The use of accelerators as mass spectrometers puts the work horse of isotope dating on a new, more useful footing.

Much of humankind's information about its prehistory still comes from meticulous examination

of the strata in which relics and artifacts are found, and from the association of things of unknown age with those for which the age can be determined. This process took a great leap forward in the nineteen-forties. That was when Willard F. Libby (whose 1960 Nobel prize recog­nized the achievement) demonstrated that the radioactive decay of carbon-14 permitted the dating with relative preci­sion of ancient organic (carbon-containing) material.

Radiocarbon dating has since then provided a series of verifiable benchmarks against which archaeological calendars could be calibrated and artifacts dated, still largely by association, but now with a far greater confidence than was ever before possible. As a measuring tech­nique, carbon-14's impact might be com­pared to the difference between the wavelength of light emitted by a krypton arc—the current international standard of length—and the distance between the king's nose and his outstretched thumb— an ancient standard yard which, over time and through many intermediate steps, krypton's light ultimately replaced.

Radiocarbon dating depends on the fact that all living things continuously ex­change carbon, in the form of carbon dioxide, with the atmosphere. Most of the carbon is common, nonradioactive carbon-12 (atomic mass of 12: six protons and six neutrons). But a tiny, fixed proportion is carbon-14 (six protons, eight neutrons), produced by a reaction between atmospheric nitrogen—nitro-gen-14 (seven protons, seven neu­trons)—and cosmic rays. An implicit assumption of radiocarbon dating is that, at any moment, over recent geologic time,

the atmospheric ratio of carbon-12 to carbon-14 has been nearly constant: about one atom in a trillion.

Once an organism dies, however, carbon exchange ceases. The amount of carbon-12 remains the same but the carbon-14, unstable from its birth, decays back to nitrogen-14 through the emission of an electron. The decay rate of a radioisotope is specified in terms of "half life"—the time it takes for half the amount of the isotope in a sample to disappear. For carbon-14, the half life is 5,730 years. If the ratio of carbon-14 to carbon-12 atoms is high (close to one to a trillion), the sample is young; if half the

original concentration is left, the object is taken to be 5,730 years old; less than that means the material is older still. The ratio ideally is a precise measure of actual age.

In the process of exploiting these char­acteristics to date an artifact, a sample is burned to isolate its carbon and a propor­tional counter, similar to a Geiger counter, is used to detect the low-energy beta (electron) emission, the carbon-14's dying "tick." The more carbon-14 present—i.e., the younger the sample— the greater the decay rate and the more ticks over a unit of time.

The arithmetic is simple: One gram of brand-new, contemporary carbon gives

MOSAIC Nnuflmhfir/Dficember 1978 43

off 14 beta emissions per minute from the total of 60 billion carbon-14 atoms in the sample. One gram, a radiocarbon half-life old, gives seven electrons per minute. If several half lives have expired, one electron per minute can be detected. To measure confidently with the technique, the decay of several thousand carbon-14 atoms has to be registered. That de­mands the destruction of enough mate­rial to enable the measurement to be made, often over a matter of hours or days. The older the sample, the more time is required for reliability and the more of the sample that must be destroyed.

Uncertainties

The technique is not without other problems. Dependent as it is on the premise that carbon-14 is produced from nitrogen-14 at a constant rate, it has come more and more to be questioned as the invariability of the cosmic-ray flux— especially the flow of solar particles (see "The Sun, the Whole Sun," Mosaic, Volume 9, Number l)—has come to be questioned. That, plus the likelihood of contaminating atmospheric carbon iso­tope ratios by nuclear weapons tests and the carbon-containing artifacts in the ground by materials originating in other strata, have led to extensive efforts to iron out the knots in the dating technique, and to improve its reliability. These have been accompanied by efforts to develop techniques of dating nonorganic artifacts, such as pottery, directly, bypassing prob­lems accompanying dating by association

with carbon-containing materials (see "Tales the Tree Rings Tell," Mosaic, Volume 8, Number 5, and "Archaeology Shifting East," Mosaic, Volume 8, Number 3).

