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“Health physics” may be defined as the protection of human beings and the environment from the harmful effects of ionizing radiation while permitting its beneficial applications. Health physicists can also be called “specialists in radiation protection.” The origin of the term “health physics” is not entirely clear.

Science and engineering depend on the quantification of variables, by measurement, calculation, or both. When we have a quantity that expresses some measure of the variable of interest, it is quite useless without the correct units. What is the acceleration on the earth due to gravity? People will often quickly answer “32” or “9.8”, but that is a meaningless answer without the appropriate units (ft/s2, m/s2.

We understood in the previous chapter that nature is made of atoms whose configurations vary. In radiation protection, our principal focus is on a particular subset of atoms that are by nature unstable. Our interest extends as well to protection of persons and the environment from radiation that may not have come from a radioactive atom (e.g., a radiation-producing machine). An important basic distinction in our discussion is the difference between radioactivity and radiation. is the process of spontaneous nuclear transformation that results in the formation of new elements. includes the various forms of particles and/or rays that are emitted by these atoms (and perhaps other secondary processes) during this nuclear rearrangement. Some atoms (in nature or artificially derived) are stable, and some are unstable. Unstable atoms rearrange themselves to achieve stability.

The particles or rays that are emitted during radioactive decay are emitted with a certain energy (which we learned how to calculate in Chapter 3), and they may or may not have charge. All of them interact with the environment into which they are released, transferring energy to that medium, and eventually dissipating all of their energy. We quantify the transfer of energy to matter in the next chapter. For now, it should be clear that such transfers of energy will have important implications for radiation biology, radiation shielding, radiation detection, and almost every practical application of radiation protection.

Interaction of all kinds of radiation with matter ultimately results in the transfer of energy, through the processes of ionization and excitation. Quantifying the transfer of energy is important to the two most fundamental areas of radiation protection: radiation dosimetry and radiation instrumentation. Understanding these two areas is essential to the practice of health physics in almost every practical application. We have already studied the forms of these interactions. Now we discuss quantitative measures of these interactions. As discussed in Chapter 2, it is important to understand the distinction between a (the parameter being measured) and a (the measure of the quantity). For example, velocity is the quantity that describes the time rate of motion of an object, and an example of the units that describe velocity is kilometers per hour (km/h).

On November 8, 1895, at the University of Wurzburg, Wilhelm Conrad Roentgen (1845–1923) was studying various physical phenomena and observed a glowing fluorescent screen in his laboratory. Studying cathode rays from an evacuated glass tube, Roentgen surmised that the fluorescence was from invisible rays originating from within the tube. The rays penetrated some opaque black paper wrapped around the tube. While considering this mystery, Roentgen realized the discovery of penetrating radiation, named “X-rays” because of its mysterious nature. This momentous event had an enormous and immediate impact on the fields of physics and medicine.

The development of radiation protection standards has occurred rather gradually over the last century. Lauriston Taylor1 has defined ten time periods in the development of radiation protection standards that characterize changes in thinking. We look at the development of these standards within the context of his time framework.

Our natural senses do not detect radiation, even at its most intense levels. A possible exception might be that at very high exposure rates, degradation of oxygen molecules can result in the formation of ozone, which can be perceived. In such a situation, however, survival of the organism is unlikely, so the detection of the hazard may not be helpful. In the systematic measurement of radiation we mostly use electronic instruments that are designed to exploit the types of interactions that radiation has with matter to produce a signal that can be detected and quantified. The basic types of detectors available for routine use have changed little in the several decades since most of the technologies were first made. Some significant changes have occurred in the sophistication of the computer-related accessories, use of global positioning technologies, and the use of computer programs for analysis of data.

We now begin two chapters on the measurement or calculation of radiation dose to humans, external and internal dose assessment. is the correct formal name for this process. In day-to-day use, however, most people will refer to this as external and internal . This is the classic historical term, in use since at least the Manhattan Project in the 1940s. The term “dosimetry” contains the suffix “metry”, which relates to metrology, which implies the measurement of physical quantities. In the last chapter, we looked at the issue of personnel dose-measuring devices. Much of external dose assessment does have to do with measurements, so the term “dosimetry” is mostly accurate. In this chapter, however, we show that a lot of work done in external dose assessment involves theoretical calculations of dose, generally with later verification using a survey meter or personnel monitoring devices.

Internal dose concepts are employed when radioactive material enters the body through any pathway, either in a workplace situation, in the nuclear medicine clinic (where the intakes are quite intentional), from eating or drinking contaminated food and water, and other such situations.