LST 1-1



1-1 What is radiation? Let us go to the dictionary! In simple terms; "radiation is energy sent out in the form of waves or particles". The definition does not specify a source for the radiation, so we may correctly conclude that it could be almost anything that sends out waves.

To illustrate: what do you do when you see a calm pond of water, perhaps with some chips of wood or leaves floating quietly on the surface? Your immediate impulse is probably irresistible -- to see how far out into that pond you can throw a pebble. From the point where the pebble hits the water, ripples radiate in rings. Those ripples are waves, and they are a form of radiation. The ripples represent the movement of some of the energy imparted by the pebble, radiating from the impact point in all directions. As each ripple reaches a chip, the chip rises to the crest of a wave.

The chip has actually been lifted a little. It is hardly necessary to point out that to lift anything -- even as small as a chip -- requires some energy. Thus, the lifting of the chip shows that the waves have energy, and that some energy has been moved from the spot where the pebble struck the water to the place where the chip was lifted. The general idea is the same for other types of waves and radiation that we will discuss.

There is one particular characteristic of all radiation that helps to identify and describe it. That is its wavelength, the distance from the crest of one wave to the crest of the next wave. Figure 1 shows a diagram of a wave. The wavelengths of the radiations that will be discussed below vary from billionths of an inch to thousands of feet.

Fig. 1

The term radiation is general; some of the more specific terms for types of radiation will be referred to below. It is not necessary that the reader understand the differences among the terms. The main reason for listing some of them is to make you familiar with the words that may appear in both technical and non-technical writings. Further, it is not necessary to understand anything special about atoms, or molecules, or electrons, or other scientific phenomena, for you to understand enough about radiation to help you make informed decisions on radiation-related issues.

Waves in water are one form of radiation. There is another large class, that we call electromagnetic radiation. These radiations are grouped together because they share many characteristics. The main distinction among electromagnetic radiations is their differing wavelengths. A list of radiations is given in Figure 2, starting at the top with radio transmission waves which have the longest wavelengths -- from one inch to several thousand yards. Next come microwaves and infra-red or heat radiation. Then comes visible light, with each color having a distinctive wavelength in the general range of ten-thousandths of an inch. As the radiation wavelengths become still shorter we go from the visible violet to the invisible ultra-violet which we hear about in connection with sunburn and other skin effects. Next come x-rays which have shorter wavelengths, followed by gamma rays which are even shorter . Actually, the wavelengths of x-rays and gamma rays may overlap. Finally there are cosmic rays, generated by particles from outer space striking the earth's atmosphere.


Non-Ionizing Electromagnetic Radiation



Infra Red (Heat)

Visible Light (Color)







Ultra Violet

Ionizing Electromagnetic Radiation


Gamma Rays

Cosmic Rays

Ionizing Atomic Particle Radiation

Beta Rays

Alpha Rays


Figure 2 Types of Radiations

Figure 2 shows several categories of radiations; the one of main interest here is ionizing radiation. Whenever ionizing radiation strikes anything - wood, iron, the human body -- anything -- it creates electrically charged particles called ions which, in turn, can have effects on matter, including living things. In various writings you may find ionizing radiation called "atomic radiation", "nuclear radiation" or "penetrating radiation". For our purposes, these terms are synonymous.

Although this report will deal with ionizing radiation, generally, it will focus on x- rays and gamma rays.

1-2 Where does radiation come from?

Today we are besieged with information and stories about all kinds of radiations and their hazards -- and occasionally we are told about some of their beneficial uses. This book will deal with the radiations that generally have an atomic or nuclear origin and are called ionizing radiations. Almost all electromagnetic radiations have atomic origins of some kind, and all can have deleterious health effects if carelessly or improperly used. However, we have known for the past sixty years how to use ionizing radiation properly and safely.

On earth, x-rays, and some gamma rays, are made by man; gamma rays also occur in nature. As noted above, x- rays, gamma rays, and cosmic rays are ordinarily included in what we call ionizing radiation; when ionizing radiation strikes atoms of any kind, it can disrupt them and cause the formation of ions and more ionizing radiations.

