We are continually bombarded by radiation from both natural and artificial sources. For example, we are exposed to infrared, ultraviolet, and visible radiation from the Sun, radio waves from radio and television stations, microwaves from microwave ovens, and X rays from various medical procedures. In addition, we are also exposed to radioactivity from the soil and other natural materials. The different energies of these various kinds of radiation are important in understanding their different effects on matter.
When matter absorbs radiation, the energy of the radiation can cause either excitation or ionization of the matter. Excitation occurs when the absorbed radiation excites electrons to higher energy states or increases the motion of molecules, causing them to move, vibrate, or rotate. Ionization occurs when the radiation removes an electron from an atom or molecule. In general, radiation that causes ionization, called ionizing radiation, is far more harmful to biological systems than radiation that does not cause ionization, called nonionizing radiation.
Most living tissue contains at least 70 percent water by mass. When living tissue is irradiated, most of the energy of the radiation is absorbed by water molecules. Thus, it is common to define ionizing radiation as radiation that can ionize water, a process requiring a minimum energy of 1216 kJ/mol. Alpha, beta, and gamma rays (as well as X rays and higher-energy ultraviolet radiation) possess energies in excess of this quantity and are therefore forms of ionizing radiation.
When ionizing radiation passes through living tissue, electrons are removed from water molecules, forming highly reactive H2O+ ions. An H2O+ ion can react with another water molecule to form an H3O+ ion and a neutral OH molecule:
The unstable and highly reactive OH molecule is an example of a free radical, a substance with one or more unpaired electrons, as seen in the Lewis structure for this molecule shown here, The presence of the unpaired electron is often emphasized by writing the species with a single dot, In cells and tissues, such particles can attack a host of surrounding biomolecules to produce new free radicals, which, in turn, attack yet other compounds. Thus, the formation of a single free radical can initiate a large number of chemical reactions that are ultimately able to disrupt the normal operations of cells.
The damage produced by radiation depends on the activity and energy of the radiation, the length of exposure, and whether the source is inside or outside the body. Outside the body, gamma rays are particularly harmful because they penetrate human tissue very effectively, just as X rays do. Consequently, their damage is not limited to the skin. In contrast, most alpha rays are stopped by skin, and beta rays are able to penetrate only about 1 cm beyond the surface of the skin (Figure 21.22). Hence, neither is as dangerous as gamma rays are, unless the radiation source somehow enters the body. Within the body, alpha rays are particularly dangerous because they transfer their energy quickly to the surrounding tissue, initiating considerable damage.
Figure 21.22 The relative penetrating abilities of alpha, beta, and gamma radiation.
In general, the tissues that show the greatest damage from radiation are those that reproduce at a rapid rate, such as bone marrow, blood-forming tissues, and lymph nodes. The principal effect of extended exposure to low doses of radiation is to induce cancer. Cancer is caused by damage to the growth-regulation mechanism of cells, inducing cells to reproduce in an uncontrolled manner. Leukemia, which is characterized by excessive growth of white blood cells, is probably the major cancer problem associated with radiation.
In light of the biological effects of radiation, it is important to determine whether any levels of exposure are safe. Unfortunately, we are hampered in our attempts to set realistic standards by our lack of understanding of the effects of long-term exposure to radiation. Scientists concerned with setting health standards have used the hypothesis that the effects of radiation are proportional to exposure, even down to low doses. Any amount of radiation is assumed to cause some finite risk of injury, and the effects of high dosage rates are extrapolated to those of lower ones. Other scientists, however, believe that there is a threshold below which there are no radiation risks. Until scientific evidence enables us to settle the matter with some confidence, it is safer to assume that even low levels of radiation present some danger.
Several different units are used for measuring radiation. The becquerel (Bq) is the SI unit for the activity of the radiation source, that is, for the rate at which nuclear disintegrations are occurring. A becquerel is defined as one nuclear disintegration per second. An older, but still widely used, unit of activity is the curie (Ci), defined as 3.7 1010 disintegrations per second, which is the rate of decay of 1 g of radium. Thus, a 4.0-mCi sample of cobalt-60 undergoes (4.0 10–3)(3.7 1010) = 1.5 108 disintegrations per second and has an activity of 1.5 108 Bq.
Two units commonly used to measure the amount of exposure to radiation are the gray and the rad. The gray (Gy), which is the SI unit of absorbed dose, corresponds to the absorption of 1 J of energy per kilogram of tissue. The rad (radiation absorbed dose) corresponds to the absorption of 1 10–2 J of energy per kilogram of tissue. Thus, 1 Gy = 100 rads. The rad is the unit most often used in medicine.
