Nuclear Clocks / Revised :: THE CARBON-14 CLOCK

THE CARBON-14 CLOCK

Carbon-14 decay is the best example of a top-only hourglass. Carbon-14 is constantly being produced in the upper atmosphere from atoms of nitrogen-14 being struck by neutrons that had their origin in COSMIC RAYS. The reaction is written:

¹4N + neutron ? ¹4C + proton

Radioactive decay then follows, with a half-life of 5800 years[9] for the ¹4C.

¹4C ? ¹4N + electron (BETA PARTICLE)

The radiocarbon emits an electron and changes back into nitrogen.

As far as anyone can tell, ¹4C was produced at a constant rate above the earth for at least 50,000 years before the first atomic bomb was exploded. In other words, the ¹4C cycle is like an hourglass in which the sand in the upper part is replenished as fast as it runs out through the hole in the waist. A process of this sort, where production equals decay, is called a SECULAR EQUILIBRIUM.

The newly produced ¹4C soon is evenly mixed with the carbon dioxide in the air, is taken up by all living plants, and then finds its way into all living animals. In effect, all carbon in living organisms contains a constant proportion of ¹4C. If any of this carbon is taken out of circulation—when a tree branch is broken off, for instance, or when a shellfish dies in the ocean—no more new ¹4C is added to that particular system, but the old ¹4C continues to run out. In effect it now starts measuring time as an hourglass should.

To illustrate secular equilibrium, one must imagine an hourglass in which the sand in the top bulb is continuously replenished—as fast as it runs out through the hole in the waist and disappears.

When we find a piece of charcoal in a cave or a piece of wood in some ancient structure, for example, we can measure the amount of carbon in it, determine how much of it is ¹4C, and then calculate back to the time when the radioactivity from the ¹4C was the same as we now find in living wood. In other words, if we assume that we know from the observed secular equilibrium how much ¹4C originally was present in living material, then we can calculate the time of death of any similar but ancient material. That is the basis of the ¹4C method of age determination.

Dinosaur tracks imprinted in rock in Navajo Canyon, Arizona, arouse the professional interest of this scientist. Fossil traces of extinct prehistoric creatures were for a long time the best clues to the age of rock formations.

A scientist using liquid nitrogen to freeze carbon dioxide gas made from a sample of ancient material that he is preparing for age determination by the carbon-14 technique.

For example, a bit of a rafter from a prehistoric cliff-dwelling or a remnant of charcoal from an ancient fire may be analyzed for its remaining ¹4C content, and its age determined accurately within the margin of a few hundred years. This fixes the time at which the wood for the rafter or the firewood was broken or cut from the living tree, and hence the period in which the men lived who used the wood.

The most useful samples for carbon-14 age determination are charcoal, wood, and shells.

Carbon-14 Counting

Carbon-14 measurements are made by taking a known amount of carbon, reducing it to a gas, and then counting the ¹4C disintegrations in the gas. This may sound simple, but in reality the measurement process is a formidable undertaking, because the amount of the ¹4C isotope in the carbon is so extremely small. (The remainder of the carbon, of course, consists of other isotopes—¹²C or ¹³C, which are stable.)

There are two basic techniques. The carbon can be:

1. Burned with oxygen to form carbon dioxide, or
2. Reduced chemically to methane or ethane, or to a carbide from which acetylene can be evolved by adding water. (See booklet cover and description on page 59.)

The first technique is the simpler, but carbon dioxide (CO2) contains only one atom of carbon per molecule, whereas acetylene (C2H2) and ethane (C2H6) each contain two. Consequently, the SPECIFIC ACTIVITY of acetylene or ethane is twice that of carbon dioxide, other things being equal. For that reason acetylene or ethane are the preferred gases in some laboratories. On the other hand, they are explosive, and that cautions other scientists into using the carbon dioxide method.

Whichever gas is used, it is first purified and then stored in a bottle for a month or so. This storage allows for decay (disappearance) of any radon, the gaseous radioactive product of uranium decay. Uranium contamination is difficult to avoid at the low radioactivity levels of ¹4C, but the half-life of radon is only 3.82 days, so that it will decay to an insignificant level in a month. After the storage period, the gas is pumped into an array of instruments known as a low-background PROPORTIONAL COUNTER, and its radioactivity is determined. This is an involved process. First there is the matter of the BACKGROUND COUNT.

