National Geographic : 1958 Aug
The National (Gographic MlIagazine old." he says. "Now the natives call the place Haunted Lake, and historians are searching for accounts of an army that may have vanished in that area." The MIagnolia Petroleum Company research center in Dallas, Texas, showed us a jar of wheat from a Moabite home in Jordan. The kernels-many of them still perfectly formed -have oxidized until they are as black as if they had been charred in an oven. Their car bon date puts them around 850 B. C.-not long after the time when Ruth the Ioabitess was gleaning the fields of Boaz. In the early stages of carbon dating, fairly large samples were needed-enough to give several ounces of carbon. Today's far more sensitive equipment makes it possible to measure the radioactivity of as little as one hundredth of an ounce of charcoal. Whether large or small, the sample is treated with acids and alkalies to remove contamina tion, then burned in a heavy glass tube. Carbon-containing gas from this combustion moves through a vacuum line-a system of tubes and flasks from which all air has been evacuated; in them it comes in contact with chemicals that extract impurities. Refined, the gas passes into the counter (page 245). Radioactivity of the sample is there meas ured by an electronic mechanism similar to the Geiger counter used to locate uranium ore. This counter is hidden inside a steel tomb, with walls sometimes nine inches thick, to shield out radioactivity from other sources. To double the safeguard, the counting tube is surrounded by an "electronic umbrella.' a ring of smaller Geiger counters that detect any cosmic rays that penetrate the shield. These counters cannot stop cosmic rays, but they turn off the main counter for an infini tesimal fraction of a second while a cosmic ray is passing through. Telling Ticks from Tocks The two kinds of counters explain the tick S..tick...tick...tok we heard in the lab oratory. The frequent soft clicks come from the cosmic ray recorder; each one signifies that 64 cosmic rays have passed through. The louder clicks represent the radioactivity of the sample itself-each one records the dis integration of a carbon atom. Actually a few of the discharges detected bv the main counter come from sources other than carbon-14. Even when the counter is empty, it is impossible to free it entirely from the slight radioactivity of the very mate rials of which it is made. These materials give off spurious counts which are referred to as the "background" of the counter. Refinements of equipment are steadily re ducing this background. One of the leading laboratories now holds background count to two discharges a minute, compared to the 35 counts per minute produced when modern carbon-14 is being tested in the same appa ratus. Background count is determined by running such "dead" samples as coal, which is so old that it contains no carbon-14 at all. This figure must be subtracted from the total count, and what is left represents the number of disintegrations of carbon atoms. When Did Hammurabi Live? Most laboratories now get reasonable re sults from a radiocarbon test overnight. How ever, the longer a test is run, the more accurate are the findings. On one set of tests, Dr. Libby ran his counters for three months. This extremely long and expensive test was an effort to settle an old dispute of much interest to historians: What are the actual dates for Hammurabi, the Babylonian king who lived nearly 2,000 years before Christ and drafted the first code of laws known to history? Here's how Ir. Libby tells the story: "The entire Babylonian calendar over a period of hundreds of years has been carefully worked out from king lists and astronomical evidence, but theories about where it should fit into the Western calendar differ by several hundred years. "To make our tests, we chose a roof beam that burned during the reign of a king who preceded Hammurabi by about 250 years. If the date of this charcoal could be determined accurately, then Hammurabi could be given his proper place in the Western calendar. "The carbon date for the charcoal came out 1993 B. C. ± 106 years. That little plus or-minus tail wagging at the end of the date is an expression of statistical error that ac companies every radiocarbon date. The longer the test is run, the smaller the plus-or-minus factor becomes: when the length of the test is increased four times, the error is cut in half. "In this case the plus-or-minus factor means that, two chances out of three, the burned beam came from a tree that died between 1887 B. C. and 2099 B. C."