Radioactivity | Research & Encyclopedia Articles

Radioactivity

Radioactivity is the process in which unstable atomic nuclei become more stable by spontaneously emitting highly energetic particles and/or energy. A sample of material is said to be radioactive if some of its atomic nuclei are emitting such radiation. The radiations emitted by unstable nuclei are capable of ionizing matter and disrupting molecules, including DNA; they are therefore a biological hazard in prolonged or intense exposures.

Radioactivity is important to society for two reasons. First, it is produced in large amounts by nuclear fission in nuclear power plants, and the safe disposal of radioactive waste is a problem. Second, radioactivity is widely used as a diagnostic and therapeutic tool in many important medical applications. It is therefore both a burden and a blessing to society.

Stable and unstable nuclei

Every atomic nucleus consists of a certain number of protons, strongly bound to a certain number of neutrons. Among the almost limitless number of kinds of nuclei that can be concocted by combining various numbers of protons with various numbers of neutrons, only certain combinations will be stable. The rest will be unstable or radioactive: they will spontaneously change their proton-neutron composition. Atoms that are radioactive are radionuclides, often called radioisotopes. (Nuclide is a generic term meaning "kind of nucleus," just as species is used to denote a specific kind of plant or animal. A radionuclide is a radioactive nuclide.)

Making different nuclides out of protons and neutrons is similar to making different kinds of molecules out of atoms. You can't just make a molecule out of any old combination of atoms and expect it to hold together indefinitely, if at all, because atoms bind together according to certain rules of chemical bonding. For example, two hydrogen atoms and one oxygen atom form a perfectly stable molecule, H₂O. But two hydrogen atoms and two oxygen atoms make the unstable molecule H₂O₂, hydrogen peroxide. This compound will slowly break down all by itself into water and oxygen. If you try to make a molecule out of two hydrogen atoms and three oxygen atoms, it won't hold together long enough for you to name it.

Similarly, the nucleus that consists of two protons and two neutrons is absolutely stable; it is a helium-4, the helium isotope of mass number 4, symbolized ⁴He. But try to use three neutrons, to make a nucleus of helium-5 (⁵He), and it'll blow itself apart after only 10^{- 21} seconds.

There are certain natural "rules" that govern how many protons and neutrons can bind together to form a stable nucleus. While these rules of nuclear binding aren't understood as deeply as are the rules for molecular bonding, their effects are well known and predictable. Nuclides that are constructed of "rule-breaking" numbers of protons and neutrons will be unstable to varying degrees. They will spontaneously break up, emitting particles and energy in order to change themselves into more stable nuclei. That is, they will be radioactive. This is an example of the general principle that nature always tries to increase the stability of a system, and in order to accomplish that it will use any avenue that is open to it--that is, any process that is energetically possible. In the case of unstable nuclei, there appear to be three options: the nucleus can split apart, it can emit particles, or it can emit pure energetic radiation. The general term for all of these changes is nuclear disintegration. Radioactivity, then, is any process by which unstable nuclei disintegrate in order to become more stable.

When a radioactive nucleus disintegrates by emitting an alpha or beta particle, it changes into a different nucleus--that is, one with a different number of protons and/or neutrons. In those cases in which the number of protons changes, the new nucleus has a different atomic number, and it therefore belongs to a different element. In other words, the radioactive atom has undergone a transmutation from one element to another. When this fact was first proposed at the beginning of the twentieth century, it was very difficult for the scientific world to accept because until that time the notion of changing one element into another had existed solely in the realm of alchemy and magic.

Any observable sample of a radionuclide will contain a huge number of atoms. As the unstable nuclei of these atoms continue to disintegrate one after the other, changing themselves into other kinds of atoms, there will be fewer and fewer of the original kind left. The more unstable that particular radionuclide is, the faster its atoms will be disappearing. Radionuclides are known that are so unstable that their half-lives are only the tiniest fractions of a second that can be measured. Others are known that are so very slightly unstable that their half-lives are trillions and quadrillions of years.

Many nuclides appear to be absolutely stable, and presumably their atoms will last forever. There are no avenues open to them, no spontaneous, energetically possible processes that could change their numbers of protons and neutrons to a more stable combination. Of the roughly 2,000 known nuclides, only 264 are stable. The rest are all radioactive in one way or another. Some of these radionuclides (such as the isotopes of uranium and radium) occur naturally on earth, but the vast majority of them have been made artificially in nuclear reactors and in particle accelerators, commonly known as "atom smashing" machines.

