UNIT
ABBREVIATION
DEFINITION
Roentgen
R
The quantity of x or gamma radiation that will
produce, in 0.001293 grams (1 cc) of
standard air, ions carrying one electrostatic
unit (esu) of charge of either sign.
Radiation Absorbed Dose
rad
The quantity of ionizing radiation of any type
that results in the absorption of 100 ergs per
gram in any material.
Roentgen Equivalent Man
rem
The quantity of ionizing radiation of any type
which, when absorbed by man, produces a
physiological effect equivalent to that
produced by the absorption of one roentgen
of x or gamma radiation.
Table 1-3. Summary of radiation units.
1-15. BACKGROUND
Ever since the discovery of x-rays by Wilhelm Conrad Roentgen in 1895 and the
discovery of natural radiation by Henri Becquerel in 1896, man has been attempting to
utilize the beneficial properties of radiation without suffering any adverse biological
consequences. As early as 1898, scientists realized that it was essential to learn more
about radiation effects on living cells. X-rays and radioactive isotopes already were
being used widely in medicine and research and many individuals were receiving
damaging exposures. In order to intelligently utilize nuclear energy, it became
necessary to understand how and when radiation could be a hazard to human health.
1-16. DAMAGE MECHANISMS
a. It is often difficult to envision how radiation, which cannot be seen, heard,
smelled, or felt, can cause injury or death to a healthy man. The answer is not fully
understood, but a partial insight into the damage mechanism is gained when we realize
that man is composed of millions of microscopic cells and that radiation damage occurs
on the cellular and subcellular level.
b. When radiation is absorbed by a living cell, the primary damage is caused by
ionization and excitation of the atoms and molecules of that cell. These interactions
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