26. Radioactivity and radiation protection 1 26. RADIOACTIVITY & RADIATION PROTECTION

recommends the use of SI units. Therefore we list SI units first, followed by cgs (or other common) units in parentheses, where they differ. • Unit of activity = becquerel (curie): 1 Bq = 1 disintegration s−1 [ = 1/(3.7 × 10 10)Ci] •Unit of absorbed dose = gray (rad): 1Gy=1joulekg−1 ( = 104 erg g−1 = 100 rad) =6.24 × 1012 MeV kg−1 deposited energy • Unit of exposure, the quantity of x- orγ-radiation at a point in space integrated over time, in terms of charge of either sign produced by showering electrons in a small volume of air about the point: =1coulkg−1of air (roentgen; 1 R =2.58×10−4 coul kg−1) =1esucm−3 ( = 87.8 erg released energy per g of air) Implicit in the definition is the assumption that the small test volume is embedded in a sufficiently large uniformly irradiated volume that the number of secondary electrons entering the volume equals the number leaving. This unit is somewhat historical, but appears on many measuring instruments. • Unit of equivalent dose (for biological damage) = sievert [ = 100 rem (roentgen equivalent for man)]: Equivalent dose in Sv = absorbed dose in grays × wR, where wR(radiation weighting factor, formerly the quality factor Q) expresses long-term risk (primarily cancer and leukemia) from low-level chronic exposure. It depends upon the type of radiation and other factors, as follows [2]: Table 26.1: Radiation weighting factors. Radiation w