Basic Physics for Radiography



Basic Physics for Radiography




Limited operators do not require an extensive background in physics, but some basic principles of physical science are essential to an understanding of x-rays and their use. This chapter covers the basic concepts of matter, energy, and electricity and relates these principles to radiography. It also discusses the nature of radiation.


If your educational background includes coursework in physics or chemistry, this chapter will provide a comprehensive review of the pertinent material. If you are unfamiliar with these subjects, it will be important for you to master them so that you can relate well to the material that follows.


Everything of a physical nature in the universe can be classified as either matter or energy. Both matter and energy can exist in several forms.



Matter


Matter is defined as anything that occupies space and has shape or form. The three basic forms of matter are solids, liquids, and gases. The quantity of matter that makes up any physical object is called its mass. Although the scientific definitions differ somewhat, mass is essentially the same thing we think of as “weight.” An object may change in form, but its mass is unchangeable. For example, a 20-lb bucket of water may freeze into a 20-lb bucket of ice or it may evaporate, resulting in 20 lb of water vapor. The form changes, but the mass remains the same.




Atoms


All matter is composed of “building blocks” called atoms. Scientists have determined that atoms may be made up of nearly 100 different subatomic particles, but only three basic particles concern us here. The fundamental particles that compose atoms are neutrons, protons, and electrons. All neutrons are identical, as are all protons and all electrons. It is the number and arrangement of these particles in the atom that account for the differences in matter.


The neutrons and protons together form the nucleus of the atom, its center. The electrons circle the nucleus in orbits called shells. A useful model for visualizing atomic structure is that of the solar system, with the nucleus as the sun and the electrons as planets in orbit around the sun (Fig. 4-1). This model was first described by Niels Bohr in 1913 and is referred to as Bohr’s atom.



Atomic particles differ from one another with respect to electric charge. Neutrons are electrically neutral (0); that is, they have no electric charge. Protons have a positive charge (+). Electrons have a negative charge (−); that is, their charge is equal to, but opposite, the charge of a proton. A particle’s charge is important, because it results in a magnetic effect. Opposite charges attract one another, seeking a neutral state. Like charges repel one another. Neutral particles neither attract nor repel and are not attracted or repelled by charged particles. Table 4-1 contains a summary of the characteristics of the fundamental atomic particles.



In its “normal” or neutral state, an atom has an equal number of protons and electrons, so the electric charges are balanced and the atom as a whole has no charge. The electrons are arranged in their orbits, with a specific number of electrons allotted to each shell. The shells are lettered alphabetically, beginning with the letter K nearest the nucleus (Fig. 4-2). From the nucleus outward, each additional shell is greater in size and can accommodate a larger number of electrons than the previous shell. Table 4-2 lists atomic shells with their letter symbols and the maximum number of electrons in each. Different types of atoms will have different numbers of electrons in their shells up to the maximum shown. From a radiography standpoint, the most important shell is the K-shell. The removal of electrons in this shell is one way in which x-rays are created.




Each of the electrons around the nucleus is in continuous motion. The distance that the shell is from the nucleus determines the energy level of the electron. The electrons are held in place by a binding energy. Electrons near the nucleus are attached with greater binding energy than those in outer shells. The binding energy of each shell varies for different atoms; larger atoms have greater binding energy than smaller ones.



Elements


The essential characteristic of an atom that determines its type is the number of protons in the nucleus. An element is a substance made up of only one type of atom; that is, all atoms of an element have the same atomic number. Scientists have identified 108 different elements. Many of these are rare, and some of them are human made. Each element has a name and a chemical symbol consisting of one or two letters. Three common elements we may be familiar with are calcium (Ca), iodine (I), and lead (Pb). Each element also has an atomic number that represents the number of protons in the nucleus. The atomic numbers for the three elements described are 20, 53, and 82. The greater the atomic number, the greater is the element’s mass and density. In radiology, a lead bullet inside a body would be easier to see on an x-ray than a calcium stone because of lead’s greater atomic number and density. The mass number of the element is the combined total of the protons and neutrons in the nucleus. One of the most important elements used in the production of x-rays is tungsten. Tungsten is the element inside the x-ray tube where the x-rays are created (discussed in Chapter 5). Tungsten (see Fig. 4-2) is represented by the symbol W and its atomic number is 74. Its mass number is 184, indicating that the nucleus contains 74 protons and 110 neutrons. The number of neutrons is determined by subtracting the atomic number from the mass number.


Two or more atoms may combine chemically to form molecules. This combination occurs with the sharing of one or more outer shell electrons between atoms. A substance that consists of only one type of molecule is called a chemical compound. Water is an example of a chemical compound. Its chemical symbol is H2O, indicating that it is made up of two atoms of hydrogen and one atom of oxygen. Substances that contain more than one type of molecule are called mixtures.



Ionization


When a neutral atom gains or loses an electron, it is called an ion and the atom is said to be ionized. This process, which is called ionization, produces an atom with an electric charge. If an electron is added to a neutral atom, electrons will outnumber the protons and the atom will have a negative charge. If an electron is removed, there will be more protons than electrons, so the atom will have a positive charge. Because the outer orbital electrons are not tightly bound to the nucleus, the application of a small amount of energy can remove an outer orbital electron from the atom (Fig. 4-3).



A familiar example of ionization is the “bad hair day” that occurs when the weather is cold and dry. The friction of a hairbrush removes electrons from atoms in the hair. In very dry air, the electrons cannot readily return to their orbits, and each hair is left with a positive charge. Since like charges repel each other, the hairs are repelled from one another and will not lie smoothly together.


The term ionization is very important in the field of radiology. X-rays cause ionization in the atoms of the human body (Fig. 4-4), a fact that explains many of the negative effects of radiation discussed later in the text.



Mar 7, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Basic Physics for Radiography
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