If we consider visible light to be the dividing line within the universe of invisible electromagnetic waves, what can we say about wavelengths on either side of the visible? Those wavelengths shorter than visible light carry more energy. The shorter wavelength radiations (think x-rays) carry enough energy that they go beyond simply bending molecules; they can actually break them. And it is the breaking of biological molecules that results in radiation’s adverse biological effects. In contrast, those radiations with longer wavelengths carry much less energy than light (think radio waves), not even enough to bend retinal or other biological molecules. If these low energy radiations have biological effects, their mechanism is more obscure and beyond what we currently understand about biology.
So the dividing line of visible light turns out to be the dividing line for adverse biological effects. Scientists call the radiations with shorter wavelengths (i.e., higher energies) the ionizing radiations because they damage molecules by ripping off electrons and producing ions.62 The longer wavelength (i.e., lower energy) radiations, in contrast, are called nonionizing because they don’t have sufficient energy to ionize anything. That’s not to say that nonionizing radiation can’t affect molecules, but their effect is primarily to shake them up so that they start bumping into each other. Scientist call this bumping of molecules kinetic energy—energy that is commonly experienced as an increase in temperature and the production of heat. So it is possible to heat something (and even burn it) with nonionizing radiation (think microwave ovens). Nevertheless, these nonionizing radiations usually do not permanently damage the molecules in the way that the ionizing radiations do. Consequently, the nonionizing types of radiation have much less, or no, potential to produce the kinds of adverse biological effects that we are most concerned about, such as genetic damage and cancer.
Before we go any further with this line of thought, it needs to be mentioned that ultraviolet (UV) radiation is a special case. The wavelengths of UV radiation lie just at the cusp between the ionizing radiations and visible light and, therefore, share properties with each. Although UV radiation does not have energy sufficient to ionize atoms, it does have sufficient energy to move the atom’s electrons to higher energy states. Such atoms are said to be in an excited state. As with all excitement, it subsides with time. But in the meantime, such atoms can undergo excited-state chemical reactions that don’t normally occur when the atoms are not excited (i.e., when they are in their so-called ground state). Excited-state reactions can produce specific types of biochemical damage that are harmful to cells. Although UV radiation is not able to penetrate tissues to any extent, it is able to cause damage at the surface. So high exposures to UV radiation can produce skin burns (i.e., sunburns) and skin cancer. Because the hazards of UV radiation are in many ways unique and largely limited to skin and eye damage, the health risk associated with UV exposure is a topic unto itself.
We need not dwell on these technical aspects of wavelength now. We will revisit wavelength issues in our discussions of health effects. Just know for now that when the question is in regard to radiation risks, wavelength matters.
WAVES AREN’T THE END OF IT
Thus far, we have discussed radiation solely in terms of electromagnetic waves and, in fact, much of the radiation that humans are exposed to is of the electromagnetic kind. There is, however, another subclass of ionizing radiation that comes in the form of high-speed atomic particles, called particulate radiation. The particulate radiations are ionizing, just like x-rays. As such, their effects on health are quite similar, as we shall see. These particles are typically produced through the decay of unstable atoms—a phenomenon known as radioactivity—and are best described in terms of their connection with radioactivity. We will now turn our sights to radioactivity.
CHAPTER 3
SEEK AND YOU SHALL FIND: RADIOACTIVITY EVERYWHERE
Name the greatest of all inventors. Accident.
—Mark Twain
A SILVER LINING
Antoine Henri Becquerel (1852–1908) was not interested in Roentgen’s x-rays or even in seeing images of his bones. But he was fascinated with fluorescence.1 Becquerel was specifically interested in determining exactly how chemicals could store visible light and release it later. In fact, his family had worked on this exact problem for decades.
Becquerel held the chair of the Department of Physics at the Musée d’Histoire Naturelle in Paris, like both his grandfather and his father had before him. All of them had studied fluorescence.2 Over many years of pursuing this multigenerational scientific passion, the Becquerel family had accumulated a large collection of minerals that had the ability to fluoresce. It was this collection that enabled Antoine Henri Becquerel to make his most important discovery, and win the Nobel Prize. To Becquerel’s dismay, however, his momentous discovery had nothing to do with fluorescence.
Becquerel had fluorescent screens and photographic films, but no Crookes tube; nor did he want one. Nevertheless, Roentgen’s report of invisible x-rays accompanying the faint visible glow from his Crookes tube made Becquerel wonder whether x-rays were also mixed in with the visible glow from fluorescence. That is, were fluorescent materials emitting those penetrating x-rays along with their nonpenetrating visible light? He designed a series of simple experiments to test his idea.
He took a piece of photographic film and sealed it tightly within thick black paper so that no light could enter to expose the film. Then he sprinkled on the black paper granules of the fluorescent material he wanted to test. He then put the whole set-up out in bright sunlight, to stimulate fluorescent emissions. At the end of the day, he developed the film hoping to find an image of the overlying fluorescent material on the developed film. He reasoned that, if the penetrating x-rays were being emitted along with the fluorescent rays, the x-rays alone and not the visible fluorescent light should penetrate the dark paper and develop the film, just as they had done for Roentgen. Consequently, if images of the granules appeared on the developed film, he would have evidence of x-ray emissions from fluorescent chemicals. Methodically he tested his entire collection of fluorescent chemicals for evidence of x-rays.
