v1.0.1 / chapter 8 of 20 / 01 sep 07 / greg goebel / public domain
* The discovery of radioactivity soon led to the characterization of nuclear radioactive decay series, knowledge that proved to have an important application: the ability to measure the age of samples of various materials. Radioactive isotopes also proved to have useful applications in medicine and manufacturing.
* In their studies of the strong force and weak interaction, theoretical physicists provided an explanation for the processes of radioactive decay that had been mapped during the preceding decades by the experimentalists. The experimentalists had been able to trace the paths of a number of radioactive series, and also put this knowledge to practical use, particularly for purposes of dating the age of the Earth.
The following tables describe various important radioactive series, giving each isotope in the series, its decay products, and its half-life. The isotopes are given with their atomic weights and atomic number; for example "U<238/92>" specifies an atomic weight of 238 and an atomic number of 92. The steps in the series are numbered because there are low-probability alternate decay pathways for some of the steps in the series. These are designated as "(alt)", along with a probability giving the proportion of times the secondary path is taken. The net effect of the branches is the same overall: in one path, a beta decay is followed by an alpha decay, in the other path an alpha decay is followed by a beta decay.
* The uranium-238 (U<238/92>) series is as follows:
step isotope decay products half-life
_______________________________________________________________
1 U<238/92> alpha + Th234/90 4,510,000,000 years
2 Th<234/90> beta + Pa234/91 24.1 days
3 Pa<234/91> beta + U234/92 6.75 hours
4 U<234/92> alpha + Th230/90 247,000 years
5 Th<230/90> alpha + Ra226/88 80,000 years
6 Ra<226/88> alpha + Rn222/86 1,600 years
7 Rn<222/86> alpha + Po218/84 3.82 days
8 Po<218/84> alpha + Pb214/82 3.05 minutes
9 Pb<214/82> beta + Bi214/83 26.8 minutes
(alt 8) Po<218/84> beta + At218/85 ( 0.04% )
(alt 9) At<218/85> alpha + Bi214/83 2 seconds
10 Bi<214/83> beta + Po214/84 19.7 minutes
11 Po<214/84> alpha + Pb210/82 1.6E-4 seconds
(alt 10) Bi<214/83> alpha + Tl210/81 ( 0.04% )
(alt 11) Tl<210/81> beta + Pb210/82 1.32 minutes
12 Pb<210/82> beta + Bi210/83 20.4 years
13 Bi<210/83> beta + Po210/84 5.01 days
14 Po<210/84> alpha + Pb206/82 138 days
(alt 13) Bi<210/83> alpha + Ti206/81 ( very rare )
(alt 14) Ti<206/81> beta + Pb206/82 4.19 minutes
15 Pb<206/82> STABLE
_______________________________________________________________
The overall reaction of the chain is:
U<238/92> -> 6 beta + 8 alpha + Pb206/82
* The uranium-235 (U<235/92>) series is as follows:
step isotope decay products half-life
_______________________________________________________________
1 U<235/92> alpha + Th231/90 710,000,000 years
2 Th<231/90> beta + Pa231/91 25.5 hours
3 Pa<231/91> alpha + Ac227/89 32,500 years
4 Ac<227/89> beta + Th227/90 21.6 years
5 Th<227/90> alpha + Ra223/88 18.2 days
(alt 4) Ac<227/89> alpha + Fr223/87 ( 1.2% )
(alt 5) Fr<223/87> beta + Ra223/88 22 minutes
6 Ra<223/88> alpha + Rn219/86 11.4 days
7 Rn<219/86> alpha + Po215/84 4 seconds
8 Po<215/84> alpha + Pb211/82 0.00178 second
9 Pb<211/82> beta + Bi211/83 36.1 minutes
(alt 8) Po<215/84> beta + At215/85 ( 0.0005% )
(alt 9) At<215/85> alpha + Bi211/83 0.0001 second
10 Bi<211/83> beta + Po211/84 2.16 minutes
11 Po<211/84> alpha + Pb207/82 0.52 second
(alt 10) Bi<211/83> alpha + Tl207/81 ( 0.3% )
(alt 11) Tl<207/81> beta + Pb207/82 4.79 minutes
12 Pb<207/82> STABLE
_______________________________________________________________
The overall reaction of the chain is:
U<235/92> -> 4 beta + 7 alpha + Pb207/82
* The thorium-232 (Th<232/90>) series is as follows:
step isotope decay products half-life
_______________________________________________________________
1 Th<232/90> alpha + Ra228/88 14,100,000,000 years
2 Ra<228/88> beta + Ac228/89 6.7 years
3 Ac<228/89> beta + Th228/90 6.13 hours
4 Th<228/90> alpha + Ra224/88 1.91 years
5 Ra<224/88> alpha + Rn220/86 3.64 days
6 Rn<220/86> alpha + Po216/84 55.3 seconds
7 Po<216/84> alpha + Pb212/82 0.14 seconds
8 Pb<212/82> beta + Bi212/83 10.6 hours
(alt 7) Po<216/84> beta + At216/85 ( 0.014% )
(alt 8) At<216/85> alpha + Bi212/83 0.0003