But radiocarbon dating has had other drawbacks, just as frustrating to archae­ologists if not quite as fundamental to the process as contamination or the in­constancy of the solar constant. First, the samples required are large: anywhere from one to ten grams. This precludes the dating of small objects or of artifacts so precious as to prohibit the destruction of any of their material. Second, the tech­nique cannot, without heroic and costly strategems, date back much beyond 40,000 years; in that time, much of the carbon-14 would already have decayed; its residual whispers become almost too faint to catch.

Within the last two years, however, a method of carbon-14 dating has been under development that seems to have overcome these limitations. Different from the standard, beta-decay approach that counts electron emissions, the new technique uses physicists' particle accelerators to find and measure directly the carbon-14 content of a sample before its radioactive atoms emit radiation and decay. In the past, the way the age of objects was measured in radioisotope dating laboratories was like waiting for a clock to tick in order to determine its existence. The new method does not require investigators to hang around for these ticks; they will measure the amount

of carbon-14 in the sample directly, counting the ticks before they happen or, more accurately, counting the tickers instead of the ticks.

With the new dating system, which employs a particle accelerator as an u l t rasens i t ive mass spec t rometer , physicists believe that samples no larger than a few milligrams of carbon will be required, and that carbonaceous relics will be datable as far back as 100,000 years. This is so because the accelerator, rather than counting the rare decay of an atom of carbon-14, counts the considerably more abundant radiocarbon atoms themselves. As a consequence, much less is needed— anywhere from a hundredth to a thousandth as much. Even in very old samples, in which the beta decays have tapered almost to nil, there is still a reasonable number of intact carbon-14 atoms. Thus ancient samples, far beyond the capability of the decay-measuring technique, are potentially datable in the accelerator-cum-spectrometer. A gram sample 100,000 years old, for instance, would still contain on the order of 300,000 to 400,000 atoms of carbon-14, a number that accelerators should be able to detect, at least in theory.

Not only does this capacity open up an astonishing array of previously impossi­ble carbon-dating opportunities, but it appears to be applicable to dating with other radioactive isotopes as well, making possible important investigations related to nuclear waste storage, paleoclima-tology, marine paleontology, cosmology and astrophysics.

To measure carbon-14/carbon-12 ra­tios in accelerators, the minute samples of carbon are boosted to energies of millions of electron volts, equivalent to velocities above 1,600 kilometers a second. Under the influence of a magnetic field, the paths of the carbon atoms are bent, to greater or lesser degrees, depending on their mass and momentum. It is the difference in mass, as expressed in the heavier carbon-14's greater resistance to deflection, that makes the differentiation possible.

Conventional mass spectrometers would not work; they can accelerate ions to only several hundred thousand elec­tron volts, which is far from enough to distinguish between carbon-14 and other mass-14 atoms and molecules.

So sensitive is the accelerator method, says physicist Harry Gove of the Univer­sity of Rochester, one of several con­tributors to the emergence of the new technology, that carbon-14 atoms as rare as one in 1016—the kind of sensitivity required to date an object 70,000 years

44 MOSAIC Novemher/DfiCfimhRr 197fi

old—have been counted with the accelerator; he calls it the equivalent of looking for—and finding—a specific grain of sand among 50,000 giant dump-trucksful.

Three discoveries The first formal, published exploration

of the potential accelerators hold for radioisotope—and specifically radio­carbon—dating came from Richard Mul-ler, an astrophysicist at the University of California's Lawrence Berkeley Labora­tory; he proposed using the Berkeley 88-inch cyclotron. Two other developments at nearly the same time—a collaborative effort by groups at the University of Rochester, the University of Toronto and the General lonex Corporation in Massachusetts, and a second joint ven­ture by scientists at Simon Fraser Univer­sity in Canada—focused on radiocarbon and made use of another kind of instru­ment, the tandem Van de Graaff accelerator.

Muller's inspiration grew out of a search in 1976 for quarks, the still-hypothetical building blocks of matter. But the search was, in effect, a search for new isotopes of hydrogen because of the theoretical possibility—pressed on Muller by Berkeley Nobelist Luis Alvarez—that the quarks carried a unit charge, as do hydrogen ions, rather than the fractional charge for which other scientists have sought (see "A Gaggle of Quarks," Mosaic, Volume 8, Number 4). For the hunt, Muller resurrected a method developed in 1939 at Berkeley by Alvarez and Robert Cornog. Alvarez and Cornog had used the then-newly developed cyclotron for the first (and until now only) time as a mass spectrometer in the successful quest for tritium, radioactive hydrogen-3.

