Section III. RADIATION UNITS
a. Although radioactivity was discovered in the late 19th century, it was originally
considered a scientific curiosity with little practical value. During this time researchers
discussed findings in terms of the effects of radiation or in terms of the type of
equipment used in a particular experiment. For example, an experimenter might
describe the type of x-ray tube and voltage used or he might describe the effects on the
skin of a man exposed to radiation. A widely used unit was the so-called erythema
dose--the amount of radiation which could cause an abnormal redness of the skin due
to capillary congestion.
b. As experiments became more precise, and the experimenters became aware
of the effects of radiation upon living tissue, the need arose for an accurate unit to
enable comparison among the various types of x-ray machines that had proliferated
during World War I. Experience gained in the war also showed the need for a careful
study of the effects of radiation on personnel; such a study would require a standard
c. For these reasons, the International Commission for Radiological Protection
(ICRP) was formed. Meeting in Geneva, Switzerland, in 1925 and again in 1928, the
ICRP recognized this need. The search for a suitable unit culminated in 1932 in the
adoption of the term "roentgen."
1-11. UNIT OF EXPOSURE
a. Named after the discoverer of x-rays, the roentgen (R) is applicable only to x-
rays 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.