9 Bi<212/83> beta + Po212/84 60.6 minutes
10 Po<212/84> alpha + Pb208/82 0.0000003 second
(alt 9) Bi<212/83> alpha + Tl208/81 ( 33.7% )
(alt 10) Tl<208/81> beta + Pb208/82 3.1 minutes
11 Pb<208/82> STABLE
_______________________________________________________________
The overall reaction of the chain is:
Th<232/90> -> 4 beta + 6 alpha + Pb208/82
* Although the radioactive series discussed above are of historical -- and,
as will be shown, practical -- interest, most elements have radioactive
isotopes, not all of which are part of any decay series. A few of the more
important radioisotopes not belonging to decay series are shown below:
isotope decay products half-life
_____________________________________________________
H<3/1> beta + He3/2 12.33 years
C<14/6> beta + N14/7 5,730 years
Na<25/11> beta + Mg25/12 1 minute
P<32/15> beta + S32/16 14.28 days
Co<60/27> beta + Ni60/28 5.27 years
Rb<87/37> beta + Sr87/38 49,000,000,000 years
Sr<90/38> beta + Y90/39 29 years
I<131/53> beta + Xe131/54 8.04 days
_____________________________________________________
Of course, short-lived radioactive isotopes are rarely found in nature except
as a transient step in a decay series, so the short-lived isotopes in this
list have to be synthesized, usually by bombarding the appropriate stable
isotope with neutrons to raise its mass number.
* Although the modern public perception of radioactivity is, with plenty of good reason, that it's a menace, in fact radioactivity has its unarguably good uses as well. One of the first technologies based on radioactivity was radioactive dating.
Radioactive dating could be regarded as the "first quantum technology", or at least the first technology that was developed with a basis in quantum theory. It had a crucial impact in a major scientific debate, the controversy over the age of the Earth and, by implication, the Universe.
Up to the scientific era, attempts to determine the age of the Earth were based on what amounted to guesswork. Very famously, in a book published in 1654, not long before his death, Archbishop James Ussher of Armagh, Ireland, calculated from the Bible that the Earth was created in 4004 BC. The Han Chinese had come up with a less conservative estimate, believing that the Earth was 23 million years old, but even this would prove to be a gross underestimate.
The first attempts by scientists to come to grips with the issue were just as far off. In the mid-18th century, the naturalist Mikhail V. Lomonosov (1711:1765), regarded as the founder of Russian science, was one of the first to undertake this exercise, suggesting in mid-century that the Earth had been created separately from the rest of the Universe several hundred thousands of years before. Lomonosov's ideas were mostly speculative, but in 1779 the French naturalist the Comte du Buffon (1707:1788) tried to obtain a value for the age of the Earth using an experiment. He created a small globe that resembled the Earth in composition and then measured its rate of cooling. This led him to estimate that the Earth was about 75,000 years old.
Very few of their colleagues paid them much mind, either leaving the question of the age of the Earth to creation tales, or if they were impious simply assuming that the Earth always had been, always would be. However, by the end of the century pioneering geologists were beginning to recognize that the layering or "strata" observed in geological deposits had been laid down sequentially and represented a "clock" of sorts, though one whose "ticks" were hard to determine for the moment.
In 1862, the physicist William Thompson of Glascow (1824:1907) published calculations that fixed the age of the Earth from 20 million to 400 million years. He assumed that the Earth had been created as a completely molten ball of rock, and determined the amount of time it took for the ball to cool to its present temperature. Geologists had trouble accepting a finite age for the Earth; biologists could accept that the Earth might have a finite age, but even 100 million years seemed much too short to be plausible. The great English naturalist Charles Darwin (1809:1882) had proposed his theory of the evolution of organisms by natural selection, which was a semi-random process that implied great expanses of time. Even 400 million years didn't seem long enough.