The quark search proved futile, but it led Muller to realize that, by using the same mass spectrometric principles, the cyclotron could be used for carbon-14 dating.

Initially, however, since the Berkeley 88-inch cyclotron was already set up for the detection of hydrogen isotopes, Muller tested the concept by using the accelerator successfully to date a sample of water of known age, with tritium (radioactive hydrogen-3; half life: 12 years) and deuterium (stable hydrogen-2) as his measuring sticks. This effort marked the first time high-energy accelerators had been used to date a sample.

Measuring carbon-14 with the cyclo­tron provided a much tougher risk, principally because of one elemental

obstacle: the presence of the common nitrogen isotope, nitrogen-14. In every carbon sample, there's a plentiful supply of nitrogen-14, which has virtually the same mass as carbon-14. Thus, unless the nitrogen can be screened out, both will be detected together. And, because of the far greater abundance of nitrogen, any rea­sonable count of carbon-14 would be all but impossible.

To get rid of the nitrogen, Muller proposed an ion absorber to filter the nitrogen out of the ion beam before the counting stage. The absorber, filled with xenon gas, capitalizes on the one signifi­cant difference between the carbon and the nitrogen: their atomic numbers, which are based on electrical charge.

The nitrogen atom contains seven protons and seven electrons; carbon— even carbon-14—has six and six. Protons carry a positive charge, electrons a negative. In the cyclotron, the highly energized carbon-14 and nitrogen-14 ions each carry a charge of plus six, the carbon having been stripped of all its electrons, the nitrogen of all but one.

In passing through the absorber, the nitrogen loses its last electron and takes on a net positive charge of seven. With this higher charge, the nitrogen ionizes more xenon than does the carbon, losing energy in the process. As a consequence, the nitrogen comes to rest sooner than does the carbon; only the carbon-14, with 15 to 30 percent less energy loss, gets through to be counted. According to Muller, the absorber currently separates

to better than one residual nitrogen atom for every 1014 carbon atoms.

Recently, Muller and co-workers Terry Mast and Edward Stephenson attempted to do some "blind" dating of samples provided by Rainer Berger, a University of California, Los Angeles, geochemist and anthropologist. Berger had already dated the samples by decay analysis but, according to design, he kept the informa­tion to himself. The first sample was put to the test and Muller made the results known to Berger; it checked out well with Berger's determination. With the second sample, however, Muller was off by thousands of years.

What caused the error, Muller suspects, was a combination of impurities in the samples and interference produced by "background" carbon-14 in the cyclotron. The cyclotron had long been used for basic research in particle physics. In those studies, Muller explains, high-energy protons may have reacted with atoms in the machine's graphite shielding, an interaction likely to produce carbon-14.

Currently, Berkeley scientists are developing and installing an external carbon-14 source which, Muller and his colleagues believe, will cut down substan­tially this unwanted "noise." In the previous trials the carbon sample, in the form of carbon dioxide gas, was injected into the cyclotron where, under an intense spark discharge, it ionized to form positively charged ions. With an external source, the carbon will enter the cyclotron as a narrow, already ionized, high-energy

MOSAIC Nnvfimhfir/Decemher 1978 45

beam. As such, very little of the prevailing background carbon-14 from the graphite lining should infiltrate the beam and disturb the results.

A second "first" While Muller was the first to publish his

ideas about putting the accelerator to work as a mass spectrometer for radiocar­bon dating, and was the first to publish an accelerator-based date, using tritium, he was not the first to succeed in using the technique to date a carbon-14 sample. That distinction belongs to the group working at the Nuclear Structure Research Laboratory at Rochester, in­cluding Harry Gove of Rochester, Albert E. Litherland of the University of Toronto and Kenneth H. Purser of the General lonex Corporation.

The investigators worked at Rochester, taking advantage of time available on its tandem Van de Graaff accelerator. That instrument consists essentially of two long accelerator chambers bisected by a large positive terminal. Negative ions from an external source are accelerated through the first half of the machine, stripped of electrons in the terminal and further accelerated as positive ions through the second half of the machine. From there they pass through a magnetic field where the carbon-12 and carbon-14 atoms are deflected into characteristic paths.