To Becquerel’s chagrin, no images were forthcoming from any of his fluorescent chemicals until he tested uranium sulfate. This compound gave him a dark image of the granules. Elated that his hypothesis was proving correct in at least one case, he proceeded to do more experiments with the uranium sulfate. One day he prepared it and the film packets as usual. But when he went to place them out in the sunlight, the skies had turned cloudy, so he decided to postpone the experiment until the next day. Unfortunately, the poor weather persisted. Impatient to get an answer, he decided to develop the film in the hope that even the weak room lighting might induce a faint image. Instead, he found a very intense image—much more intense than could be explained by room light. At this point he realized that he had neglected an essential experimental control: he had never shown that the image from uranium sulfate granules actually required prior exposure to light. Consequently, he prepared a control where he completely covered the uranium sulfate so it would experience no light exposure at all. To his astonishment, the resulting photographic film still had the same image of the granules.
Adding to the mystery, Becquerel soon found that even nonfluorescent uranium compounds exposed the films. The only common denominator in the experiments seemed to be the presence of uranium atoms. Neither prior sun exposure nor fluorescent properties of the compound were required. The presence of uranium atoms alone was necessary and sufficient to expose the film.3
By this time Becquerel knew his hypothesis about x-rays and fluorescence was wrong. But exactly how was it wrong? He was confused. Ultimately, Becquerel had no other option but to conclude that uranium atoms spontaneously emit some type of invisible radiation with properties similar to x-rays. Further experiments confirmed this supposition and supported the notion that the invisible penetrating radiation and the visible nonpenetrating fluorescence were completely unrelate
d phenomena; the invisible penetrating radiation resulted from a nuclear process, and the visible nonpenetrating radiation from a chemical process. The co-occurrence of these two properties in uranium sulfate was a fortuitous coincidence.4 That coincidence allowed Becquerel to discover radioactivity.
Luck had once again favored a prepared mind, and Becquerel’s place in scientific history was secure. In 1903, he was awarded the Nobel Prize in Physics for his discovery, just two years after Roentgen received his for discovering x-rays. To further honor him, the standard international unit for measuring radioactivity would eventually be named a becquerel.5
Still, Becquerel never became the celebrity that Roentgen was. This was probably for three reasons. First, he shared his 1903 Nobel Prize with Marie and Pierre Curie, who were the colorful husband and wife scientific team that discovered radium—a highly radioactive element that they found in uranium ore—and would soon capture the public’s imagination.6 Second, unlike the easily obtainable Crookes tubes, uranium was very scarce, so few scientists could easily perform the same kinds of experiments that Becquerel had. In fact, uranium was thought to be scarce even up to the time of the invention of the atomic bomb.7 We now know it to be one of the more common and widely distributed elements on Earth; more common than silver or gold. Third, the photographic images produced by uranium radioactivity were very diffuse, and the medical utility of radioactivity not immediately apparent. It wasn’t until radium was purified and separated from the uranium ore that the use of radioactive substances in medicine started to grow.8
Although Becquerel appreciated the parallels between his discovery of radioactivity and Roentgen’s discovery of x-rays, he either was not aware of the danger of x-rays or he did not realize that he had enough radioactivity on hand to produce similar health effects. He continued to work with radioactivity without any protection. One day, he put a vial of radioactive material in his vest pocket where it remained for several hours. Later, he found the skin on his abdomen had been burned.9 He suddenly had a new respect for the stuff.
Becquerel’s biggest stroke of personal luck may have been that he stopped working with radioactivity very soon after he discovered it. Radioactivity had been only a side interest of his, and he thought the big discoveries with radiation had all been made. Having discovered radioactivity in 1896, he was largely out of the radioactivity business within a couple of years. He published his last paper on the subject in 1897 and moved on to new things.10 He left it to others to hammer out the scientific details. Other than that one-time radiation burn on his belly, Becquerel was not known to have suffered any ill health due to his brief work with radioactivity. His short tenure in that research field, coupled with the fact that his uranium samples had relative weak radioactivity levels, likely meant that his body’s lifetime radiation dose from his exposures to radioactivity was quite modest, and was probably restricted mostly to his hands.
HOT ROCKS: URANIUM
By definition, radioactivity is the ability of an atom to spontaneously release radiation without any stimulation from light, electricity, or any other form of energy. It is a property intrinsic to the nucleus of an atom and resistant to modification by any outside forces. Uranium was the first radioactive substance to be discovered, and still is one of the best known, but dozens of elements exist in hundreds of different radioactive forms known as radioisotopes. Many of these radioisotopes are mixed with and cannot be easily separated from their nonradioactive forms. And some of the elements with natural radioisotopes are essential to life, including carbon, the element that plays a critical role in all biochemical synthesis pathways, and potassium, an essential element in cells and blood. Thus all living things are radioactive, to some extent. Radioactivity inside and outside of human bodies exposes humans to a continuous bath of low-level radiation.