In a lecture in 1869, Darwin's great advocate, Thomas H. Huxley (1825:1895), attacked Thompson's calculations, suggesting they were based on faulty assumptions. Huxley was correct and in fact Thompson's estimates would prove far too short, but Thompson had at least attempted to root the debate in facts instead of than speculation, and apply a little rigor to it.
He certainly managed to provoke a long and productive debate. The German physicist Hermann von Helmholtz (1821:1894) and the American astronomer Simon Newcomb (1835:1909) joined in by independently calculating the amount of time it would take for the Sun to condense down to it current diameter and brightness from the nebula of gas and dust from which it was born. They came up with a value of 100 million years, which seemed to set an upper limit on the age of the Earth that was consistent with Thompson's calculations. However, they assumed that the Sun was only glowing from the heat of its contraction. They knew of no other ways for it to produce its energy.
* By the turn of the century, Thompson had been made Lord Kelvin in appreciation of his many scientific accomplishments. He had reason to feel confident of himself, and the fact that multiple attempts to determine the age of the Earth seemed to show that it was about 100 million years old led him to feel very certain that his estimates were correct. The geologists could only weakly suggest that Kelvin didn't have all the facts, and still believed that the Earth was far older than 100 million years.
The discovery of radioactivity upset the status quo, since it provided a new source of energy that undermined the assumptions of Lord Kelvin's analysis, and implicitly those of Helmholz and Newcomb. In 1901 two German schoolteachers, Julius Elster and Hans F. Geitel, had detected radioactivity in the air and then in the soil. Other investigators found it in rainwater, snow, and groundwater. Robert J. Strutt of Imperial College, London, found traces of radium in many rock samples, and concluded that the Earth contained more than enough radioactive material to keep it warm for a long, long time.
Strutt's work created controversy in the scientific community. Lord Kelvin spoke for those with a vested interest in the older estimates, fighting a stubborn rear-guard action in public against the new findings up to his death in 1907, though he admitted in private that his calculations had been shown to be incorrect. Geologists and biologists who had stuck by their gut feelings that the Earth was much older than 100 million years were relieved to have been at least partly vindicated.
Rutherford and Soddy's work on radioactive decay provided a key to a more accurate estimate of the age of the Earth. Some radioactive materials have short half-lives, some have long half-lives. Uranium, thorium, and radium have long half-lives and so persist in the Earth's crust, but those radioactive elements with short half-lives have generally disappeared. This suggested that it might be possible to measure the age of the Earth by determining the relative proportions of radioactive materials in geological samples.
* The pioneers in applying radioactive decay to dating were Bertram B. Boltwood (1870:1927), a young chemist just out of Yale, and the energetic Rutherford. Boltwood had conducted studies of radioactive materials as a consultant, and when Rutherford lectured at Yale in 1904, Boltwood was inspired to describe the relationships between elements in various decay series. That same year, Rutherford had also taken the first step toward radioactive dating by suggesting that the alpha particles released by radioactive decay could be trapped in a rocky material as helium atoms. At the time, Rutherford was only guessing at the relationship between alpha particles and helium atoms; he wouldn't prove the relationship for another four years.
Soddy and Sir William Ramsay (1852:1916), then at University College in London, had just determined the rate at which radium produces alpha particles, and Rutherford proposed that he could determine the age of a rock sample by measuring its concentration of helium. He dated a rock in his possession to an age of 40 million years by this technique. This assumed that the rate of decay of radium as determined by Ramsay and Soddy was accurate, and that helium didn't escape from the sample over time. Rutherford's scheme was inaccurate, but it was a useful first step.
Boltwood focused on the end products of decay series. In 1905, he suggested that lead was the final stable product of the decay of radium. It was already known that radium was an intermediate product of the decay of uranium. Rutherford joined in, outlining a decay process in which radium emitted five alpha particles through various intermediate products to end up with radium, and speculated that the radium-lead decay chain could be used to date rock samples. Boltwood did the legwork, and by the end of 1905 had provided dates for 26 separate rock samples, ranging from 92 to 570 million years. He did not publish these results, which was fortunate because they were flawed by measurement errors and poor estimates of the half-life of radium. Boltwood refined his work and finally published the results in 1907.
Boltwood's paper pointed out that samples taken from comparable layers of strata had similar lead-to-uranium ratios and that samples from older layers had a higher proportion of lead, except where there was evidence that lead had leached out of the sample. However, his studies were flawed by the fact that the decay series of thorium was not understood at the time, which led to incorrect results for samples that contained both uranium and thorium. His calculations were still far more accurate than any that had been performed to that time. Refinements in the technique would later give ages for Bolton's 26 samples of 250 million to 1.3 billion years.