History of radioactivity

In December 1895, the German physicist Roentgen, Wilhelm Roentgen (1845-1923) announced his discovery of mysterious penetrating rays--he called them x rays, and we still do--that could go right through an object such as a human hand, making an image of the bones on a photographic plate placed behind the hand. Because these rays came from fluorescent (glowing) spots in Roentgen's glass vacuum tubes, scientists immediately began to test a wide variety of fluorescent materials--materials that glow after being exposed to light--to see if they also emitted x rays.

In early 1896, the French physicist Henri Becquerel (1852-1908), working in Paris, was examining many chemical substances that were known to be fluorescent. He first exposed them to bright sunlight to make them fluoresce and then observed whether they emitted any rays that could go through light-proof paper and expose a photographic plate that was wrapped inside. On one gray February day, he put some samples away in a drawer until he could expose them on the next sunny day. To his surprise, he found that these samples left an image on a photographic plate that was also stored in the drawer. Apparently, the samples were emitting some kind of penetrating radiation without even having to be exposed to light. He soon found that only compounds that contained the element uranium had this ability to emit radiation, and it didn't matter what chemical compounds the uranium atoms were in. Becquerel had discovered that uranium is radioactive.

What was startling about this discovery was that the uranium atoms were apparently a source of energy all by themselves; they didn't have to be "activated" by absorbing light energy or anything else. Where the uranium atoms were getting their energy was the big For more than 50 years, scientists had believed in the law of conservation of energy: that energy simply cannot come from nowhere. But they also believed strongly that atoms were unchangeable. How could uranium atoms be giving off radiation without changing?

In December 1897, Marie Curie (1867-1934), a Polish chemist working on her doctoral thesis at the Sorbonne in Paris, began to try to find out where this mysterious uranium energy was coming from. In the process, she discovered two new elements that are millions and billions of times more radioactive than uranium: radium and polonium. By inventing new chemical techniques for separating radioactive elements, Marie Curie laid the foundation for all of today's applications of radioisotopes in industry and medicine. She was the world's first radiochemist.

Marie Curie's work was an important step toward the understanding of radioactivity. However, it remained for Ernest Rutherford (1871-1937) and Frederick Soddy (1877-1956) to suggest in 1902 that radioactivity represents an actual disintegration of atoms. Later, Rutherford also discovered the atomic nucleus and the proton. In 1931 he was named Baron Rutherford of Nelson (the town in New Zealand where he was born) in recognition of his scientific achievements.

Lord Rutherford found two kinds of rays coming from the radioactive uranium atoms: a slightly penetrating kind that wouldn't even go through paper and a somewhat more penetrating kind that would go through thin sheets of metal. He called them alpha rays and beta rays, respectively. Later, a very penetrating, third kind of radiation, called gamma rays, was discovered. These three types of radiation, symbolized by the Greek letters , , and , still represent the three major types of radioactivity. We now know that alpha and beta "rays" are actually high-speed subatomic particles, while gamma rays are electromagnetic energy waves of pure energy.

Types of radioactivity

Nuclear chemists and physicists have found that certain combinations of neutrons and protons seem to make the most stable nuclei. In general, the most stable nuclei will be those that (a) contain nearly equal numbers of protons and neutrons, but with more neutrons than protons, and that (b) have an even (rather than odd) total number of protons plus neutrons. Nuclei that deviate too much from these rules will be unstable to various degrees.

The three kinds of radioactivity, alpha, beta, and gamma, come from nuclei that deviate in three different ways from the stability rules: alpha particles come from nuclei that have too many protons plus neutrons (that is, they are simply too big and heavy); beta particles come from nuclei that have too many protons or too many neutrons; and gamma radiation comes from nuclei that simply have too much energy.

Nuclei are known that contain up to 266 neutrons and protons, a very large number. That's a lot of particles to be packed into a volume that has a radius of only 10^{- 12} centimeter, especially since almost half of the particles are protons--positively charged particles that, being all of the same charge, are trying hard to repel each other. So when a nucleus is too "big and fat," it tries to reduce by shooting off some of its particles. The most energetically favorable combination of particles that it can shoot off is a tight little package of two protons and two neutrons. Two protons and two neutrons, bound together, constitute a nucleus of helium-4. This nucleus, when shot out at high speed by an unstable nucleus, is called an alpha () particle.

Alpha particles, containing two protons, have a charge of +2. So when an alpha particle is shot off into the surrounding matter by a radioactive nucleus, it will interact very strongly with the negatively charged electrons in the matter. This has two effects: (1) the alpha particle tears electrons off many atoms as it passes through, that is, it ionizes many atoms, and (2) it slows down and stops very quickly because the ionization process uses up its energy. Alpha particles, therefore, cannot penetrate very far through matter before they slow down completely and stop. Even a sheet of paper will stop most alpha particles.

