Measuring Radioactivity

The radioactivity of a substance is measured by how many decays per unit time occur. Two popular units of the activity are used: the curie (Ci), defined as

and the becquerel (Bq), defined as

Note that, by themselves, these units do not measure accurately how dangerous a given amount of radiation might be for humans - for medical purposes, other units of radioactivity reflecting this aspect are more appropriate. One of these, the Gray (Gy), is the amount of energy delivered to the tissue per unit mass - a related unit is the rad, equal to 0.01 Gy:

    To quantitatively measure the biological damage done by some radiation, a factor called the relative biological effectiveness (RBE) is introduced. X-rays of energy 200 keV are defined to have an RBE of one, and all other radiation sources are compared to this standard.

 

Table 13.2: RBE values of various types of radiation

Radiation	RBE
Electrons	1 - 1.5
Slow neutrons	3-5
Protons	10
Alpha particles (helium nuclei)	20
heavy ions	20

 

The biologically equivalent dose, or simply equivalent dose, is measured in Sieverts (Sv) and is defined as

which is a more accurate measure of the biological danger posed by a given type of radiation. Another common unit to measure this in is the röntgen equivalent man (rem), defined as