* Although Boltwood published his paper in a prominent geological journal, the geological community had little interest in radioactivity at the time, seeing it generally as a matter for the physicists to puzzle over and otherwise of no particular relevance. Boltwood gave up work on radioactive dating and went on to investigate other decay series. Rutherford remained mildly curious about the issue of the age of the Earth but did little work on it. Robert Strutt tinkered with Rutherford's helium method until 1910 and then also gave up on it. However, Strutt's student Arthur Holmes (1890:1965) became interested in the concept of radioactive dating and continued to work on it after everyone else had given up.
Holmes focused on lead dating, since he regarded the helium method as unpromising. He performed measurements on rock samples in 1911 and concluded that the oldest was about 1.6 billion years old. These calculations were not particularly trustworthy. For example, he assumed that the samples had contained only uranium and no lead when they were formed. The discovery of isotopes also complicated his work considerably, so much so that many geologists felt radioactive dating would never be workable. Holmes felt he could improve his techniques and he plodded ahead with his research, publishing papers on the subject before and after the First World War.
His work was generally ignored until the 1920s, though in 1917 Joseph Barrell, a professor of geology at Yale, redrew geological history as it was understood at the time to conform to Holmes' findings in radioactive dating. Barrell's research determined that the layers of strata had not all been laid down at the same rate, and so the old trick of using current rates of geological change could not provide accurate timelines of the past history of the Earth. Holmes' his persistence finally began to pay off in 1921, when the speakers at the yearly meeting of the British Association for the Advancement of Science came to a rough consensus that the Earth was a few billion years old, and that radioactive dating was credible.
No great push to embrace radioactive dating followed, however, and the die-hards in the geological community stubbornly resisted. They had never welcomed attempts by physicists to intrude in their domain, and had successfully ignored them to that time. The growing weight of evidence finally tilted the balance in 1926, when the National Research Council of the US National Academy of Sciences finally decided to address the question of the age of the Earth by appointing a committee to investigate.
Holmes, being one of the few people on Earth who was trained in radioactive dating techniques, was a committee member, and in fact wrote most of the final report. The report concluded that radioactive dating was the only reliable means of pinning down geological time scales. Questions of bias were deflected by the great and exacting detail of the report. It described the care in which measurements were made, and defined not only the methods of used, but also their "error bars" and limitations.
* The decay of radioactive materials can take place through alpha decay, with an isotope losing two neutrons and two protons, or through beta decay, with an isotope losing an electron, converting a neutron into a proton. Radioactive or "radiometric" dating works by determining the relative concentrations of the "parent" and "daughter" isotopes in a decay process. Radiometric dating schemes are regarded as accurate because radioactive decay is not influenced by most common physical processes: it is neither slowed nor accelerated by heat, pressure, or magnetic and electric fields. It can be accelerated by radioactive bombardment, but such bombardment tends to leave evidence of its occurrence.
Furthermore, the processes that form specific materials are often very conveniently selective as to what isotopes they incorporate during their formation. In the ideal case, the material will incorporate a parent isotope and reject the daughter isotope. This ensures that the only daughter isotopes found through examination of a sample are those that were created by decay since the sample was formed. However, if a material that selectively rejects the daughter isotope is heated, any daughter isotopes that have been accumulated over time will be lost through diffusion, setting the isotopic "clock" to zero. The temperature at which this happens is known as the "blocking temperature" and is specific to a particular material.
Although radiometric dating is accurate in principle, the accuracy is very dependent on the care with which the procedure is performed. The possible confounding effects of initial contamination of parent and daughter isotopes have to be considered, as well as the effects of any loss or gain of such isotopes since the sample was created. Accuracy is enhanced if measurements are taken on different samples taken from the same rock body but at different locations. This permits some compensation for variations and for errors.
Of course, the half-life of the parent isotope has to be known, and the measurements of the parent and daughter isotopes have to be accurate as well. Radiometric dating can be performed on samples as small as a billionth of a gram using a mass spectrograph, or its more modern derivative, the mass spectrometer, which provides an electronic readout of isotope accumulations.
