Intensity and absorbed dose rate.

Abstract

For the radiologist and for the investigator using radiation beams, the two most important and commonly used concepts in the quantitative measurement of radiation are the output of the source of radiation and the absorbed dose produced in tissue by the absorption of the radiation. In more absolute and precise terminology, output refers to the intensity of a radiation beam, while absorbed dose refers to the energy absorbed in a unit mass of the absorbing material. These are distinct quantities, the following definitions being taken from the recommendations of the International Commission on Radiological Units (1953) (1). “Intensity of radiation is the energy flowing through unit area perpendicular to the beam per unit time. It is expressed in ergs per square centimeter per second or watts per square centimeter.” “Quantity of radiation is the time integral of intensity. It is the total energy which has passed through unit area perpendicular to the beam and is expressed in ergs per square centimeter or watt-seconds per square centimeter.” “Absorbed dose of any ionizing radiation is the amount of energy imparted to matter by ionizing particles per unit mass of irradiated material at the place of interest. It shall be expressed in rads.” It should be noted that no definition of dose was agreed on, so that the term “absorbed dose” is the only one having a precise meaning. In this discussion we shall employ the defined term, “absorbed dose,” and shall mean this even when the expression “dose” is used. “The rod is the unit of absorbed dose and is 100 ergs per gram.” The erg is 0.63 × 1012 electron volts and is equal to the amount of energy required on the average by an electron to ionize approximately 20 billion atoms of air. The use of absolute units achieves the ideal of definite and unambiguous terms of measurement. However, the recommended use of absolute units raises practical questions: How can the output and absorbed dose be expressed in absolute units? What is the relationship of our conventional ionization measurements to absolute intensity and absorbed dose rate? Consideration of these questions will aid the exposition of the significance of and distinction between absorbed dose rate and intensity. Historically, various methods have been used for the detection of radiation. These include action on fluorescent materials, photographic film, and color pastilles, resistance of irradiated selenium, and the production of ionization in gases such as air. Of these methods, ionization measurements have been primarily employed for indirect quantitative determinations of both intensity and absorbed dose rate. When x-rays are absorbed in any medium, energy is imparted to secondary electrons.