Initially our discussions will deal with x-rays, the kind of radiation that most of us are aware of in our individual lives. All x-rays of concern on earth are man-made, produced in powerful high voltage equipment. The x-rays can be turned on and off readily -- a feature that distinguishes them from the gamma rays from radioactive sources. If something is irradiated with x-rays, some secondary x rays may be produced during the irradiation. But once the high voltage is turned off, the initial x-rays are stopped, so are the secondaries, and there is no radiation left.

X rays, because of their very short wavelength, are capable of passing through materials -- for example, flesh, water, wood, or iron. They can be most effectively stopped by heavy materials like lead or by substantial thicknesses of concrete. Gamma rays are essentially the same as x rays except that they are produced in the nucleus of the atom. The gamma rays in nature come mainly from naturally occurring radioactive materials, like radium or some product of radium, or from radioactive potassium -- about both of which more will be said later. Other gamma ray emitting nuclei can be produced by man in very high voltage generators. Once a material has been made radioactive in this way, its emission of radiation lasts for a characteristic period; that period may range from fractions of a second to centuries depending on the material involved. Some elements ( iron or oxygen, for example) are made up of several almost identical atoms called isotopes. If the isotopes give off ionizing radiation they are called radioisotopes. Isotopes are distinguished from one another by numbers: for example, potassium-40 and potassium-39 are isotopes of the element potassium.

Radionuclide Half Life


4.5 billion years


24,000 years


30 years


5.3 years


8 days


7.4 seconds

Figure 3 Half-life

Once a material has been made radioactive, its emission of radiation cannot be turned off or destroyed. It has to "self-destruct" naturally. But, as noted above, different radioactive materials self-destruct or lose their strength over different periods of time. These periods are measured in what are called half-lives: A half-life is the time it takes for the material to lose one-half of its radioactivity. For example, the half-life of radium-226 is 1622 years, which means that a given quantity of radium will lose half of its strength in 1622 years, and half of its remaining strength in another 1622 years, and so on. This process is illustrated in Figure 3, above.

Some other radioactive materials that are mentioned frequently in general writings and the media are listed, with their half lives, in Figure 3. For example, among the natural radioactive materials that will be discussed later in this book is uranium-238, from which several other natural radioactive materials are derived. Uranium-238 has a half life of 4,500,000,000 years. One of the products of uranium-238 is radium-226. A product of radium is radon-222, a radioactive gas, with a half life of 3.83 days.

Among the man-made radioactive materials are plutonium-239, with a half life of 24,000 years; Cesium-137 with a half life of 30 years; Cobalt-60 with a half life of 5.3 years; and iodine-131 with a half life of 8 days. Because of the chemical affinity of Iodine for the thyroid gland and the relatively short half-life of Iodine-131, that radioisotope is used as a drug to treat certain diseases involving the thyroid. Its radioactivity drops to a negligible value in a length of time that is reasonable for such of medical treatments

1-3. What are the benefits of radiation? The gifts that ionizing radiation offers to man are great, but we must be aware that few gifts are free of costs. In the final balance, unless the benefits from its use exceed the costs (or risks) we should forego the gift.. For example, consider the sun -- a nuclear furnace that emits all kinds of radiation, including ionizing radiation. We do not need to dwell on the value of the sun's infrared radiation (heat), or the visible or ultraviolet radiation (light). These are a part of our natural world and we could not live without them. At the same time, it must be pointed out that any of these natural radiations also carry a potential for harm. Too much infrared, could cause everything to burn up. Too much light in the visible spectrum could cause blindness, and too much ultraviolet could cause severe sunburn or skin cancer.

Because the public is most familiar with hearing about, or experiencing, x-ray examinations, a few examples of their use will be noted here and discussed in more detail later. X-rays are widely used in medicine today to visualize outlines and structures in the body and the body organs, to detect foreign bodies, to help in the repair of broken bones, and to detect diseases. For many years, x-rays played a major diagnostic role in reducing the threat of tuberculosis.

