Chemical of the Week

ENVIRONMENTAL RADIATION

We are exposed to nuclear radiation every day. Some radiation comes from natural sources and some is anthropogenic (i.e., a result of human activity). Natural sources include cosmic radiation, radiation from lighter, unstable nuclei produced by the bombardment of the atmosphere by cosmic radiation, and radiation from heavy, unstable nuclei produced by the decay of long-lived nuclides in the earth's crust. Artificial sources include medical procedures, commercial products, and fallout from nuclear testing.

Nuclear radiation can cause biological damage because it is highly energetic. Nuclear radiation loses its energy when it passes through matter by ionizing the absorbing material. For this reason, nuclear radiation is called ionizing radiation. In the ionization process, neutral atoms in the absorbing material lose electrons, forming positive ions. Frequently, the ejected electrons possess sufficient energy to cause other atoms to ionize. The average amount of energy required to ionize an atom is 35 electron volts (eV). (1 eV is the amount of energy acquired by an electron accelerated in an electric field of 1 V; 1 eV is equal to 1.6 10-19 joules (J).) The energy of a single particle from a nuclear decay can be as high as 8 million electron volts (8 MeV), and one 8 MeV particle can produce 2 105 ions.

The magnitude of the radioactivity in a sample can be expressed as an activity, exposure, or absorbed dose. Activity is the number of nuclei that decay (disintegrate) per unit time. The most common unit of activity is the curie (Ci), which is defined as 3.7 1010 disintegrations per second. (Marie Curie discovered Ra-226, and 1 Ci is the activity of 1 gram of Ra-226.) The SI unit of activity is 1 disintegration per second or becquerel. Exposure is the amount of ionization caused by radioactive material. One roentgen is the amount of radiation that produces ions with a total charge of 1 electrostatic unit in 1 cm3 of dry air. (1 roentgen is equal to 2.58 104 Coulomb/kg of air). Absorbed dose is the amount of energy absorbed by a substance exposed to ionizing radiation. One radiation absorbed dose or rad is equal to 1 105 J/g. Different kinds of radiation cause different biological effects for the same amount of energy absorbed. For this reason, roentgen equivalent in man or rem was introduced. One rem is equal to one rad multiplied by a factor, Q, that accounts for the relative biological effect of radiation on humans. For X-rays, Q ~ 1, while for particles and fast neutrons, Q ~ 20.

Table of Units

The ionizing power of radiation depends on the type of radiation. An alpha particle, which is a helium nucleus, 4He2+, is relatively massive and ionizes virtually every atom in its path. However, alpha particles lose most of their energy after traveling only a few centimeters in air or less than 0.005 mm in aluminum. A beta particle, which is an electron, is relatively light and ionizes only a fraction of the atoms in its path. However, beta particles can travel more than a meter in air or several millimeters in aluminum.

For most people, cosmic radiation is the major source of adsorbed radiation. At sea level, the average person absorbs 26 millirem (mrem) per year. The atmosphere shields the surface of the earth from cosmic radiation; however, for each 100-meter increase in elevation, the dosage absorbed increases by about 1.5 mrem per year. A person traveling by commercial jet aircraft on a long flight, such as Los Angeles to London, can receive as much as 10 mrem during the flight.

When cosmic radiation interacts with gases in the atmosphere, it causes nuclear transformations that release neutrons and protons. These neutrons and protons interact with other nuclei in the atmosphere, producing radioactive nuclei, such as carbon-14 and tritium (3H). Carbon-14 is responsible for less than 1 mrem per year of absorbed radiation in humans, and tritium about 1 microrem.

Long-lived radioisotopes in the earth's crust are also a source of radiation. Potassium is one of the most abundant elements, and an essential component of food. Potassium-40 makes up 0.019% of all potassium, and has a half-life of 1.3 109 years. The average absorbed dose for a person from external potassium-40 is about 12 mrem per year, while that from internal potassium-40 is about 20 mrem per year.

For more information about environmental radiation, see "Radioactivity in Everyday Life," in the May, 1997, issue of the Journal for Chemical Education, page 501.


