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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):

\begin{displaymath}1 \hbox{\sl ~rad\/} \equiv 100 \hbox{\sl ~erg\/}/{\sl g} \end{displaymath}

(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:''

\begin{displaymath}1 \hbox{\sl ~gray\/} \equiv 100 \hbox{\sl ~rad\/} \equiv 1 {\sl ~J\/}/{\sl kg}. \end{displaymath}

Early work on radiation hazards was based on X-ray exposure8 and the units used were always r\oentgen (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 (r\oentgen equivalent to man), defined by

\begin{displaymath}\hbox{\sl ~rem\/} \equiv \hbox{\rm ~RBE} \times \hbox{\sl ~rad}. \end{displaymath}

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:

\begin{displaymath}1 \hbox{\sl ~seivert\/} \equiv 100 \hbox{\sl ~rem}. \end{displaymath}

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

next up previous
Next: Effects Up: How Bad is How Much of What, and Previous: How Bad is How Much of What, and
Jess H. Brewer
1999-11-05