to 400. A typical alpha particle has a Q of four.
[Wilhelm Roentgen (1845-1923) discovered an unknown emission (X-rays) from cathode ray tubes. It still happens today - that's how X-rays are made today. Incidentally, your TV screen is a cathode ray tube, and it emits many times the radiation we get from nuclear power plants. Somehow this fact escapes notice of the TV doomsayers.]