Perhaps the biggest advantage of the tandem Van de Graaff is that it averts the cyclotron's nitrogen contamination prob­lem. That comes about because the machine employs a negative ion source, in contrast to the cyclotron's positive ion input, and because nitrogen is one of the relatively few elements that do not form stable negative ions.

In the testing of a sample, the carbon under investigation is mixed with potas­sium bromide, which acts as a binder. The binder makes it possible to press the carbon, under a ton of pressure, onto the surface of an aluminum cone. The cone is then bombarded with positive cesium ions, which "sputter" or displace carbon atoms from the cone surface, losing electrons in the process. The carbon picks up these electrons and, with an initial energy of 20,000 electron volts, begins its sojourn through the accelerator.

During their initial trials, the collab­orators at Rochester used the Van de Graaff to run a test of graphite, a form of fossilized carbon so ancient as ordinarily to be devoid of all carbon-14 traces. The group obtained a date of 70,000 years. This was then the limit of the machine's

sensitivity, owing to its own background carbon-14, a consequence of 12 years of nuclear physics research. In subsequent runs, Gove and his colleagues measured samples provided by Meyer Rubin of the U.S. Geological Survey, the ages of which had already been established by decay counting. The dates proved reasonably congruent with each other. Perhaps the most significant aspect of these runs was the size of the samples used: 3.5 to 15 milligrams, a hundredth to a thousandth the size of samples required by the Libby technique.

(The basic dating measurement is the ratio of carbon-14 to carbon-12. Ideally, scientists would like to count these different ions simultaneously as a means of making the technique even more precise; such an ability would be held by several accelerators, for which designs have already been drawn up, dedicated solely to carbon-14 dating. With the present set-up, the measurements are made at ten-minute intervals; at Roch­ester, for instance, the main selecting magnet is set first for mass 12; ten minutes later it is switched to mass 14. A dedicated system, if one is built, would measure carbon-14 and carbon-12 within milliseconds of each other, improving accuracy while avoiding much of the background carbon-14 problem.)

Not long after Muller published his proposition and the Rochester group registered the first successful carbon-14 date, a team of Canadian scientists—Earle Nelson and Ralph Korteling of Simon Fraser University and William Stott, John McKay and Dennis Burke of McMaster University—crediting Muller's work, also came to realize the potential of the Van de Graaff's negative ions for avoiding the nitrogen contamination problem. Using the McMaster tandem Van de Graaff, they successfully dated a sample of 19th century white spruce.

The "dream lists" Thus far, no valuable undated sample

has been given over to the accelerator for dating. The work until now—in the United States, Canada and abroad, too, in France and in England—has been directed

University of Rochester

at demonstrating the feasibility of the technique and getting the kinks out of the system. But with the developmental pace heating up—and with new, dedicated machines expected to come into operation soon—it will not be long before many long-wished-for dating opportunities will begin to be explored.

For instance, there are hundreds of archaeological and geological sites throughout the world where too little organic carbon has been available for analysis by conventional means. With the accelerator, as little as a fleck of charcoal the size of a match head is potentially datable. C. V. Haynes, an anthropologist at the University of Arizona, recalls that, in 1959, the first reliable radiocarbon date for Paleo-Indian hunters in northern Colorado was dredged from 1.5 grams of charcoal, a preparation and testing span of over 500 man-hours. Had the new accelerator technology been available, the necessary milligram quantities could have been gathered and tested in an hour.

Another promising application for radiocarbon analysis of very small samples is in dating precious religious and archaeological artifacts; that only a tiny part of the object need be removed is an obvious advantage. Thus, says Haynes some of the most important Paleolithic cave paintings may be directly datable. Recently, the Rochester group, in a test run, dated the linen wrapping of a 2,000-year-old Egyptian mummy, using a one-inch-long piece of thread. A similar test is being contemplated for the celebrated and venerated Shroud of Turin, a linen wrapping carrying the mysterious im­print of a human form and said to be the innermost layer of the cloth in which Christ's body was enfolded. Accurate dating of the shroud would eliminate one area of uncertainty concerning its origin, but conventional radiocarbon dating, which would consume too much of the relic, has never been considered.

