create one coulomb of charge of each polarity in one kilogram of matter. the roentgen (r) is an obsolete traditional unit of exposure, which represented the amount of radiation required to create one electrostatic unit of charge of each polarity in one cubic centimeter of dry air. 1 roentgen = 2.58×10−4 c/kg. however, the effect of ionizing radiation on matter (especially living tissue) is more closely related to the amount of energy deposited into them rather than the charge generated. this measure of energy absorbed is called the absorbed dose: the gray (gy), which has units of (joules/kilogram), is the si unit of absorbed dose, and it is the amount of radiation required to deposit one joule of energy in one kilogram of any kind of matter. the rad is the (obsolete) corresponding traditional unit, equal to 10 millijoules of energy deposited per kilogram. 100 rad = 1 gray. the equivalent dose is the measure of the biological effect of radiation on human tissue. for x-rays it is equal to the absorbed dose. the roentgen equivalent man (rem) is the traditional unit of equivalent dose. for x-rays it is equal to the rad, or, in other words, 10 millijoules of energy deposited per kilogram. 100 rem = 1 sv. the sievert (sv) is the si unit of equivalent dose, and also of effective dose. for x-rays the "equivalent dose" is numerically equal to a gray (gy). 1 sv = 1 gy. for the "effective dose" of x-rays, it is usually not equal to the gray (gy). ionizing radiation related quantities quantity unit symbol derivation year si equivalent activity (a) becquerel bq s−1 1974 si unit curie ci 3.7 × 1010 s−1 1953 3.7×1010 bq rutherford rd 106 s−1 1946 1,000,000 bq exposure (x) coulomb per kilogram c/kg c⋅kg−1 of air 1974 si unit röntgen r esu / 0.001293 g of air 1928 2.58 × 10−4 c/kg absorbed dose (d) gray gy j⋅kg−1 1974 si unit erg per gram erg/g erg⋅g−1 1950 1.0 × 10−4 gy rad rad 100 erg⋅g−1 1953 0.010 gy equivalent dose (h) sievert sv j⋅kg−1 × wr 1977 si unit röntgen equivalent man rem 100 erg⋅g−1 × wr 1971 0.010 sv effective dose (e) sievert sv j⋅kg−1 × wr × wt 1977 si unit röntgen equivalent man rem 100 erg⋅g−1 × wr × wt 1971 0.010 sv