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.