sources, and finally, to implement the proper control measures to ensure safe utilization
of these sources.
c. In order for him to fully appreciate the techniques involved in
implementing such measures, the preventive medicine specialist must understand the
principles upon which these techniques are based. Knowledge of basic radiological
health will form a foundation upon which these techniques can be better understood.
1-5. STRUCTURE OF MATTER
a. Man has long wondered about the structure of matter. As far back as 500
BC, there were many imaginative ideas concerning the true nature of matter. These
ideas were in many ways little more than philosophical notions. The Greek,
Empedocles, believed that all matter was composed of four basic substances or
elements--earth, air, fire, and water. Other Greeks, the atomists, such as Epicurus and
Democritus, thought that all matter consisted of elemental, indivisible units called atoms.
b. With the advent of scientific methods, man discovered the basic substances
of which all matter is composed--the natural elements. Water was separated into
hydrogen and oxygen. Air was found to be basically a mixture of oxygen and nitrogen.
They and the other elements that were discovered could not be further divided into
simpler substances; thus, the name element, a basic substance. There are 92 natural
elements. These include, for example, iron, sulfur, aluminum, carbon, sodium, and
chlorine. Several others have been produced artificially, for instance, plutonium.
c. Scientific analysis has shown that the Greek atomists were, in a sense,
correct in that elements are composed of basic units or atoms. Each element has its
own characteristics and its own characteristic atoms. An element is a substance, which
cannot be separated into simpler substances by ordinary chemical means. An atom is
the smallest unit of an element that possesses all the characteristics of the element.
d. The Greek atomists were, however, in one sense incorrect. Atoms have been
found to be divisible; atoms have been divided into more fundamental particles called
electrons, protons, and neutrons. The electron was first discovered as the basic unit of
electricity. It is a very tiny, negatively-charged particle considerably lighter than an
atom. The proton is a positively-charged particle having exactly the same magnitude of
charge as the electron; however, it is much larger than the electron in mass, being
approximately 1,840 times the electron mass. A gram of protons contains roughly 6 x
1023 protons. After the discovery of the electron and the proton, the neutron was
predicted as the particle that would be formed if a proton and electron were closely
combined. It would thus be neutral in charge. In 1932, the neutron was actually
discovered as predicted.