Another especially appealing possibility raised by small-sample dating is a more reliable way of determining the age of bone. For years scientists have tried, with only limited success, to isolate carbon from collagen, a protein bound in bone

46 MOSAIC November/December 1978

MOSAIC November/December 1978 47

and connective tissue. In many cases, however, there is either too little collagen to analyze or it has degraded or become contaminated. As a result, scientists have been greeting many bone dates with skepticism. There are two amino acids specific only to bone protein. The more abundant of the two, hydroxyproline, is made up of 46 percent carbon. If a few milligrams of pure hydroxyproline could be separated from a bone sample, enough carbon would theoretically be available for highly precise dating. Until now, by conventional dating techniques, to get enough hydroxyproline from many paleontological sites would have required a whole mammoth skeleton. "Now [with an accelerator] perhaps we could get reliable results from just a toe bone," says Haynes, "and that would certainly make the vertebrate paleontologist much hap­pier than would grinding up a whole skeleton." Success with this approach would make possible accurate dating of many of the bone specimens that are critical to questions relating to the peopling of the New World (see "The Earliest Known Americans," Mosaic, Volume 8, Number 2) or the time of emergence of Homo sapiens (see "Shifting Perspectives on Early Man," Mosaic, Volume 9, Number l ) .

Earth, ocean, climate Another exciting area of research that

stands to reap considerable benefit from accelerator dating has to do with predict­ing geologic hazards, such as earthquakes, volcanic eruptions, mud flows and landslides. As Meyer Rubin of the U.S. Geological Survey explains it, the dating of faults, lava flows and other pertinent geological formations (e.g., terraced terrain produced by repeated earthquake-induced upheavals) established a time­table of previous and possibly periodic events. Such data are employed by the U.S.G.S. to help make recommendations about sites for nuclear power reactors, nuclear waste disposal, erection of earthen dams and the establishment of recreational communities.

What has confounded the effort, Rubin says, is the difficulty of obtaining ap­propriate carbon samples which, because of weathering and oxidation, are often available only in trace amounts. With the conventional method, Rubin and his colleagues have often had to lug back and burn many pounds of soil to extract enough carbon for just one run. "With the accelerator," he says, "all we'll need to do is dig up a fleck's worth of charcoal with a penknife."

Rubin, who has been supplying test samples to the Rochester group, says he is very optimistic about the progress being made in refining carbon-14 dating. He thinks it will be only a matter of months until he will feel confident enough to turn over his file of rare geological samples to the Rochester group, samples much too small for dating by radioactive decay.

Still other areas of potential application relate to oceanography, ice core dating and atmospheric chemistry. Case in point: the time it takes for a volume of surface ocean water to sink to the bottom. The contemporary measure is about 1,200 years, as determined by comparing carbon-14 concentrations in deep water to those at the surface; existing tech­niques are adequate to that. But in­vestigators also want to know the numbers for past epochs, back to the peak glacial periods. With a method for dating small samples of marine shells in cores of seafloor sediment, that should become possible, says Wallace S. Broecker, an oceanographer and climatologist at Columbia University's Lamont-Doherty Geological Observatory.

According to Broecker, there are two kinds of shells side by side in the sea-bottom sediments: those of organisms that inhabited the surface, and thus have the carbon-14/carbon-12 ratio found in surface waters, and those of benthic or bottom-dwelling species whose shells record the isotope ratio of deep water. By running radiocarbon dates on both kinds of shells, at successive levels along a sediment core, and measuring the shell-to-shell differences, says Broecker, it should be possible to establish a con­tinuous record of the rate of water turnover. By thus reconstructing past ocean dynamics, another piece could be added to the still-incomplete puzzle of climate change (see "Climate: How Large an Unknown in the Food Equation?" Mosaic, Volume 6, Number 3, and "Climate, Weather, Aridity," Mosaic, Volume 8, Number 1).

Broecker says shell dating is really the only way to get at this record. A problem until now has been that, while shells of surface creatures are found in great abundance in sediment cores, shells of benthic origin are scarce; perhaps one in every 100 shells in the core comes from a bottom dweller. The prohibitively large sample requirements of conventional techniques preclude any attempt to carry out dating studies.