URANIUM: A RADIOACTIVE CLOCK

How old is the Earth, the solar system, or a piece of charcoal from an ancient campfire? Until the beginning of the 20th Century, geologists had no method by which to determine the absolute age of a material. The age of the earth was believed to be at most tens of millions of years. Not long after the discovery of radioactivity in 1896, scientists realized that radioactive decay constitutes a "clock" capable of measuring absolute geologic time. By 1907, the discovery that lead was the final product of uranium decay provided evidence that geologic time should be measured in billions of years.

Uranium occurs in numerous minerals, such as pitchblende (UO3UO2PbO) and carnotite (K2O2U2O3V2O53H2O), and is more plentiful in the Earth's crust than mercury or silver. Uranium was first isolated in 1841 by the reduction of uranium(IV) chloride with potassium.

4 K + UCl4  4 KCl + U

Uranium is sufficiently radioactive to expose a photographic plate in one hour. Naturally occurring uranium contains 14 isotopes, all of which are radioactive. The three most abundant are U-238 (99.28%), U-235 (0.71%), and U-234 (0.006%). In contrast to chemical reactions, where the isotopes of an element behave similarly, in nuclear reactions, isotopes behave quite differently. For example, of the three most abundant uranium isotopes, only U-235 undergoes fission.

U-238 decays by alpha emission to Th-234.
238U  234Th + 4He t = 4.5 109 years

The product of this reaction, Th-234, is also radioactive and undergoes beta decay.
234Th  234Pa + -1e t = 24 days

Protactinium-234 also decays by emitting a beta particle. These two reactions are the beginning of a series of 14 nuclear decay steps, referred to as the uranium decay series. After the emission of 8 alpha particles and 6 beta particles, the stable isotope Pb-206 is produced. The intermediate isotopes are called "daughters", and have half-lives that range from 1.6 10-4 seconds for Po-214 to 2.5 105 years for U-234. Two other radioactive series occur in nature, one that starts with U-235 and the other with Th-232.

U-238 Series

The uranium decay series has been used to estimate the age of the oldest rocks in the Earth's crust. The ratio of U-238 to Pb-206 in a rock changes slowly as the U-238 in the rock decays. Because the half-life of U-238 is 20,000 times that of the next longest half-life in the series, the rate of decay of U-238 is the rate-determining step in the conversion of U-238 to Pb-206. The rate of radioactive decay is first order in the amount of decaying isotope.

Rate Equation

In this equation, N0 is the number of U-238 atoms initially present in the sample, N is the number of U-238 atoms in the sample after a length of time t has elapsed, and k is the rate (decay) constant, which is equal to 0.693 divided by the half-life. If no Pb-206 was initially present in the sample, then N0 is equal to the sum of the number of atoms of U-238 and Pb-206.

Two other radioactive clocks are used for dating geologic time. One is potassium-40, which decays by electron capture to argon-40.

40K + -1e  40Ar t = 1.3 109 years

The other is rubidium-87 which emits a beta particle to form Sr-87.
87Rb  87Sr + -1e t = 5.7 1010 years

These radioactive "clocks" are more useful for dating rock samples than uranium because potassium and rubidium are more widely distributed in rock samples.

All radiochemical methods of dating have uncertainties associated with them. Several assumptions are made in determining an age. The most significant assumption is that the sample is a closed system, which is to say that no parent or daughter isotopes were gained or lost by the sample over time. Another assumption involves the amount of daughter isotope present at the time the sample was formed. For rare isotopes, this is generally assumed to be zero. The strongest evidence for the age of a sample is obtained when two different radiochemical dating methods produce the same result. Because the chemical properties of daughter products are very different, any geological transformation of a rock sample will have very different effects on the sample's daughter isotope content. Potassium and rubidium frequently occur together in rock samples, making this pair particularly important for radiochemical dating.

Radiochemical dating of samples from the Earth's crust yield a maximum age of about 3.5 109 years; however, the earth is believed to be older than this. The oldest meteorites and moon rocks are 4.5 109 years old. If these other members of the solar system were formed at the same time, then the Earth may also have formed 4.5 billion years ago. The isotopic composition of lead supports this conclusion. Of the four lead isotopes, only Pb-204 is not produced by radioactive decay of parent U-238, U-235, or Th-232. Comparing the isotopic composition of lead in the Earth's crust to t hat of meteorites free of uranium and thorium indicates that about 4.5 billion years of U and Th decay would be required to produce the Pb isotope ratios found on Earth.