Calculating Radiation Doses

Radiation dosimetry is the quantitative description of the effect of radiation on living tissue. The absorbed dose of radiation is defined as the energy delivered to the tissue per unit mass. The SI unit of absorbed dose is called the gray, Gy:

1 Gy=1 J/Kg

although the rad is more commonly used:

1 rad =0.01 Gy

Absorbed dose by itself is not an adequate measure of biological effect because equal energies of different kinds of radiation cause different amounts of biological effect. This varaiation is dexcribed by a numerical factor called 'the relative biological effectiveness' or 'quality factor' of each type of radiation. X rays with energyof 200KeV are defined to have a quality factor of 1, and the effects of other radiations can be compared experimentally. The table shows approxinate values of the quality factor for several different types of radiation.


Quality Factor

X rays and Gamma rays



1 – 1.5

Slow neutrons

3 – 5





Heavy ions


These values are approximate and depend on the energy of the radiation and the kind of tissue absorbing the radiation.

The biological effect is the product of the absorbed dose and the quality of the radiation. This quanity is called the equivalent dose and is measured in either Sieverts (Sv):

Equivalent Dose (Sv) =Quality Factor * Absorbed Dose (Gy)

or rem:

Equivalent Dose (Sv) =Quality Factor * Absorbed Dose (rad)

1 rem =0.01 Sv

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