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pediatric practices. This put infants at risk of thyroid cancer years later. Radium-224 was
given intravenously to treat tuberculosis of the bone, resulting in an excess incidence of
bone malignancies. Thorotrast ® was also used as an intravenous radiographic contrast
medium in Europe, Japan, and the United States, producing excess malignancies of the
liver, bile ducts, spleen, brain, and bone in all age groups. Children and adults were also
subjected to radiation exposure as part of experimental research in people that would not
have met today’s ethical standards.
In 1956, the U.S. National Academy of Sciences—National Research Council and the
U.K. Medical Research Council published similar conclusions about the potential hazards
and the late effects of radiation. Since that time, the uses of ionizing radiation have come
under increasing scrutiny and regulation.
Physical Principles of Ionizing Radiation
Atomic Structure
Atoms have two distinct parts that are made up of three main particles. Protons and
neutrons are found in the nucleus, which is the tiny, dense center that contains almost the
entire mass of the atom. Electrons surround the nucleus, moving in paths called orbitals.
Protons. Protons have a mass of about 1.7 × 10 -27 kg, which is assigned the value of 1
atomic mass unit (AMU). Protons carry an electrical charge, which is assigned a value of
+1. The density of an atom is determined by the number of protons in the nucleus. For
example, uranium has 92 protons, making it much denser than hydrogen, which has only
one proton.
Neutrons. Neutrons also have a mass of about 1 AMU, differing from the proton by