* The uranium-lead radiometric dating scheme is one of the oldest available, as well as one of the most reliable. It has been refined to the point the error in dates of rocks about three billion years old is no more than two million years. Uranium-lead dating is best performed on the mineral "zircon" (ZrSiO4), though it can be used on other materials. Zircon incorporates uranium atoms into its crystalline structure as substitutes for zirconium, but strongly rejects lead. It has a very high blocking temperature, and is very chemically inert.
One of its great advantages is that a sample provides two clocks, one based on uranium-235's decay to lead-207 with a half-life of about 4.5 billion years, and one based on uranium-238's decay to lead-206 with a half-life of about 700 million years, providing a built-in crosscheck that allows accurate determination of the age of the sample even if some of the lead has been lost.
* Several other radiometric techniques are used for long-term dating. Potassium-argon dating involves the decay of potassium-40. Potassium-40 has a half-life of 1.25 billion years, decaying either to calcium-40 (88.8% of the time) or argon-40 (11.2% of the time). The calcium decay path is no good for dating because the rock sample will likely contain calcium to begin with. However, since argon is a gas, if the rock is melted it will escape, resetting the "clock". It is possible to have air bubbles trapped in molten lava, but atmospheric argon, while mostly argon-40, also includes a fixed trace of argon-36, and that ratio can be used to compensate for atmospheric bubbles.
Potassium-argon dating is conceptually simple -- just measure the ratio of potassium-40 and argon-40, then do the appropriate half-life curve calculation -- and the long half-life makes the method applicable to the oldest stones. Radioactive potassium-40 is common in micas, feldspars, and hornblendes, though the blocking temperature is fairly low in these materials -- about 125 degrees Celsius.
There is also an "argon-argon" dating scheme, which involves the neutron bombardment of a sample to convert potassium-39 to argon-39, and then the sample is heated to drive out argon-39 and argon-40 for measurement. It's really measuring the same ratios as potassium-argon and the two are often used together as a crosscheck.
Rubidium-strontium dating is based on the beta decay of rubidium-87 to strontium-87, with a half-life of about 50 billion years. This scheme is used to date old igneous and metamorphic rocks, and has also been used to date lunar samples. Blocking temperatures are so high that they are not a major concern. Since the half-life is so very long, rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample.
There are a few other less prominent techniques used for very long-term dating:
* There are a number of other dating techniques that have short ranges and are so used for historical or archaelogical studies. One of the best-known is the carbon-14 radiometric technique.
Carbon-14 is a radioactive isotope of carbon-12, with a very short half-life of 5,730 years. In other radiometric dating methods, the heavy parent isotopes were synthesized in the explosions of massive stars that scattered materials through the Galaxy, to be formed into planets and other stars. The parent isotopes have been decaying since that time, and so any parent isotope with a short half-life should be extinct by now.
Carbon-14 is an exception. It is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere. The carbon-14 ends up as a trace component in atmospheric carbon dioxide (CO2). An organism acquires carbon from carbon dioxide during its lifetime. Plants acquire it through respiration and photosynthesis, and animals acquire it from consumption of plants and other animals. When the organism dies, the carbon-14 will begin to decay, and the proportion of the isotope left when the remains of the organism is examined provides an indication of the date of its death. Carbon-14 radiometric dating has a range of about 50,000 years.
The rate of creation appears to be roughly constant, as cross-checks of carbon-14 dating with other dating methods show it gives consistent results. However, local eruptions of volcanoes or other events that source large amounts of carbon dioxide can reduce local concentrations of carbon-14 and give inaccurate dates. The releases of carbon dioxide into the biosphere as a consequence of industrialization have also depressed the proportion of carbon-14 by a few percent, though somewhat in compensation it was increased by above-ground nuclear tests that were conducted into the early 1960s.
* Another relatively short-range dating technique is based on the decay of uranium-238 into thorium-230, a process with a half-life of 80,000 years. It is accompanied by a sister process, in which uranium-235 decays into protactinium-231, which has a half-life of 34,300 years. While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sedinents, from which their ratios are measured. The scheme has a range of several hundred thousand years.
* Archaeologists use "tree-ring dating (dendrochronology)" to determine the age of old pieces of wood. Trees grow "rings" on a yearly basis, with the spacing of rings being wider in good growth years than in bad growth years. These spacings can be used to help pin down the age of old wood samples, and also give some hints to climate change. The technique has a range of only about 4,000 years, however.
Although determining geologic time by measuring the rate of deposition of sediments is not reliable over the long term, it is still useful for certain scenarios, such as the deposition of layers of sediment on the bottom of a stable lake. The approach is now known as "varve analysis", where the term "varve" means a layer or layers of sediment.
