X-ray - Units of Measure and Exposure

Units of Measure and Exposure

The measure of X-rays ionizing ability is called the exposure:

• The coulomb per kilogram (C/kg) is the SI unit of ionizing radiation exposure, and it is the amount of radiation required to 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 sievert (Sv) is the SI unit of equivalent dose, and of effective dose, which for equivalent dose of X-rays is numerically equal to the gray (Gy), and for effective dose of X-rays is usually not equal to the gray (Gy).
• The Roentgen equivalent man (rem) is the traditional unit of equivalent dose. For X-rays it is equal to the rad or 10 millijoules of energy deposited per kilogram. 1 Sv = 100 rem.