The background count of an instrument is the number of pulses (counts) it will give per unit time when there is no radioactive sample in it. These counts are caused by cosmic rays, by radioactive contamination always found in the vicinity of the counter, or by any contamination inside the instrument. In ¹4C counting, all these sources of background must be reduced to negligible levels. This is done in a number of ways.

For one thing, the whole assembly is constructed with surrounding walls of lead or iron more than a foot thick. Such a shield will stop all the GAMMA RAYS coming from radioactive contamination in the laboratory and much of the cosmic radiation; high-energy cosmic rays and all neutrons still will get through. Therefore, there is an ANTICOINCIDENCE RING inside the lead shield. This is a cylindrical space completely surrounded by GEIGER COUNTERS that are connected to each other and to the SAMPLE COUNTER in the middle. With this arrangement, when the sample counter and any ring counter discharge simultaneously, it is a signal that the pulse triggering this response was caused by some energetic particles, such as a cosmic ray, passing through the whole assembly. A pulse recorded simultaneously on two counters is automatically rejected from the counting mechanisms. Some instruments have been designed with a cylinder of paraffin immediately inside the anticoincidence ring, to slow down neutrons so that they can be captured, and with a final shield of highly purified mercury between two cylinders of selected steel to hold out even more unwanted radiation.

A carbon-14 counter. Sketch (above) shows arrangement of components in photo (left).

Finally, the sample counters are made of specially selected metal tubing that is extremely low in radioactive content, with a fine wire stretched down the middle. (In some recent designs, the anticoincidence ring and sample counter are combined in a single cylindrical housing with a thin foil of metallized plastic between them.) A thin glass filling tube connects the sample counter with the outside world.

Detail of one type of sample-counting tube for carbon-14 work. Carbon-14 in benzene gas molecules is placed in the central cell and mixed with a fluid that scintillates, or emits light flashes, when exposed to radiation. Photomultiplier tubes convert the flashes to electric signals.

The radiocarbon-bearing gas is pumped into the sample counter through the filling tube, and all the counts resulting from its disintegrations are recorded electronically. The age of the sample is calculated from the NET COUNTING RATE (the sample counting rate minus the background); the lower the counting rate the higher the age. The upper limit of the age that can be measured is determined by the STATISTICAL ERROR (that is, by the measure of the instrument accuracy) in the net count. In very old samples this error may be great enough so that the calculated age of the sample may have little or no meaning.

Carbon-14 Results

OBJECTS DATED BY RADIOCARBON

Hair of an Egyptian woman. 5020 ± 290 years old.

Linen wrapping from the Dead Sea Scroll containing the Book of Isaiah. 1917 ± 200 years old.

Peruvian rope. 2632 ± 200 years old.

Preglacial wood found in Ohio. More than 20,000 years old.

Rope sandal found in an eastern Oregon cave. One of a pair of 300 pairs found in this cave. 9035 ± 325 years old.

Carbon-14 is by far the most widely used method of measuring geologic time. It has become the mainstay of archeology and geology for studies of events of the past 50,000 years or so, and also has wide applications in climatology, ecology, and geography. It would be difficult to pick out the most significant example of the use of this method, but one important contribution has been in study of the early inhabitants of North America. With the aid of ¹4C it has been possible to date human living sites from many points in the western United States. The first appearance of these sites, about 11,500 years ago, apparently coincided with the time when a land bridge was open from Asia to America over what is now the Bering Strait. An ice-free passage extended from this bridge through present-day Alaska and western Canada to the United States. This may have been the route taken by the first immigrants to America—a population of mammoth-hunters, who made the characteristic flint Clovis arrow and spear points.

A Clovis arrow point chipped from flint by the earliest men on the American continent. The photograph is actual size.

By about 11,000 years ago, these Clovis people had spread across the area of the United States and into Mexico. It may have been they who killed off the mammoths and then gradually assumed the characteristics of the Folsom culture. The Folsom people were bison-hunters, and long were thought to have been the first population in America. It was with the use of ¹4C that it finally was possible to place these two cultures in proper sequence—the Clovis first—and to correlate them with major natural changes, especially the advance and retreat of glaciers across the continent.

Clyx.com