Not all forms of radiation have the same efficiency for damaging biological materials. For example, a rad of alpha radiation can produce more damage than a rad of beta radiation. To correct for these differences, the radiation dose is multiplied by a factor that measures the relative biological damage caused by the radiation. This multiplication factor is known as the relative biological effectiveness of the radiation, abbreviated RBE. The RBE is approximately 1 for gamma and beta radiation, and 10 for alpha radiation. The exact value of the RBE varies with dose rate, total dose, and the type of tissue affected. The product of the radiation dose in rads and the RBE of the radiation gives the effective dosage in units of rem (roentgen equivalent for man):
The SI unit for effective dosage is the Sievert (Sv), obtained by multiplying the RBE times the SI unit for radiation dose, the gray; hence, 1 Sv = 100 rem. The rem is the unit of radiation damage that is usually used in medicine.
The effects of short-term exposures to radiation appear in Table 21.7. An exposure of 600 rem is fatal to most humans. To put this number in perspective, a typical dental X ray entails an exposure of about 0.5 mrem. The average exposure for a person in one year due to all natural sources of ionizing radiation (called background radiation) is about 360 mrem.
The radioactive noble gas radon has been much publicized in recent years as a potential risk to health. Radon-222 is a product of the nuclear disintegration series of uranium-238 (Figure 21.4) and is continually generated as uranium in rocks and soil decays. As Figure 21.23 indicates, radon exposure is estimated to account for more than half the 360-mrem average annual exposure to ionizing radiation.
Figure 21.23 A graph of the sources of the average annual exposure of the U.S. population to high-energy radiation. The total average annual exposure is 360 mrem. (Data from "Ionizing Radiation Exposure of the Population of the United States," Report 93, 1987, National Council on Radiation Protection).
It is interesting to consider the interplay between the chemical and nuclear properties of radon that make it a health hazard. Being a noble gas, radon is extremely unreactive and is therefore free to escape from the ground without chemically reacting along the way. It is readily inhaled and exhaled with no direct chemical effects. However, the half-life of 222Rn is short: 3.82 days. It decays through alpha-particle loss into a radioisotope of polonium:
Because radon has such a short half-life and alpha particles have a high RBE, inhaled radon is considered a probable cause of lung cancer. Even worse, however, is the fact that the decay product, polonium-218, is an alpha-emitting solid that has an even shorter half-life (3.11 min) than radon-222:
The atoms of polonium-218 can become trapped in the lungs, where they continually bathe the delicate tissue with harmful alpha radiation. The resulting damage is estimated to result in as many as 10 percent of lung cancer deaths.
The U.S. Environmental Protection Agency (EPA) has recommended that radon-222 levels in homes not exceed 4 pCi per liter of air. Homes located in areas where the natural uranium content of the soil is high can have levels much greater than that. As a result of public awareness, radon-testing kits are readily available in many parts of the country (Figure 21.25).
Potassium ion is present in foods and is an essential nutrient in the human body. One of the naturally occurring isotopes of potassium, potassium-40, is radioactive. Potassium-40 has a natural abundance of 0.0117 percent and a half-life of t1/2 = 1.28 109 yr. It undergoes radioactive decay in three ways: 98.2 percent is by electron capture, 1.35 percent is by beta emission, and 0.49 percent is by positron emission. (a) Why should we expect 40K to be radioactive? (b) Write the nuclear equations for the three modes by which 40K decays. (c) How many 40K+ ions are present in 1.00 g of KCl? (d) How long does it take for 1.00 percent of the 40K in a sample to undergo radioactive decay?
SOLUTION (a) The 40K nucleus contains 19 protons and 21 neutrons. There are very few stable nuclei with odd numbers of both protons and neutrons.
(b) Electron capture is capture of an inner-shell electron by the nucleus:
Beta emission is loss of a beta particle by the nucleus:
Positron emission is loss of by the nucleus:
(c) The total number of K+ ions in the sample is
Of these, 0.0117 percent are 40K+ ions:
(d) The decay constant (the rate constant) for the radioactive decay can be calculated from the half-life using Equation 21.20:
The rate equation, Equation 21.19, then allows us to calculate the time:
That is, it would take 8.51 billion years for just 1.00 percent of the 40K in a sample to decay.