But, as for so many other things -- certain medicines, for example -- x-rays, if used carelessly or in ignorance might increase the risk of cancer and even injure or kill a patient. However, we now know so much about ionizing radiation, and how to minimize our exposure to it, that there is no longer need to fear its proper uses. We can minimize improper uses through education, training, and enforcement of the rules for use that have been developed over the last fifty years. All of the ionizing radiations discussed above can now be used for a wide range of useful purposes with a high degree of safety.

This brings up the question of "safety" -- a term that is very much in the minds of everyone today. Safety, as a concept, has been misstated to the public at times and is misunderstood much of the time.

1-4. Is radiation safe ? There is no such thing as perfect safety. To understand this, let us again turn to the dictionary. Safe or safety is defined as, "the state of not being liable to danger or injury of any kind; free from hurt, injury, or damage". It must be obvious from this very precise definition that there can be no such thing in the practical world as complete and total safety. Even as you lean back in your chair reading this book, you are at risk from something, for example, tipping over backward and breaking an arm.

Since there can be no complete safety, it is true that in everything we do there is some element, however small, of possible hazard or risk. Thus, in each decision we make to take an action, there is probably an element of chance, some small gamble -- some risk -- to be considered. Most of the time we do not think about it; we make our decisions unconsciously. Stop to think of it sometime. If you cross a street when there is only a little traffic, you watch for a traffic break; if you step across a small stream, you look for a dry spot on the other side so you will not slip. In each case, an essentially unconscious decision has been made to minimize the risk.

Whether we stop to think about it or not, some choice of action may be relatively more or less safe than the alternative. The use of x-rays affords a good example of relative safety. A routine x-ray examination to detect a possible cancer might involve a minuscule chance of, itself, causing some injury. (This is the kind of a chance that may exist in the taking of any ordinary drug). However, the cancer, if left undetected, could be fatal. The risk of not having the examination could be vastly greater than any radiation risk from the examination itself. This is frequently referred as the risk/benefit consideration. When you take an exceedingly small radiation risk for the highly positive benefit of detecting the tumor so it can be cured, the benefit of the diagnostic procedure enormously outweighs whatever small risks it might entail.

And yet, over the past twenty years or so, there have been cases of persons resisting, or even refusing, a standard x-ray or nuclear radiation examination because they had heard or read somewhere that all radiation is dangerous. They have done this blindly, in reaction to some rumor, news item, or gossip. There is no doubt in my mind that some people have lost their lives because they didn't understand the risk/benefit relationship involved. Put another way, you have the choice. If, to avoid a minuscule x-ray risk, you refuse a diagnostic x-ray examination prescribed by a competent physician, you take the chance of allowing a fatal disease to remain undetected. If you accept the physician's advice along with a minuscule x-ray risk, you will either find evidence of disease and enter into treatment, or learn that you are free of the suspected disease. Either result is far better than not knowing at all.

Scientists cannot yet completely rule out the extremely small chance of some injurious effect from a very small exposure to radiation -- like that from a diagnostic x-ray examination. On the other hand, if such injuries do occur, they are rare, and scientists have so far been completely unsuccessful in finding and identifying them, just because they are so rare. Millions of dollars have been expended on research by the world's leading medical doctors and scientists, seeking positive evidence of significant injuries to man from the low levels of radiation exposure associated with most medical x-ray examinations. None has been found.

1-5. How were radiation and its effects discovered?. In 1895 , Professor Wilhelm Conrad Roentgen discovered x rays. Before then, he, and many other scientists around the world, were studying electrical discharges in gases, similar to the colorful discharges now seen in neon signs and fluorescent lighting. However, in those early years, not much was understood about what was happening, and there was great curiosity about the beautiful electrical displays that were observed in the partially evacuated discharge tubes.

On the day of Prof. Roentgen's discovery, there were several scraps of metal, covered with fluorescent material, lying about on his work bench. He noticed that, even when the tube was operated inside a closed cardboard box, some of these scraps began to glow when he turned the tube on and stopped glowing when he turned the tube off. He quickly concluded that whatever caused the glowing originated inside of his vacuum tube. Prof. Roentgen then realized that he had discovered a new phenomenon or a new kind of radiation. Because it was unknown -- and because in mathematics, X is used to stand for an unknown quantity -- he called these rays, "x- rays".

