10 March 2009
A technique that collects X-ray data from solid,
opaque masses such as human bodies and uses a computer to
construct a three-dimensional image.
The people behind the invention:
Godfrey Newbold Hounsfield (1919- ), an English
electronics engineer who shared the 1979 Nobel Prize in
Physiology or Medicine
Allan M. Cormack (1924-1998), a South African-born American
physicist who shared the 1979 Nobel Prize in Physiology or
James Ambrose, an English radiologist
The invention: Self-contained system making it possible to record and repeatedly play back sound without having to thread tape through a machine. The person behind the invention: Fritz Pfleumer, a German engineer whose work on audiotapes paved the way for audiocassette production Smaller Is Better The introduction of magnetic audio recording tape in 1929 was met with great enthusiasm, particularly in the entertainment industry, and specifically among radio broadcasters. Although somewhat practical methods for recording and storing sound for later playback had been around for some time, audiotape was much easier to use, store, and edit, and much less expensive to produce. It was Fritz Pfleumer, a German engineer, who in 1929 filed the first audiotape patent. His detailed specifications indicated that tape could be made by bonding a thin coating of oxide to strips of either paper or film. Pfleumer also suggested that audiotape could be attached to filmstrips to provide higher-quality sound than was available with the film sound technologies in use at that time. In 1935, the German electronics firm AEG produced a reliable prototype of a record-playback machine based on Pfleumer’s idea. By 1947, the American company 3M had refined the concept to the point where it was able to produce a high-quality tape using a plastic- based backing and red oxide. The tape recorded and reproduced sound with a high degree of clarity and dynamic range and would soon become the standard in the industry. Still, the tape was sold and used in a somewhat inconvenient open-reel format. The user had to thread it through a machine and onto a take-up reel. This process was somewhat cumbersome and complicated for the layperson. For many years, sound-recording technology remained a tool mostly for professionals. In 1963, the first audiocassette was introduced by the Netherlands-based PhilipsNVcompany. This device could be inserted into a machine without threading. Rewind and fast-forward were faster, and it made no difference where the tape was stopped prior to the ejection of the cassette. By contrast, open-reel audiotape required that the tape be wound fully onto one or the other of the two reels before it could be taken off the machine. Technical advances allowed the cassette tape to be much narrower than the tape used in open reels and also allowed the tape speed to be reduced without sacrificing sound quality. Thus, the cassette was easier to carry around, and more sound could be recorded on a cassette tape. In addition, the enclosed cassette decreased wear and tear on the tape and protected it from contamination. Creating a Market One of the most popular uses for audiocassettes was to record music from radios and other audio sources for later playback. During the 1970’s, many radio stations developed “all music” formats in which entire albums were often played without interruption. That gave listeners an opportunity to record the music for later playback. At first, the music recording industry complained about this practice, charging that unauthorized recording of music from the radio was a violation of copyright laws. Eventually, the issue died down as the same companies began to recognize this new, untapped market for recorded music on cassette. Audiocassettes, all based on the original Philips design, were being manufactured by more than sixty companies within only a few years of their introduction. In addition, spin-offs of that design were being used in many specialized applications, including dictation, storage of computer information, and surveillance. The emergence of videotape resulted in a number of formats for recording and playing back video based on the same principle. Although each is characterized by different widths of tape, each uses the same technique for tape storage and transport. The cassette has remained a popular means of storing and retrieving information on magnetic tape for more than a quarter of a century. During the early 1990’s, digital technologies such as audio CDs (compact discs) and the more advanced CD-ROM (compact discs that reproduce sound, text, and images via computer) were beginning to store information in revolutionary new ways. With the development of this increasingly sophisticated technology, need for the audiocassette, once the most versatile, reliable, portable, and economical means of recording, storing, and playing-back sound, became more limited. Consequences The cassette represented a new level of convenience for the audiophile, resulting in a significant increase in the use of recording technology in all walks of life. Even small children could operate cassette recorders and players, which led to their use in schools for a variety of instructional tasks and in the home for entertainment. The recording industry realized that audiotape cassettes would allow consumers to listen to recorded music in places where record players were impractical: in automobiles, at the beach, even while camping. The industry also saw the need for widespread availability of music and information on cassette tape. It soon began distributing albums on audiocassette in addition to the long-play vinyl discs, and recording sales increased substantially. This new technology put recorded music into automobiles for the first time, again resulting in a surge in sales for recorded music. Eventually, information, including language instruction and books-on-tape, became popular commuter fare. With the invention of the microchip, audiotape players became available in smaller and smaller sizes, making them truly portable. Audiocassettes underwent another explosion in popularity during the early 1980’s, when the Sony Corporation introduced the Walkman, an extremely compact, almost weightless cassette player that could be attached to clothing and used with lightweight earphones virtually anywhere. At the same time, cassettes were suddenly being used with microcomputers for backing up magnetic data files. Home video soon exploded onto the scene, bringing with it new applications for cassettes. As had happened with audiotape, video camera-recorder units, called “camcorders,” were miniaturized to the point where 8-millimeter videocassettes capable of recording up to 90 minutes of live action and sound were widely available. These cassettes closely resembled the audiocassette first introduced in 1963.
