UNIT OF EXPOSURE

a. Named after the discoverer of x-rays, the roentgen (R) is applicable only to xrays and gamma radiation. The roentgen is defined in terms of the ionizations produced by x and gamma radiation in air.

b. As radiation passes through air, it will interact with the gas atoms and produce ionizations. In each ionization, an ion pair is formed, consisting of an electron and a positive ion. The charge on the electron is equal in magnitude to that on the positive ion but opposite in sign; if the two are allowed to recombine, the charges will be neutralized and a neutral atom will result. If, however, an electric field is present, the two ions will not recombine but will move in opposite directions, eventually to be collected by the electrodes that created the electric field. When the ions are collected, they will neutralize a small portion of the charge originally placed on the electrodes. The amount of charge that is thus neutralized may be measured; this fact is the basis for the definition of the roentgen.

c. One roentgen is that quantity of x or gamma radiation which will produce one electrostatic unit of charge of either sign in 0.001293 grams of air at standard temperature and pressure or 2.584 x 10-4 coulomb/kg of air.

d. The electrostatic unit (esu) of charge equals, approximately, the electrical charge of two billion electrons. The mass of air specified in the definition is the mass of approximately one cubic centimeter (cc) of air under conditions of standard temperature and pressure. By referring to the definition, then, we can see that one roentgen produces approximately two billion ionizations in each cubic centimeter of air. Of course, the measuring chamber need not have a volume of exactly one cubic centimeter. Increasing the chamber volume to 10 cubic centimeters, for example, would simply mean that one roentgen would cause approximately 20 billion ionizations in this volume, capable of neutralizing 10 esu of charge.

e. Conversion factors of this kind have been utilized to arrive at alternate forms of the definition of the roentgen:

(1) 773.4 esu per gram of air.

(2) 1.611 x 1012 ion pairs per gram of air.

(3) 87.8 ergs per gram of air.

f. The alternate form, paragraph (3) above, is derived from paragraph (2) above and the laboratory measurements of the amount of energy required to cause one ionization in air. As the measurements are extremely difficult to make, the value given represents an estimate.

g. The roentgen is still used in the measurement of x and gamma radiation. As these types of radiation travel in straight lines and will easily penetrate body tissue, it is meaningful to talk of exposure--the quantity of radiation, which passes through a given volume in space. Exposure is based upon the ability of the radiation to produce ionizations in air; however, as is the case of light rays measured by an exposure meter, the radiation need not actually interact with the body tissue or with anything else. h. For this and other reasons, it was soon recognized that the concept of exposure was severely limited in its application. It was not defined for radiation other than x and gamma and it was not directly related to damage produced in living tissue. The "absorbed dose" concept was therefore developed; it is based on the quantity of ionizing radiation, which actually interacted with matter, producing ionizations and releasing energy. As the unit of energy in the metric system is the erg, the natural unit for absorbed dose was ergs released per gram of material.