There is a further prospect of con­tributing to an accurate profile of the earth's paleoclimate, the record of past

climate locked in the ice sheets of Greenland and Antarctica (see "Tales the Ice Can Tell," Mosaic, Volume 9, Number 5), as well as in ocean sediment cores. By measuring the concentrations of oxygen-18, a stable isotope, in relation to the common oxygen-16 along a deep-ice core, paleoclimatologists have been able to establish the air temperature at which the snow layers formed. (Generally, the colder the air temperature, the less oxygen-18 in the sample.) Moreover, all major volcanic events are also recorded in the snow by way of airborne volcanic ash. A number of investigators suspect that climate change and glacial periods are strongly influenced by the frequency of volcanic eruptions. Accurate dates for various layers of the ice core have been difficult to nail down, however, and there has been a considerable amount of guesswork in the assigning of ages to core strata.

One way to improve this speculative timetable and assign more precise dates to the ice layers is to measure the carbon-14 in bubbles of carbon dioxide trapped as the snow fell through the atmosphere of the time. With conventional dating methods, a substantial volume of ice—as much as several tons—has been required to get enough carbon for an age deter­mination and, not surprisingly, a limited amount of carbon dating work on ice cores has been done. Reducing sample size by a factor of 1,000, as accelerator dating promises, would certainly encourage attempts to date the core as a means of sharpening the record on past climate cycles.

Another Oceanographic phenomenon, the rate and manner in which particulate matter falls through the ocean from the surface, is of interest to a number of scientific disciplines: to biologists because it is a mechanism for carrying food to deep-dwelling organisms and has much to do with the maintenance of marine ecosystems; to geologists because par­ticulate drift is the dominant means of transfer of chemical substances from one place in the water column to another and, ultimately, into the sediment.

Lately, with the development of sedi­ment traps—essentially, boxes with open doors that can be moored at various levels in the ocean for periods ranging from a few months to a year—it is possible to capture a volume of particulate samples sufficient for accelerator-dating needs. The prospects were improved, in­advertently, by the large-scale nuclear testing of the mid-nineteen-fifties. Those tests increased the carbon-14/carbon-12

48 MOSAIC November/December 1978

ratio in the atmosphere and upper ocean, and the organic material formed by photosynthetic marine organisms in the ocean's upper 100 meters during the last two decades contains as much as 20 percent more carbon-14 than that seen in samples from earlier years. Thus, by looking at the carbon-14/carbon-12 ratio in deep-dwelling organisms, for example, it should be possible to establish food transit time.

Yet another idea to come to light in the wake of carbon-14 accelerator dating progress is the possibility that the origin and fate of trace gases in the atmo­sphere—carbon monoxide, for example— can be determined. Once it was thought that automobiles were the principal source of atmospheric carbon monoxide. But now there is speculation that the oxidation of methane released by natural organic processes may be at least as responsible.

Dating small samples would help resolve that important environmental issue; carbon monoxide from auto emis­sions contains essentially no carbon-14, all its carbon having come from fossil fuels. But carbon monoxide from methane, most of it recently emitted by organic materials, would have contem­porary carbon-14 levels. If it were to be shown that the current atmospheric carbon monoxide reads as if it had an average age of 5,730 years, it would mean that half the carbon monoxide was from fossil fuels—that human contributions were matching nature's in the production of carbon monoxide, rather than outstrip­ping her by ten times or more as has been proposed.

To make appropriate carbon-14 deter­minations of atmospheric carbon monox­ide by conventional methods, however, requires the milking of thousands of cubic feet of air for carbon monoxide content, a situation that leaves a great deal of room for contamination; small-sample spec­troscopy would vastly increase the reliability of the results.