The invention: Atechnique that measures the radioactive decay of carbon 14 in organic substances to determine the ages of artifacts as old as ten thousand years. The people behind the invention: Willard Frank Libby (1908-1980), an American chemist who won the 1960 Nobel Prize in Chemistry Charles Wesley Ferguson (1922-1986), a scientist who demonstrated that carbon 14 dates before 1500 b.c. needed to be corrected One in a Trillion Carbon dioxide in the earth’s atmosphere contains a mixture of three carbon isotopes (isotopes are atoms of the same element that contain different numbers of neutrons), which occur in the following percentages: about 99 percent carbon 12, about 1 percent carbon 13, and approximately one atom in a trillion of radioactive carbon 14. Plants absorb carbon dioxide from the atmosphere during photosynthesis, and then animals eat the plants, so all living plants and animals contain a small amount of radioactive carbon. When a plant or animal dies, its radioactivity slowly decreases as the radioactive carbon 14 decays. The time it takes for half of any radioactive substance to decay is known as its “half-life.” The half-life for carbon 14 is known to be about fifty-seven hundred years. The carbon 14 activity will drop to one-half after one half-life, onefourth after two half-lives, one-eighth after three half-lives, and so forth. After ten or twenty half-lives, the activity becomes too low to be measurable. Coal and oil, which were formed from organic matter millions of years ago, have long since lost any carbon 14 activity. Wood samples from an Egyptian tomb or charcoal from a prehistoric fireplace a few thousand years ago, however, can be dated with good reliability from the leftover radioactivity. In the 1940’s, the properties of radioactive elements were still being discovered and were just beginning to be used to solve problems. Scientists still did not know the half-life of carbon 14, and archaeologists still depended mainly on historical evidence to determine the ages of ancient objects. In early 1947,Willard Frank Libby started a crucial experiment in testing for radioactive carbon. He decided to test samples of methane gas from two different sources. One group of samples came from the sewage disposal plant at Baltimore, Maryland, which was rich in fresh organic matter. The other sample of methane came from an oil refinery, which should have contained only ancient carbon from fossils whose radioactivity should have completely decayed. The experimental results confirmed Libby’s suspicions: The methane from fresh sewage was radioactive, but the methane from oil was not. Evidently, radioactive carbon was present in fresh organic material, but it decays away eventually. Tree-Ring Dating In order to establish the validity of radiocarbon dating, Libby analyzed known samples of varying ages. These included tree-ring samples from the years 575 and 1075 and one redwood from 979 b.c.e., as well as artifacts from Egyptian tombs going back to about 3000 b.c.e. In 1949, he published an article in the journal Science that contained a graph comparing the historical ages and the measured radiocarbon ages of eleven objects. The results were accurate within 10 percent, which meant that the general method was sound. The first archaeological object analyzed by carbon dating, obtained from the Metropolitan Museum of Art in New York, was a piece of cypress wood from the tomb of King Djoser of Egypt. Based on historical evidence, the age of this piece of wood was about fortysix hundred years. A small sample of carbon obtained from this wood was deposited on the inside of Libby’s radiation counter, giving a count rate that was about 40 percent lower than that of modern organic carbon. The resulting age of the wood calculated from its residual radioactivity was about thirty-eight hundred years, a difference of eight hundred years. Considering that this was the first object to be analyzed, even such a rough agreement with the historic age was considered to be encouraging. The validity of radiocarbon dating depends on an important assumption— namely, that the abundance of carbon 14 in nature has been constant for many thousands of years. If carbon 14 was less abundant at some point in history, organic samples from that era would have started with less radioactivity. When analyzed today, their reduced activity would make them appear to be older than they really are.Charles Wesley Ferguson from the Tree-Ring Research Laboratory at the University of Arizona tackled this problem. He measured the age of bristlecone pine trees both by counting the rings and by using carbon 14 methods. He found that carbon 14 dates before 1500 b.c.e. needed to be corrected. The results show that radiocarbon dates are older than tree-ring counting dates by as much as several hundred years for the oldest samples. He knew that the number of tree rings had given him the correct age of the pines, because trees accumulate one ring of growth for every year of life. Apparently, the carbon 14 content in the atmosphere has not been constant. Fortunately, tree-ring counting gives reliable dates that can be used to correct radiocarbon measurements back to about 6000 b.c.e. Impact Some interesting samples were dated by Libby’s group. The Dead Sea Scrolls had been found in a cave by an Arab shepherd in 1947, but some Bible scholars at first questioned whether they were genuine. The linen wrapping from the Book of Isaiah was tested for carbon 14, giving a date of 100 b.c.e., which helped to establish its authenticity. Human hair from an Egyptian tomb was determined to be nearly five thousand years old.Well-preserved sandals from a cave in eastern Oregon were determined to be ninety-three hundred years old. A charcoal sample from a prehistoric site in western South Dakota was found to be about seven thousand years old. The Shroud of Turin, located in Turin, Italy, has been a controversial object for many years. It is a linen cloth, more than four meters long, which shows the image of a man’s body, both front and back. Some people think it may have been the burial shroud of Jesus Christ after his crucifixion. Ateam of scientists in 1978 was permitted to study the shroud, using infrared photography, analysis of possible blood stains, microscopic examination of the linen fibers, and other methods. The results were ambiguous. A carbon 14 test was not permitted because it would have required cutting a piece about the size of a handkerchief from the shroud. Anew method of measuring carbon 14 was developed in the late 1980’s. It is called “accelerator mass spectrometry,” or AMS. Unlike Libby’s method, it does not count the radioactivity of carbon. Instead, a mass spectrometer directly measures the ratio of carbon 14 to ordinary carbon. The main advantage of this method is that the sample size needed for analysis is about a thousand times smaller than before. The archbishop of Turin permitted three laboratories with the appropriate AMS apparatus to test the shroud material. The results agreed that the material was from the fourteenth century, not from the time of Christ. The figure on the shroud may be a watercolor painting on linen. Since Libby’s pioneering experiments in the late 1940’s, carbon 14 dating has established itself as a reliable dating technique for archaeologists and cultural historians. Further improvements are expected to increase precision, to make it possible to use smaller samples, and to extend the effective time range of the method back to fifty thousand years or earlier.