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also called the Q (quality factor), or RBE (relative biological effectiveness of the
radiation).

D eq   D abs R w R .
R

For photons and electrons, the quality factor is defined to be 1; for neutrons,
it ranges from 5 to 20 depending on the energy of the neutron (10); for alpha
particles, it is 20.

Thus, 1 gray (100 rad) of alpha particles is currently judged to have an effect
on living tissue that is twenty times more than 1 gray (100 rad) of x rays.

The equivalent dose was expressed in terms of a unit called the rem, an
acronym for roentgen equivalent man. (The term equivalent simply meant that an absorbed
dose expressed in rem would have equivalent biological effects, regardless of the type of radiation.
Thus, 10 rem of x rays should have the same biological effect as 10 rem of neutrons absorbed by
the same part of the body.).

The modern unit is the sievert (abbreviated Sv and named for the prominent
Swedish radiologist, Rolf Sievert), which is equal to 100 rem. Thus, an equivalent
dose of 200 rem would today be expressed as 2 sieverts.

Table 13.3 - The Radiation Biological Effectiveness of Various Forms of Radiation
Radiation W R (RBE)

x-rays and -rays 1
fast-moving neutrons or protons 10
-particles or heavy ions 20

4) Effective dose
Finally, the biological effect of radiation depends on the type of tissue being
irradiated.
As with different types of radiation, a weighting or quality factor is introduced
depending on the type of tissue. The more sensitive the tissue is to radiation, the
higher the factor (table 13.4).
The effective dose is the sum of the equivalent doses of the various types of
irradiated tissue, each properly weighted for its sensitivity to radiation.

D еff  D eq T  w T .

T
Tissue weighting factors wT are determined from the relative incidence of
cancers in different tissues in the Japanese survivors of the atomic bombs.
Units of measurement of effective dose consistent with units of measurement
equivalent dose. It also measured in sievert or rem.

Table 13.4 - VALUES OF THE COEFFICIENTS OF RADIATION RISKS FOR DIFFERENT ORGANS AND

TISSUES

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