Other isotopes While carbon-14 dating opportunities

are currently generating the most excite­ment, there are many scientists who believe that accelerator dating with other naturally occurring radioisotopes will turn out to be just as important. Beryllium-10, for instance, seems well suited to some fundamental problems in geology. With a long half life (1.5 million years), hence a very low level of radioac­tive emission, beryllium-10 cannot readily be detected by any available decay-

counting technique. It is a different story, of course, with accelerators, which allow for direct measurement of the undecayed beryllium-10 atoms. Berkeley's Richard Muller, who has been awarded the National Science Foundation's 1978 Alan T. Waterman Award, has done some preliminary cyclotron work on beryllium-10. And a French group, headed by G. M. Raisbeck of the Rene Bernas Laboratory, in Orsay, has reported the successful measurement of the isotope in 1,000- and 5,000-year-old antarctic ice. Beryllium-10 may well become "the carbon-14 of geology," Muller declares. (Other radio isotopes , including those of uranium, thorium, potassium and rubidium, have geological applications; but, with half lives in the range of hundreds of millions to billions of years, they still leave a gap in the millions-of-years range to be filled. Standard spec-trometric techniques are employed in their application to such problems as the age of rock.)

One troublesome question beryllium-10 dating may help resolve is the origin and rate of growth of manganese nodules, mysterious, spoorlike structures found widely scattered on the ocean floor. From one to six inches in diameter, the nodules principally comprise iron and manganese oxides. They are also rich in nickel, cobalt and copper, and have been receiving considerable attention from marine min­ing interests.

Apparently the nodules accrete very slowly, at a rate of about three millimeters every million years. "If that's the case," says Lamont's Broecker, "to get as large as they are they must have been around anywhere from 10 to 30 million years. If we can date all the way through [the nodule] we can tell whether it grew continuously or not; this can help tell us about processes that have been going on in and on the ocean floor."

Beryllium-10 appears to be an ideal vehicle for such a study. Produced by cosmic rays, the isotope settles out of the atmosphere and, being insoluble in water, drops to the ocean floor where traces of it are incorporated in the nodules. (Carbon-14, with a half life of 5,730 years, would be totally useless here. What is required is a radioisotope with a half life comparable to the age of the event or process under scrutiny.)

Beryllium-10, together with another cosmogenic isotope, aluminum-26 (half life: 740,000 years), may also help settle some cosmic issues. (See "Stellar On­togeny: from Dust...," and "Stellar On­togeny: ...to Ashes," Mosaic, Volume 9,

Number 3.) For instance, it has been contended that a supernova exploded in the vicinity of earth a million years ago, but there has been some question about the validity of the data supporting this supposition. Berkeley's Muller, a prac­ticing cosmologist and astrophysicist, suggests that the issue may be resolved by looking for beryllium-10 and aluminum-26 "peaking" at certain appropriate strata in the ocean cores. Similarly, analysis of core strata should have much to tell about the fluctuations in intensity of cosmic ray impingement upon the earth over much of the planet's lifetime.

Another isotope of growing importance is chlorine-36 because, with a half life of 310,000 years, it can serve as a means of determining the age of groundwater. Produced by cosmic ray action (on atmos­pheric argon-40), chlorine-36 falls out in rain and enters the earth's water cycle.

Groundwater dating is of concern not only in relation to whether or not ir­replaceable fossil water is being "minced" (see "Modest Technologies," Mosaic, Volume 8, Number l ) , but also in relation to such questions as nuclear waste storage. As Rochester's Harry Gove puts it: "We must be sure that, even if subterranean nuclear repositories even­tually leak and get into the water, these radionuclides won't get to the surface. If we can show, and chlorine-36 can help show us, that the underground water is a million years old, say, then we can rest easy that the water hasn't moved in a long time, that it isn't likely to and that [storage] shouldn't pose any real hazard." (The Rochester group has recently been able to detect chlorine-36 in natural water samples, down to 10"~15 concentrations.)

This list of potential applications could go on. Perhaps that should come as no surprise. After all, for decades scientists have taken to drawing up "dream lists" of problems that would lend themselves to solution were there ever to become avail­able a sensitive technique for measuring long-lived radioisotopes directly. This once-remote wish has now become a near-reality. And, as the exciting promise of particle accelerator dating comes nearer to fruition, there is little hyperbole in suggesting that the next three decades may well produce a quantum increase in knowledge about the past, as Libby's initial invention of radiocarbon dating itself has done over the last three decades. •

National Science Foundation contributions to the research reported in this article are through its Nuclear Physics Program,

MOSAIC November/December 1978 49