Chapter 10 Nuclear Physics

Copyright © Houghton Mifflin Company 10-2 Section 10.1: Symbols of the Elements An element is the fundamental atom by which all molecules are made. They are symbolized by their Latin names. Ex: CopperCuCuprum Gold Au Aurum IronFeFerrum SodiumNa Natrium

Copyright © Houghton Mifflin Company 10-3 Section 10.2: The Atomic Nucleus Atom consists of protons, neutrons, and electrons. The proton was discovered by Rutherford in The electron was discovered by J. J. Thomson in The neutron was discovered by James Chadwick in 1932.

Copyright © Houghton Mifflin Company 10-4 The nucleus of the atom consists of protons and neutrons. They are nucleons. The charge on proton = +e = 1.6x c The charge on electron = -e = -1.6x c The neutron has no net charge.

Copyright © Houghton Mifflin Company 10-5 In the atom, each particle is allowed to take on integer spin, +/-1, +/-2,+/-3,… etc. Positive for spin up, and negative for spin down. Furthermore, each of these particles is thought to consist of even smaller particles, which are partly matter and partly energy, called quarks.

Copyright © Houghton Mifflin Company 10-6 The proton, is thought to be made of two spin up quarks, each with a charge of +2/3e, and one spin down quark with a charge -1/3e. Each quark has a charge of a fraction of the charge on the proton and electron, e.

Copyright © Houghton Mifflin Company 10-7 The electron is thought to consist of three spin down quarks each of which has a charge of -1/3e. The neutron is thought to consist of two spin down quarks and one spin quark.

Copyright © Houghton Mifflin Company 10-8 Figure 10.2 Rutherford's Alpha-Scattering Experiment Alpha particles are helium nuclei

Copyright © Houghton Mifflin Company 10-9 Nuclei of different elements are constructed by adding protons and neutrons to its nucleus. The atomic number is the number of protons in the nucleus. The atomic mass is measured in atomic mass units (amu). 1 amu = (1/12) (mass of the C 12 nucleus) For lighter nuclei, this is approximately equal to the number of protons + the number of neutrons.

Copyright © Houghton Mifflin Company Each element is then symbolized according to the following system:

Copyright © Houghton Mifflin Company Two nuclei that have the same atomic number are of the same element Two nuclei that have different atomic numbers are of different elements. Two nuclei that have the same atomic number but different atomic mass, are isotopes of each other.

Copyright © Houghton Mifflin Company Three isotopes of hydrogen are Protium (1 proton, 0 neutrons), Deuterium (1 proton, 1neutron), and Tritium (1 proton, 2 neutrons)

Copyright © Houghton Mifflin Company Figure 10.5a: A Mass Spectrometer

Copyright © Houghton Mifflin Company Types of forces in nature: a. The electromagnetic force: acts on magnets and electric charges. It has infinite range. Can be attractive or repulsive. b. The gravitational force: acts on all matter. It is infinite in range, and is always attractive.

Copyright © Houghton Mifflin Company c. The strong force: Acts on nucleons. It is very strong, but its range is only of the size of the nucleus ( m). d. The weak force: Acts on all particles, but in particular the neutron. It is involved in beta decay, and its range is less than that of the strong force.

Copyright © Houghton Mifflin Company Relative strengths of forces outside the nucleus: Strongest 1. Electromagnetic force 2. Gravity 3. Strong force (nonexistent) Weakest4. Weak force (nonexistent)

Copyright © Houghton Mifflin Company Relative strengths of forces within the nucleus. Strongest1. Strong force 2. Weak force 3. Electromagnetic force Weakest4. Gravity

Copyright © Houghton Mifflin Company Section 10.3: Radioactivity and Half-Life Radioactivity is the spontaneous decay of a parent nucleus into one or more daughter nuclei with the emission of a by-product nucleus. Example: Parent daughter By-product

Copyright © Houghton Mifflin Company In alpha decay, the by product is a helium nucleus. This type of decay is produced by a nucleus that is too big for its nucleons. The atomic mass reduces by 4, and the atomic number by 2.

Copyright © Houghton Mifflin Company In beta decay, the by-product is an electron. In this type of decay, a neutron in the nucleus spontaneously disintegrates into a proton and an electron. This is produced by a nucleus that has too many neutrons relative to protons. The atomic number increases by 1, but the atomic mass stays the same. Example:

Copyright © Houghton Mifflin Company In gamma decay the by-product is a photon (gamma particle). This is produced by a nucleus that is just too energetic. As the nucleus releases its excess energy, a photon of the appropriate wavelength is produced. The atomic number and atomic mass do not change. Example:

Copyright © Houghton Mifflin Company In positron emission, the by-product is a positron. A positron is an electron with a positive charge, +e. It the antiparticle of the electron. This type of decay is produced when a proton in the nucleus of the parent spontaneously decays into a neutron, with the emission of a positron. The atomic number increases by one and the atomic mass stays the same. Example:

Figure 10.7 The Three Components of Radiation from Heavy Radionuclides Radionuclides are the parents of decay

Figure 10.8 The Decay of Uranium-238 to Lead 206

Figure 10.9 A Plot of Number of Neutrons (N) Versus Number of Protons (Z) for the Nuclides

Copyright © Houghton Mifflin Company The half-life is the time taken for the number of parent nuclei to decrease to half of its initial value. the time taken for the number of daughter nuclei to equal the number of parent nuclei.

Figure The Decay Curve for Any Radionuclide The decay rate = = (constant) (#of parent nuclei)

Copyright © Houghton Mifflin Company Section 10.4: Nuclear Reactions Nuclear Reactions involve two or more nuclei of different elements to produce other elements. The first nuclear reaction was discovered by E. Rutherford.

Copyright © Houghton Mifflin Company The transuranium elements are elements with atomic number greater than that of uranium (92). They are stable, but must be synthesized by bombardment by alpha particles, or protons.

Copyright © Houghton Mifflin Company Section 10.5: Nuclear Fission Fission is the process in which a large nucleus is “split” into two or more intermediate-size nuclei, with the emission of neutrons, and the conversion of mass into energy. (SWT p. 242.)

Copyright © Houghton Mifflin Company A nuclear chain reaction is a triggered chain of events leading to an uncontrolled release of neutrons and a large amount of energy.

Figure Subcritical and Supercritical Masses When the flux of neutrons is just enough to start the chain reaction, the system is said to be at critical mass.

Copyright © Houghton Mifflin Company A nuclear reactor is a controlled nuclear chain reaction.

Copyright © Houghton Mifflin Company Section 10.6: Nuclear Fusion Nuclear fusion is the process in which smaller combine to form larger ones, with the release of energy. (SWT p. 246) The deuterium-tritium reaction:

Copyright © Houghton Mifflin Company The binding energy = the amount of energy needed to pull the nucleus apart. = (mass lost)c 2 Stable nuclei tend to be toward higher binding energy.

Copyright © Houghton Mifflin Company Figure The Relative Stability of Nuclei