Quantum Physics

952 Chapter 29 Nuclear PhysicsHere, we see that when 12 N decays into 12 C, a particle is produced which is identicalto the electron except that it has a positive charge of e. This particle is calleda positron. Because it is like the electron in all respects except charge, the positronis said to be the antiparticle of the electron. We will discuss antiparticles further inChapter 30; for now, it suffices to say that, in beta decay, an electron and an antineutrinoare emitted or a positron and a neutrino are emitted.Unlike beta decay, which results in a daughter particle with a variety of possiblekinetic energies, alpha decays come in discrete amounts, as seen in Figure 29.8b.This is because the two daughter particles have momenta with equal magnitudeand opposite direction and are each composed of a fixed number of nucleons.ENRICO FERMI, Italian Physicist(1901–1954)Fermi was awarded the Nobel Prize in1938 for producing the transuranicelements by neutron irradiation and forhis discovery of nuclear reactions boughtabout by slow neutrons. He made manyother outstanding contributions to physics,including his theory of beta decay, thefree-electron theory of metals, and thedevelopment of the world’s first fissionreactor in 1942. Fermi was truly a giftedtheoretical and experimental physicist. Hewas also well known for his ability topresent physics in a clear and excitingmanner. “Whatever Nature has in store formankind, unpleasant as it may be, menmust accept, for ignorance is never betterthan knowledge.”APPLICATIONCarbon Dating of theDead Sea ScrollsNational Accelerator LaboratoryGamma DecayVery often a nucleus that undergoes radioactive decay is left in an excited energystate. The nucleus can then undergo a second decay to a lower energy state—perhaps even to the ground state—by emitting one or more high-energy photons.The process is similar to the emission of light by an atom. An atom emits radiationto release some extra energy when an electron “jumps” from a state of high energyto a state of lower energy. Likewise, the nucleus uses essentially the same methodto release any extra energy it may have following a decay or some other nuclearevent. In nuclear de-excitation, the “jumps” that release energy are made by protonsor neutrons in the nucleus as they move from a higher energy level to a lowerlevel. The photons emitted in the process are called gamma rays, which have veryhigh energy relative to the energy of visible light.A nucleus may reach an excited state as the result of a violent collision withanother particle. However, it’s more common for a nucleus to be in an excitedstate as a result of alpha or beta decay. The following sequence of events typifiesthe gamma decay processes:125 B : 12 6C * e [29.17]126 C * 12: 6 C [29.18]Equation 29.17 represents a beta decay in which 12 B decays to 12 C * , where theasterisk indicates that the carbon nucleus is left in an excited state following thedecay. The excited carbon nucleus then decays to the ground state by emitting agamma ray, as indicated by Equation 29.18. Note that gamma emission doesn’tresult in any change in either Z or A.Practical Uses of RadioactivityCarbon Dating The beta decay of 14 C given by Equation 29.15 is commonly usedto date organic samples. Cosmic rays (high-energy particles from outer space) inthe upper atmosphere cause nuclear reactions that create 14 C from 14 N. In fact,the ratio of 14 C to 12 C (by numbers of nuclei) in the carbon dioxide molecules ofour atmosphere has a constant value of about 1.3 10 12 , as determined by measuringcarbon ratios in tree rings. All living organisms have the same ratio of 14 Cto 12 C because they continuously exchange carbon dioxide with their surroundings.When an organism dies, however, it no longer absorbs 14 C from the atmosphere,so the ratio of 14 C to 12 C decreases as the result of the beta decay of 14 C. It’stherefore possible to determine the age of a material by measuring its activity perunit mass as a result of the decay of 14 C. Through carbon dating, samples of wood,charcoal, bone, and shell have been identified as having lived from 1 000 to 25 000years ago. This knowledge has helped researchers reconstruct the history of livingorganism—including human—during that time span.A particularly interesting example is the dating of the Dead Sea Scrolls. Thisgroup of manuscripts was first discovered by a young Bedouin boy in a cave atQumran near the Dead Sea in 1947. Translation showed the manuscripts to bereligious documents, including most of the books of the Old Testament. Becauseof their historical and religious significance, scholars wanted to know their age.Carbon dating applied to fragments of the scrolls and to the material in which