A similar scheme of counting layers is used with glacial ice deposits to provide dates back to within about a hundred thousand years. It should be noted that the top layers of a glacier are very thick, but they get thinner due to pressure as they get deeper, and it can be hard to tell the lowest layers apart. The accuracy of the scheme falls off gradually for more than 40,000 years.
Another technique used by archaelogists is to inspect the depth of penetration of water vapor into chipped obsidian (volcanic glass) artifacts. The water vapor creates a "hydration rind" in the obsidian, and so this approach is known as "hydration dating" or "obsidian dating", and is useful for determining dates as far back as 200,000 years.
Yet another technique is based on the fact that natural sources of radiation in the environment will knock loose electrons in, say, a piece of pottery, and these electrons will accumulate in defects in the material's crystal lattice structure. When the sample is heated, at a certain temperature it will glow from the emission of electrons released from the defects, and this glow can be used to estimate the age of the sample to a threshold of a few hundred thousand years.
* Finally, "fission track dating" involves inspection of a polished slice of a material to determine the density of "track" markings left in it by radioactive decay of uranium-238 impurities. The uranium content has to be understood, but it can be determined by placing a plastic film over the polished slice of the material, and then bombarding it with slow neutrons. This causes induced fission of U235, as opposed to spontaneous fission of U238. The fission tracks produced by this process are recorded by the plastic film. The uranium content of the material can then be calculated so long as the neutron dose is known.
This scheme has a maximum range of about a million years and works best with micas, tektites (glass fragments from volcanic eruptions), and meteorites. However, the dates may be inaccurate if the sample was heated to high temperatures in the past as blocking temperatures are generally low, or if the sample was exposed on the surface of the Earth, where it was bombarded with cosmic rays.
* Along with dating schemes, radioactive isotopes have a range of useful applications in research, medicine, and industry. One major practical application of radioactive isotopes is as "tracers", allowing interactions to be followed at the atomic and molecular level. The best-known use of radioactive tracers is in medicine, in which tiny quantities of a low-level radioactive isotope are injected into a patient to observe where they go. The general approach is to synthesize molecules using a radioactive isotope of an atom normally used in these molecules, and then use a radiation detector to see where the molecule ends up. For example, ordinary table salt (NaCl) can be synthesized with radioactive Na<11/25> and used to trace the flow of the circulatory system to hunt for blockages.
Carbon dioxide (CO2) that was synthesized using radioactive C<6/14> was an important tool to unraveling plant photosynthesis, the process by which a green plant converts water and carbon dioxide into oxygen and the sugar glucose. The fact that biosystems may be highly selective in their uptake of certain elements makes tracers very valuable: iodine, for example, concentrates in the thyroid gland, the liver, as well as some areas in the brain, and so radioactive I<53/131> is often used to investigate thyroid and liver problems, as well as to hunt for brain tumors. Conveniently, thallium tends to accumulate in healthy heart muscle tissue, making radioactive Tl<81/201> a good tracer; and technetium tends to accumulate in diseased heart muscle tissue, with radioactive Tc<43/99> allowing doctors to map the functions of the heart.
The same technique is used in biomedical research, for example in tracing processes of metabolism in bacteria; in agriculture, for example in tracing the flow of irrigation water using water molecules synthesized with H3 (tritium), or tracing the flow of fertilizers; and in ecology, for example in tracing the flow of materials through an ecosystem. Chemists have long used tracers to help unravel the paths of chemical reactions of all sorts.
One modern variation on biomedical radioactive tracing is "positron emission tomography (PET)", which maps the locations of positron-emitting isotopes of a short-lived radioactive tracer that are injected into a patient; it is often used to image the brain. A positron emitter can be linked to water molecules, which are then pumped through the blood around the body and into the brain. The positrons hit electrons and are mutually annihilated, producing gamma rays that fly off in opposite directions through the body.
The PET scanner includes an array of scintillation sensors that emit light when hit by a gamma ray. The light emitted by the scintillator is amplified by a photomultiplier tube and measured. Since the two gamma rays fly off in opposite directions, only inputs that occur at the same time from sensors on opposite sides of the array are recorded, with the rest discarded as noise.
For reasons not quite understood at present, different types of mental activity change the blood flow through different sections of the brain, and these changes can be tracked by PET to follow brain activity. Other researchers have tagged positron emitters onto glucose (blood sugar) molecules, which allows PET to trace glucose metabolism. This also maps brain activity.