The remarkable thing is that within a few days after his announcement of this new effect, experimenters all over the world were producing x-rays with equipment that they had had in their laboratories for years.

Within a few weeks after Roentgen's announcement of the discovery of x-rays, a French scientist, Henri Poincare, reasoned that there might be some connection between the rays from Roentgen's tubes that made certain minerals glow, and something in the same minerals that would spontaneously produce the same glow or phosphorescence. A colleague, Henri Bequerel, undertook a systematic study of such minerals, including those containing uranium and potassium. After some initial experiments, he placed some of these minerals on photographic film in a dark drawer and found dark spots on the developed films where the minerals had been placed. He soon realized that he had discovered a type of radiation that was very much like Roentgen's x-rays. Becquerel had, in fact, discovered natural radioactivity, and he reported this about a year after Roentgen's discovery.

At about this time, Pierre and Marie Curie began trying to isolate the actual sources of the radiation Becquerel had discovered. Their work involved collecting and refining many tons of a uranium ore known as pitchblende. In December 1898, they announced the discovery of some very strong radiation from one of its constituents, a metal that they named "radium". After many years of research and production efforts, enough radium was isolated in pure form to enable it to be used in the experimental treatment of cancer, and for some commercial purposes. It was, however, enormously expensive, costing about $75,000 (in 1920 dollars) for 1 gram (1/35th of an ounce). At this rate one ounce would have cost about $2,500,000. However, even more than 20 years later, in 1920, there was less than an ounce of Radium available in all commerce. Subsequently, with the discovery of large quantities of radium-bearing ore in Africa, the price dropped and radium began to be widely used in medicine.

There was no reason to suspect any particular danger from rays that couldn't even be seen, so early researchers worked freely and feverishly with x-ray tubes. Thomas Edison, in this country, and others elsewhere, were exposed to unlimited amounts of radiation, unaware of any problem until they developed severe skin ulceration. Edison was one of the first to report this effect. Various safety precautions were immediately introduced, and experimenters began to be more careful. Most of the safety precautions involved placing some kind of material, such as lead, between the source and the experimenter. While this practice did not completely shield the experimenters, it certainly reduced their exposures.

Literally within months of the discovery of x rays, x-ray tubes were being manufactured all over the world. They came into almost immediate use in medicine -- assisting in the examination of bones, teeth, and other parts of the human body. Many of the early medical x-ray practitioners put themselves at risk of injury because they did not know how to quantify their radiation exposure nor how to evaluate its potential effects. The early x-ray physicians received some exposure during each patient examination, and they examined many patients. Individual patients must have had some exposures that were fairly large compared to those of today's patient and which may have lasted as long as thirty minutes (exposures today may take as little as 1/120th of a second). Few, if any, of these early patients are known to have been seriously harmed by x-radiation.

The widespread use of radium soon led to some of the same problems for the physicians administering it as had been seen with the use of x-rays, and similar protective practices were soon adopted. This meant that there was a good twenty to thirty years of experience on safety with radiation from radioactive materials before the discovery of nuclear fission and artificial radioactivity in the late 1930's

Great improvements were made in the gas x-ray tubes during the first twenty years of their use. However, the tubes remained temperamental in their operation, and they could not operate for extended periods. This situation changed radically in 1914, when Dr. William D. Coolidge, of the General Electric Company in the United States, developed the hot-cathode x-ray tube, which is still in use today. These new tubes operated steadily and reliably, and their radiation output was many times greater than that of the early x-ray tubes.

The introduction of Coolidge's new tubes into medicine and industry coincided with the start of the First World War and the large demand for the use of x rays by the military. Under the stressful conditions of combat, physicians using x-rays worked hour after hour, looking for shrapnel in wounded soldiers, setting broken bones, and examining other injuries. They often worked with their hands and forearms in the x-ray beam while they manipulated medical tools. The pressures of battlefield need took first place, and physicians had little time to consider the risks to themselves. As a result of the large x-ray exposures caused by these practices, many doctors received serious injuries, losing fingers, hands, arms, and some, their lives.