All nuclides that are heavier than bismuth (atomic number 83, mass number 209) are too heavy to be stable; they are radioactive and emit alpha particles. Among the commonly known alpha emitters are various isotopes of polonium, radium, thorium, uranium and all of the transuranium elements.

When a nucleus emits an alpha particle, it decreases its mass number—the total number of protons plus neutrons--by four units: it loses two protons and two neutrons. However, losing the two protons also decreases its nuclear charge or atomic number by two units, transforming it into the nucleus of a different element, two spaces to the left in the periodic table. For example, when a radium nucleus (atomic number 88) emits an alpha particle, it becomes a nucleus of radon (atomic number 86).

²²⁶Ra ⁴He + ²²²Rn + Energy

(In these symbols, the superscript is the mass number, the total number of protons and neutrons in the nucleus.) The released energy is mostly in the form of kinetic (movement) energy of the alpha particles, which are emitted at speeds of around one-tenth the speed of light, depending on the particular radionuclide that is emitting them.

Alpha particles are both highly energetic and relatively highly charged, so they disrupt many atoms and molecules along their paths before they come to a stop. But they are not much of a hazard to living things because they stop so soon. They can only penetrate about a thousandth of an inch (0.03 mm) of aluminum, for example. Human skin will stop them, so they can't penetrate far enough to disrupt the cells in any vital organs. If alpha-emitting radionuclides are inhaled into the lungs or ingested into the stomach, however, they can do their damage locally to highly susceptible kinds of tissues, and can therefore be very dangerous. Inhaling radonradon gas has been blamed for many lung cancers in uranium miners, for example. There is radon gas in uranium mines because the disintegration of uranium leads to radium, which then forms radon as shown in the equation above. The radon is not yet stable, and it emits alpha particles itself.

A second means of disintegration that is open to too-heavy nuclides is spontaneous fissionspontaneous fission. Most of the transuranium elements have isotopes that disintegrate by fissioning (splitting) in addition to emitting alpha particles.

As previously stated, a nucleus must have roughly equal numbers of protons and neutrons in order to be stable. If a nucleus has too many protons for its number of neutrons, it will be radioactive. Simply shooting out an unwanted proton turns out to require more energy than the nucleus has to give, except in a few very rare cases. (Lutetium-151, which has an extremely large number of protons compared with its number of neutrons, has actually been observed to emit protons from some of its nuclei.)

If a nucleus with too many protons could transform one of them into a neutron, however, it would be improving its proportion of protons to neutrons by simultaneously losing a proton and gaining a neutron. And that is what it does, but because the proton has a positive charge and the neutron is neutral, the nucleus somehow has to get rid of a positive charge. It does that by creating and emitting a positronpositron, also known as a positive beta particle. For example, potassium-40, an isotope of potassium that constitutes 1.2% of all potassium atoms in nature (including those in our own bodies), is radioactive and emits positrons.

⁴⁰K ⁺ + ⁴⁰Ar + Energy

Positrons are emitted at perhaps nine-tenths the speed of light, depending on the radionuclide that is emitting them. The positron is a light particle, identical to an ordinary electron except that its charge is +1 instead of -1. Theory predicts that every particle has an opposite, called an antiparticleantiparticle, and the positron is the antiparticle of the electronelectron. Antiparticles don't last long in our world of ordinary particles, however. As soon as a positron (or any antiparticle) meets an ordinary electron (or its ordinary counterpart), the two particles annihilate each other: they both disappear in a puff of energy.

Positrons emitted from radioactive nuclei are light-weight and have only a single unit of charge, so they don't interact as strongly with matter as alpha particles do. They can penetrate farther into matter--about a tenth of an inch of aluminum--before slowing down and annihilating. Therefore, they are also a greater hazard to humans than alpha particles are.

Another kind of radioactivity that accomplishes the same thing as positron emission is electron capture. This is the process in which a proton is converted into a neutron by the nucleus capturing a negative electron from one of the inner orbits of its atom. No particles are emitted, but some x rays are, due to the now-missing atomic electron. Radionuclides that have too many protons often disintegrate by both methods: some of the nuclei by positron emission and some by electron capture.

If a nucleus has too many neutrons in relation to its number of protons, it will try to become more stable by decreasing its number of neutrons. Ejecting a neutron is energetically unfavorable, however, except in one or two rare cases. (Lithium-11 has been observed to emit neutrons.) However, a nucleus with too many neutrons can convert one of them into a proton. To do this, it would have to find an extra positive charge, because the neutron is neutral and the proton is positively charged. But an object can also increase its positive charge by throwing out a negative charge, and that is what the nucleus does: it creates and emits a negative beta particle, which is identical to an ordinary electron except that it comes from a nucleus instead of from the outer parts of an atom.