Incidentally, non-radioactive isotopes may also be used as tracers in some cases, with samples tested by mass spectrometry to determine if they have incorporated a high proportion of the isotope. This is done when safety is a concern. For example, "heavy water" with nonradioactive H2 (deuterium) atoms can be used as a tracer, though interestingly at very high concentrations heavy water tends to be troublesome to organisms as well; apparently it's just different enough in its properties from ordinary water to cause difficulties. It is also possible to build distinctive molecular "tags" by taking a molecular structure that normally incorporates a number of hydrogen atoms and substituting deuterium in some subset of them.
Tracers have uses in industrial research and manufacturing. Tests of lubricants in engines may use engine parts that have been irradiated by neutron bombardment to become slightly radioactive. The level of wear of these parts can then be determined by the concentration of radioactivity in the oil due to metal particles. Incidentally, though it is stretching the concept of a radioactive tracer a bit, the purity of a material sample is sometimes measured by what is known as "neutron activation analysis": the sample is heavily irradiated and then carefully observed to check for the radioactive signatures of certain specific contaminants.
Oil is often pumped through long-distance pipelines. Since the flow is more or less continuous, it is difficult to determine when a particular batch of oil pumped into the pipeline reaches the distant terminal end. The batch can be marked with a radioactive tracer to allow it to be detected. Radioactive tracers can also be used to help find leaks in pipelines.
In multicolor textile printing, radioactive isotopes may be used in a similar way to control dye contamination: if cloth is dyed various colors, it will be sent through a sequence of rollers and dye baths, and the dye from one stage will gradually build up in the dye bath of the stage until it reaches the level where the "pirate" color corrupts the color of the dye in the next stage. Dye baths have to be changed every now and then, but of course nobody wants to do it any more often than necessary. The trick is to put radioactive phosphorus-32 into the dye bath of the pirate color, and monitor the buildup "downstream" with radiation detectors.
* There is a grab-bag of other applications of radioactive isotopes. Radioactive isotopes have been used in a more aggressive medical role for cancer therapy. Radioactive cobalt -- Co<27/60> -- was used for some time to treat cancers that could not be surgically removed, by exposing the cancer to a source of penetrating radiation. This procedure is now generally out of date because cobalt-60 is so nasty to handle. Radiation therapy continues to be practiced, but intense X-ray sources are used instead.
Irradiation with radioactive isotopes has been used occasionally to help preserve foods, though this is another procedure that has generally gone out of fashion. One particularly interesting use of radioisotopes is the sterilization of male screwworm flies, which are a nuisance to animal husbandry. Since screwworms only mate once, sterilizing the males can wipe out the population of flies in a location.
Radium used to be used to illuminate watch dials by energizing fluorescent paint, but it was a threat to workers handling it and has been replaced by tritium sources for low-level illumination applications. One interesting application of this technology is sights for firearms to allow the shooter to draw a bead in low-light conditions.
Many household smoke detectors include an "ionization detector" containing a small quantity of americium -- Am<95/241> -- to detect smoke. The americium is stored at the top of the ionization detector, which looks like a small inverted dome with slots around it. There is a conductive plate on the top of the detector and a conductive plate around the dome, and these form two contacts of an electronic detector circuit. Under normal circumstances, the radioactivity of the americium ionizes the air inside the dome, completing the electrical circuit. If smoke filters into the dome, it will dampen the ionization, breaking the circuit and causing an alarm to go off.
One of the better-known uses of radioactive isotopes is in "radioisotope thermoelectric generators (RTGs)", which are electric power units that use the heat released by the decay of radioactive isotopes -- usually plutonium, Pu<94/238> -- to provide electric power, with the conversion generally being performed by a "thermoelectric converter", a solid-state system that is discussed in a later chapter.
The best-known application for RTGs is in space exploration. They are often used to power deep-space probes that venture beyond the orbit of Mars, where the sunlight is no longer powerful enough to make solar-power arrays practical. However, the US Viking Mars landers of the 1970s also used RTGs, as did the Lunar Module manned Moon landers from the US Apollo missions. These RTGs are built to a very high standard to prevent them from burning up after a launch accident. Incidentally, small quantities of radioactive materials have also been used as heating elements in space probes to protect systems from freezing up.
RTGs have also been used in terrestial applications. "Plutonium cells", which are basically RTGs on the scale of a watch battery, have been used in implanted heart pacemakers to give them a lifetime of about ten years before they have to be surgically replaced. The Soviets were very enthusiastic about large RTGs and built them to power lighthouses and remote weather stations.
