Quantum Physics

29.6 Nuclear Reactions 955wounds on the head that could have been caused by a crown of thorns and anotherwound in the side that could have been the cause of death. Skepticism over theauthenticity of the shroud has existed since its first public showing in 1354; in fact, aFrench bishop declared it to be a fraud at the time. Because of its controversialnature, religious bodies have taken a neutral stance on its authenticity.In 1978 the bishop of Turin allowed the cloth to be subjected to scientific analysis,but notably missing from these tests was carbon-14 dating. The reason for thisomission was that, at the time, carbon-dating techniques required a piece of clothabout the size of a handkerchief. In 1988 the process had been refined to thepoint that pieces as small as one square inch were sufficient, and at that timepermission was granted to allow the dating to proceed. Three labs were selectedfor the testing, and each was given four pieces of material. One of these was apiece of the shroud, and the other three pieces were control pieces similar inappearance to the shroud.The testing procedure consisted of burning the cloth to produce carbon dioxide,which was then converted chemically to graphite. The graphite sample wassubjected to carbon-14 analysis, and in the end all three labs agreed amazingly wellon the age of the shroud. The average of their results gave a date for the cloth ofA.D. 1 320 60 years, with an assurance that the cloth could not be older thanA.D. 1 200. Carbon-14 dating has thus unraveled the most important mystery concerningthe shroud, but others remain. For example, investigators have not yetbeen able to explain how the image was imprinted.Santi Visali/The IMAGE BankFigure 29.11 The Shroud of Turinas it appears in a photographic negativeimage.29.5 NATURAL RADIOACTIVITYRadioactive nuclei are generally classified into two groups: (1) unstable nucleifound in nature, which give rise to what is called natural radioactivity, and (2)nuclei produced in the laboratory through nuclear reactions, which exhibitartificial radioactivity.Three series of naturally occurring radioactive nuclei exist (Table 29.2). Eachstarts with a specific long-lived radioactive isotope with half-life exceeding that ofany of its descendants. The fourth series in Table 29.2 begins with 237 Np, atransuranic element (an element having an atomic number greater than that ofuranium) not found in nature. This element has a half-life of “only” 2.14 10 6 yr.The two uranium series are somewhat more complex than the 232 Th series(Fig. 29.12). Also, there are several other naturally occurring radioactive isotopes,such as 14 C and 40 K, that are not part of either decay series.Natural radioactivity constantly supplies our environment with radioactive elementsthat would otherwise have disappeared long ago. For example, because theSolar System is about 5 10 9 years old, the supply of 226 Ra (with a half-life of only1 600 yr) would have been depleted by radioactive decay long ago were it not forthe decay series that starts with 238 U, with a half-life of 4.47 10 9 yr.U Uranium23892 4.47 10 982 PbN140Rae – e – Ac224 Ra135220 Rn216 Po212Pb 130e – 212Bi208 TlPoe – 125 PbZ80 85 90Figure 29.12 Decay series beginningwith 232 Th.TABLE 29.2The Four Radioactive SeriesSeries Starting Isotope Half-life (years) Stable End ProductActinium2357.04 10 829.6 NUCLEAR REACTIONSIt is possible to change the structure of nuclei by bombarding them with energeticparticles. Such changes are called nuclear reactions. Rutherford was the first toobserve nuclear reactions, using naturally occurring radioactive sources for the20620792 U 82 Pb23220890 Th 82 Pb23793 Np 83 BiThorium 1.41 10 10Neptunium 2.14 10 6 209232Th228 Th