A common emitter of negative beta particles is carbon-14, the radioactive isotope of carbon that is found in all living plants and animals.

¹⁴C ^- + ¹⁴N + Energy

Negative beta particles penetrate matter in the same way as positive beta particles do, except that they don't annihilate.

A nucleus can have an unusually large amount of internal energy, just as the electrons in a whole atom can. In both cases, we say that the nucleus or the atom is excited, or in an excited state. A nucleus can find itself in an excited state when, for example, it has just been created through the disintegration of another radioactive nucleus. Just as an excited atom can dispose of its excess energy by emitting x rays, an excited nucleus can emit gamma rays. Gamma rays are electromagnetic radiation just like x rays except that they are generally of higher energy. When a nucleus emits gamma rays, the composition of its protons and neutrons does not change.

Most radionuclides that emit alpha and beta particles also emit gamma rays. This is because the nuclei into which they are converted are often created in excited states; these excited nuclei immediately get rid of their excess energy by emitting gamma rays. This is an important safety consideration because gamma rays are extremely penetrating and can cause biological damage all the way through the body. Almost any radioactive substance must be assumed to be emitting highly penetrating gamma rays, even if the substance is known to be "only" an alpha or beta emitter.

Radioactivity in nature

All of the elements heavier than bismuth (atomic number 83) are completely radioactive--that is, they have no stable isotopes. We still find quite a few of the elements in nature, either because they have such long half-lives that they haven't completely died out since the earth was formed some 4.5 billion years ago, or because they are constantly being produced by the disintegration of uranium or thorium, whose half-lives are indeed comparable to the age of the earth. (By an interesting coincidence, uranium-238, the principal isotope of uranium, has a half-life of 4.47 x 10⁹ years, just about equal to the age of the earth. Thus, there is almost exactly half as much uranium left on earth today as there was when the earth was formed.)

There are three series of naturally occurring heavy radionuclides. Each one begins with a radionuclide that is long-lived enough to have survived since the earth was formed. By a sequence of alpha and beta disintegrations, it transforms itself into a series of other radionuclides, until it becomes a stable isotope.

One natural radioactive series begins with uranium-238 (atomic number 92), which undergoes a long sequence of disintegrations, producing radioactive isotopes of several elements until it reaches the stable isotope, lead-206. A second series of disintegrations begins with uranium-235 (half-life 7.04 x 10⁸ years) and winds up as stable lead-207. The third series begins as thorium-232 (half-life 1.41 x 10¹⁰ years) and winds up at stable lead-208.

Along the way, these disintegration series produce radioactive isotopes of protactinium, thorium, actinium, radium, francium, radon, astatine, polonium, bismuth, lead, thallium and mercury. The first eight of these are those heavier-than-bismuth elements that we find in nature. Depending on the relationship between the half-lives of the radionuclides and the half-lives of their predecessors and successors in the sequences that produce them, various amounts of these elements exist on earth at the present time.

In addition to the heavy radioactive elements, 18 lighter elements have radioisotopes that are found in nature because their half-lives are long compared with the age of the earth. Two others, hydrogen-3 (tritium) and carbon-14, are constantly being produced by special processes. All of the naturally occurring radionuclides, both heavy and light, contribute to a certain amount of radioactivity to which everyone on earth is always being exposed, regardless of humanity's activities in nuclear technology.

Synthetic radioactivity

Of the roughly 1,700 radioactive nuclides that are known, only about 70 occur in nature. The rest have all been made synthetically: either they were found in the nuclear debris of man- made nuclear fission, or they have been produced in particle accelerators. During nuclear reactions carried out in accelerators, the numbers of protons and neutrons in atomic nuclei can be changed, thereby transforming them into different, and sometimes new, radionuclides. In fact, the elements with atomic numbers 43, 61, and 85 (technetium, promethium, and astatine, respectively) were unknown on earth until some of their radioactive isotopes had been produced synthetically. In addition, all of the elements with atomic numbers higher than uranium's (92) were discovered by making them synthetically in particle accelerators.

Applications of radioactivity

Both natural and synthetic radionuclides, generally referred to as radioisotopes, are of enormous value in science and industry, and in medical research, diagnosis, and therapy. Applications of radioisotopes are based primarily on two facts: radioactivity can be detected with such astounding sensitivity--the disintegration of single atoms can actually be detected--that extremely tiny amounts of radioactive material can be followed through complex biological and industrial processes by keeping track of where the radiation goes; and the radiations from radioactive materials can be used to destroy living cells, such as harmful microorganisms and human cancer cells.

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