* Of course, radioactive isotopes have their malevolent uses. The idea of using radioactive isotopes as a weapon is not new, having been discussed in science-fiction stories as far back as 1940. However, handling radioactive isotopes is so troublesome that no formal military organization would consider bothering with it, all the more so because such a "radiological weapon" would be not very focused in its use and would lead to major "clean-up" consequences after the shooting stopped.
Unfortunately, in the modern age of international terrorism the idea has come to the top, with terrorist groups attempting to get their hands on radioisotopes to build "dirty bombs", which use conventional explosives to disperse the radioisotopes and render a site unusable for a long time. Soviet RTGs have been a particular worry, since they were scattered over a large number of widely separated ground sites, and since the collapse of the USSR many of these sites have been more or less abandoned. Efforts have been made to ensure that the RTGs are collected or otherwise accounted for.
* Although radioactive dating is generally regarded as a reliable indicator of age by the mainstream science community, there is a faction of critics that believes the Earth is only a few thousand years old and so the results of radioactive dating must be in error. Although the critics are not taken very seriously by the scientific community, they are very energetic in promoting their views, and it is worthwhile to discuss those views here.
Some of the criticisms are bogus on the face of it. One approach, for example, was that there was no way to reliably measure a half-life of a half-billion years from a sample of a material with a low decay rate. In reality, a kilogram of such a material has so many atoms in it that such a decay rate would still result in at least billions of decays a year, a rate which is perfectly measurable. Except for isotopes with very long half-lives, in the tens of billions of years, the half-lives of all the radioactive isotopes used for dating are known to within about 2%; the rest are easily known within 10%. Claims that radioactive dating schemes have been "calibrated" from the estimated ages of fossils, making geological dating a circular exercise, are simply fabrications.
A subtler approach is to suggest that the methodology of radioactive dating is suspect, in particular that the isotopic ratios of samples have been modified by various processes. In reality, the processes that can alter isotopic ratios are in general well characterized. There is also the possibility of experimental error, which is not only conceivable given that radioactive dating is not a casual procedure, but has clearly happened in some cases.
The biggest rebuttal to claims of widespread methodological error is the fact that multiple datings of a wide range of samples have been performed by a large number of research teams using several separate radioactive dating methods -- and have given consistent results. If there were errors in the times produced by multiple different clocks in several different locations, it would be difficult to understand why the errors all give pretty much the same time. There are no gross anomalies in the big chronological picture provided by the various dating methods and the large number of experiments using them.
Anomalies do crop up on occasions in specific samples, for example a rock blasted out of a volcano during an eruption might prove very old. However, the anomalies are unusual and have explanations varying from plausible to airtight: the rock from the volcano was actually an old surface rock that got caught up in the eruption.
Possibly the most far-reaching argument proposed by the critics is that radioactive decay rates were much more rapid in the past than they are now, giving a misleading age of the Earth. This notion is hard to refute as such, but the same could be said of the statement: "If pigs had wings, they could fly." To suggest that radioactive decay rates have slowed down over time is to suggest that physicists have got some very basic things very wrong, and there's no evidence to show that's the case: nobody's discovered a winged pig.
If radioactive decay rates were so much faster in the past, it would a stretch to claim that they have slowed down so much that nobody can detect further slowing in the present. Current measurements show no sign of a change in decay rates any greater than about one part in 10^11 per year. There is, incidentally, a similar argument that claims that astronomical observations showing an old Universe are skewed because the speed of light has been slowing down drastically -- but it's also a "pigs with wings" argument not supported by any observations.
* Some of the critics have developed their own dating methods to show the Earth is only a few thousand years old:
The blatantly obvious flaw in this reasoning is to assume that human population has been growing at the modern rate through all history, when in fact it was likely more or less constant before the invention of agriculture, and probably declined every now and then when plagues or harsh climate intervened. By the same logic, if mice were assumed to double their numbers every year, in about 60 years they would carpet the Earth, and 30 years later would weigh as much as the Earth itself -- proving that the Earth can't be even as much as a century old. This is an old argument, and since it's so easily demolished it would seem the critics would have given up on it by now, but surprisingly it seems to still be in circulation.
The mainstream scientific consensus remains that the Earth is about 4.5 billion years old. Some geologists are puzzled as to why that value is an issue one way or another to anybody, one saying: "I have no reason whatsoever to want the age of the Earth to be any more or less than it happens to be. I would take great delight in proving the Earth is only 10,000 years old, if it were possible to do so."