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Units

The basic unit of radiation dose used to be the "rad," defined in terms of the energy deposited by ionizing radiation per unit mass of exposed matter (e.g. flesh or bone):

$$1 \hbox{\sl ~rad\/} \equiv 100 \hbox{\sl ~erg\/}/{\sl g}$$

(g means gram here.) More recently, for some reason this nice mnemonic unit has been officially supplanted by yet another "personal name SI unit" in honour of British physicist and radiation biologist Louis Harold Gray (1905-1965) - the "gray:"

$$1 \hbox{\sl ~gray\/} \equiv 100 \hbox{\sl ~rad\/} \equiv 1 {\sl ~J\/}/{\sl kg}.$$

Early work on radiation hazards was based on X-ray exposure⁸ and the units used were always rntgen (after the scientist by that name), which are about the same as rad for X-rays only, and are virtually unused today. Later it was found that even the rad was too simple; different types of radiation (e.g. neutrons) were found to be more (or less) destructive than X-rays for different types of tissues, so an empirical "fudge factor" called the Relative Biological Effectiveness (RBE) was invented to account for these differences (averaged over all body parts, of course, which decreased its usefulness). The RBEs of $\gamma$-rays, X-rays and $\beta$-rays (fast electrons) are all 1 by definition; thermal neutrons have an average RBE of 3; fast neutrons (on average), protons and $\alpha$-rays (⁴He nuclei) all have RBEs of 10; and fast heavy ions have an RBE of 20.⁹ A new unit was then constructed by combining the RBE with the dosage in rads, namely the rem (rntgen equivalent to man), defined by

$$\hbox{\sl ~rem\/} \equiv \hbox{\rm ~RBE} \times \hbox{\sl ~rad}.$$

The "R" in the preceding paragraph stands for rem and the "mR" for millirem - one thousandth of a rem.

Today the standard international unit for measuring "effective dosage" is the seivert, named after Rolf Sievert (1898-1966), a pioneering Swedish radiation physicist. Converting between rem and seivert is just like converting between rad and gray:

$$1 \hbox{\sl ~seivert\/} \equiv 100 \hbox{\sl ~rem}.$$

Now that all mnemonic content has been deleted from the names of the units associated with radiation dosage, you may expect these names to stick.¹⁰