Becquerel possessed uranium sulfate within his collection of minerals simply because it fluoresced. It was the only mineral in his fluorescent mineral collection that also happened to be significantly radioactive, so Becquerel would not have discovered radioactivity if he didn’t have uranium sulfate. But there is another coincidence about uranium that Becquerel did not discover, and it would turn out to be even more important. Uranium is one of the very few radioactive elements that can also undergo spontaneous nuclear fission—a natural process where an atom’s nucleus suddenly splits into two or more smaller parts.11 Spontaneous nuclear fission is the property that made uranium the key to the development of a nuclear bomb. Thus, this convergence of fluorescence, radioactivity, and fission into a single element could be viewed as one of the luckiest coincidences of nature … or one of the unluckiest. Had uranium’s radioactive properties not been discovered in 1896, it is highly unlikely that the atomic bomb could have been developed in time for deployment during World War II by any nation, and humanity’s loss of innocence regarding nuclear warfare would have either been averted entirely or at least greatly postponed.
RADIOACTIVE DECAY
Why are some elements radioactive and others not? To answer this question we must first consider what makes an atom stable (i.e., nonradioactive). As most high-school science students know, the nucleus of the atom contains a mixture of two subatomic particles. These are the positively charged particles called protons and the uncharged particles termed neutrons; the latter are electrically neutral (i.e., they have no charge). Positive charges repel each other, so the nucleus would not remain together were it not for the presence of the neutrons, which dilute the positive charge to the point that the nucleus can remain intact and stable.
As it turns out, for most atoms, the nucleus is stable when the number of protons nearly equals the number of neutrons.12 If an atom’s nucleus diverges too much from this one-to-one ratio in either direction, the nucleus is likely to be unstable. The farther from a one-to-one ratio, the more unstable it will be, and the more likely the situation will be corrected. If an atom has too many protons, a proton is converted into a neutron. If it has too many neutrons, a neutron is converted into a proton. This process is called radioactive decay. By decaying, the atom moves closer to a one-to-one ratio, and closer to stability. If after one decay, the atom still has an excess of either protons or neutrons, the process can repeat itself until ultimately the nucleus is stable. (Such a sequence of decays of radioactive elements on the way to a stable atomic nucleus is called a decay chain.)
To illustrate this concept, let’s look at some different nuclear forms of carbon. Stable carbon has six protons and six neutrons (known as carbon-12, because it has a total of 12 nuclear particles). If a carbon atom has six protons and seven neutrons (carbon-13), the extra neutron can be tolerated and the atom is still stable. However, if the carbon atom has six protons and eight neutrons (carbon-14), the neutron burden is too high, the atom is radioactive, and it decays by converting a neutron into a proton.13 This results in seven protons and seven neutrons—a one-to-one ratio—and a stable nucleus. Ironically, the new stable atom is no longer carbon! This is because an element’s chemical identity is determined specifically by its number of protons. All carbon atoms have six protons, with a varying number of neutrons. Because our new stable atom now has seven protons it has become a nitrogen atom. So, the consequence of this radioactive decay is that an unstable carbon-14 atom has been converted into a stable nitrogen-14 atom. This, fundamentally, is how radioactive decay works, and atoms that can potentially decay because of their unstable nuclei are said to be radioactive.
We will soon learn more about neutrons in the atomic nucleus. But while we are discussing carbon-12, carbon-13, and carbon-14, we might as well clarify some terminology that is commonly used, but frequently confused. These three different versions of the same element (carbon) are called isotopes, from the Greek word meaning “in the same place.” They are in the same place in that they all have six protons and are all, therefore, carbon, as explained previously. Nonetheless, they differ from one another in their neutron numbers—six, seven, and eight, for carbon-12, carbon-13
, and carbon-14, respectively. Elements that differ only in their neutron numbers are said to be isotopes of one another. These isotopes can be either stable (e.g., carbon-12 and carbon-13) or unstable and, therefore, radioactive (e.g., carbon-14). The radioactive isotopes are often called radioisotopes. So as we move through our story, know that radioisotopes are just the versions of elements that happen to be radioactive because they have an unstable ratio of neutrons to protons in their nucleus. Remember also that all elements have proton-neutron combinations that are radioisotopes. Some are very common and easily found in our environment, while others are so rare and fleeting that they need to be artificially produced in order to study them.
PARTICULATE RADIATION
You might well have noticed that as carbon-14 decays to nitrogen-14, the charge of the nucleus has gone from 6+ to 7+ (i.e., six protons become seven). You may also remember from your high-school physics that charge can neither be created nor destroyed (law of conservation of charge). So how can the creation of an additional positive charge in this atom be accounted for?
What has not yet been mentioned is that when the carbon-14 decays, its surplus neutron (charge = 0) changes into both a proton (charge = 1+) and an electron (charge = 1−). So the net charge is zero (1+ + 1− = 0), and there actually is a conservation of charge. But the negatively charged electron is ejected from the atom in the form a high-speed particle known as a beta particle. This beta particle is but one representative of the family of radiation types called particulate radiation.
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