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Transcript
Chapter 32 Nuclear Physics Chapter Introduction The nucleus of an atom is incredibly small. The diameter of the nucleus is about 100,000 times smaller than the diameter of the atom. Although small, the nucleus contains more than 99.9% of the mass of an atom, because each of its constitutes (protons and neutrons) is about 1800 times more massive than an electron. We will see that many nuclei, particularly those of the heavier elements, are unstable and spontaneous break apart, or disintegrate, into other nuclei. This spontaneous disintegration is called radioactive decay, and it is accompanied by the emission of certain types of particles and high-energy photons. While these particles and photons can be very dangerous to individuals and to the environment, radioactive decay also has many beneficial uses. In this chapter, we will study the common types of radioactive decay and what they imply about nuclear structure. Basic Terms Nucleus The nucleus of an atom consists of protons and neutrons, which are collectively referred to as nucleons. Proton A proton has a positive electrical charge which, in magnitude, is equal to the charge carried by an electron (e = 1.602 177 33 × 10-19 C). Its mass mp = 1.672 6231×10-27 kg. Neutron A neutron is an electrically neutral particle whose mass is slightly larger than that of a proton, mn = 1.674 9286 × 10-27 kg. Atomic number The atomic number Z is the number of protons in the nucleus. Atomic mass number (nucleon number) The atomic mass number (or nucleon number) A is the total number of protons and neutrons in the nucleus: A = Z + N, where N is the number of neutrons. Isotopes Nuclei that contains the same number of protons, but a different number of neutrons, are called isotopes. Strong nuclear force The strong nuclear force is the force of attraction between nucleons and is one of the fundamental forces of nature. This force balances the electrostatic force of repulsion between protons and holds the nucleus together. The strong nuclear force has a very short range of action and is almost independent of electric charge. Binding energy The binding energy of a nucleus is the energy required to separate the nucleus into its constitute protons and neutrons. The binding energy is equal to (∆m)c2, where c is the speed of light in a vacuum and Delta m is the mass defect of the nucleus. Mass defect The mass defect is the amount by which the sum of the individual masses of the protons and neutrons exceeds the mass of the parent nucleus. Atomic mass unit When specifying nuclear masses, it is customary to use the atomic mass unit (u), which is one-twelfth of the mass of a 12C atom. Radioactivity The Radioactivity is a process in which unstable nuclei spontaneously decay by breaking apart or rearranging their internal structures. Naturally occurring radioactivity produces alpha rays, beta rays, and gamma rays. Alpha particle Alpha rays consist of positively charged particles, each particle being the 42He nucleus of helium. Beta particle There are two kinds of beta rays. One consists of negatively charged particles which are electrons, the other consists of positively charged particles which are positrons that have the same mass as electrons but carry a charge of +e. Gamma ray Gamma rays are high-energy photons. Transmutation If a radioactivity parent nucleus disintegrates into a daughter nucleus that has a different atomic number, one element has been converted into another element, the conversion being referred to as a transmutation. Neutrino The Neutrino is an electrically neutral particle that has near zero or zero mass. The neutrino travels near or at the speed of light and is emitted along beta particles.