Slide 1 / 33 Slide 2 / 33 The Nucleus Proton: Nuclear Physics The charge on a proton is +1.6x10 -19 C. and Nuclear The mass of a proton is 1.6726x10 -27 kg. Reactions Neutron: The neutron is neutral. The mass of a neutron is 1.6749x10 -27 kg. Slide 3 / 33 Slide 4 / 33 The Nucleus The Nucleus Proton and neutrons are collectively called nucleons. Nuclei with the same number of protons are the same element but if they have different numbers of neutrons they The number of protons in a nucleus is called the atomic are called isotopes. number and it is designated by the letter Z. For many elements, there are a few different isotopes that The number of nucleons in a nucleus is called the atomic occur naturally. mass number and it is designated by the letter A. Natural abundance is the percentage of a certain element The neutron number, N, is given by N = A - Z. that occurs as a certain isotope in nature. To specify a nuclide we use the following form: Many isotopes that do not occur in nature can be created in a laboratory with nuclear reactions. where X is the chemical symbol for the element. Slide 5 / 33 Slide 6 / 33 The Nucleus The Nucleus Nuclear masses are specified in unified atomic mass The approximate size of nuclei was originally determined units (u). On this scale a neutral carbon atom with 6 by Rutherford. We say approximate because of wave- protons and 6 neutrons has a mass of 12.000000 u. particle duality. The size of nuclei is a little fuzzy. 1 u = 1.6605 x 10 -27 kg = 931.5 MeV/c 2 We can get a rough size of a nucleus by scattering high speed electrons off of it. Rest Mass The approximate radius of a nucleus is given by: MeV/c 2 Object kg u r # (1.2 x 10 -15 m)(A 1/3 ) 9.1094 x 10 -31 0.00054858 0.51100 Electron 1.67262 x 10 -27 1.007276 Proton 938.27 Where A is the number of nucleons (not the area). Hydrogen Atom 1.67353 x 10 -27 1.007825 938.78 1.67493 x 10 -27 1.008665 Neutron 939.57
Slide 7 / 33 Slide 8 / 33 Binding Energy and Nuclear Forces Binding Energy and Nuclear Forces The total mass of a nucleus is always less than the sum of the masses of its protons and neutrons. Where has all this mass gone? It has become energy! (Energy, such as radiation or kinetic energy.) The difference between the total mass of the nucleons and the mass of the nucleus is called the total binding energy of the nucleus. In energy units, the total binding energy is given by: E = Δmc 2 This binding energy is the amount of energy needed to be put into the nucleus in order to break it apart into protons Figure by MIT OpenCourseWare. From Meyerhof. and neutrons. Slide 9 / 33 Slide 10 / 33 Binding Energy and Nuclear Forces Radioactivity The force that binds the nucleons together is called the strong nuclear force. · Radioactivity is the spontaneous emission of This is a very strong but a close range force. It is nearly radiation by an atom. zero if the distance between nucleons is more that 10 -15 m. · It was first observed by Henri Becquerel. The Coulomb (Electric) Force is a long range force. Since protons repel at larger distances, neutrons are needed in · Marie and Pierre Curie also studied it. nuclei with a large number of protons. There is another nuclear force called the weak nuclear force which governs radioactive decay. Slide 11 / 33 Slide 12 / 33 Radioactivity Radioactivity · Three types of radiation were discovered by Ernest · a particles accelerate with the E-field, so they are positive Rutherford: · b particles accelerate against the E-filed, so they are negative · a particles · b particles · g rays are unaffected by the E-field, so they have no charge · g rays
Slide 13 / 33 Slide 14 / 33 Alpha Decay Radioactivity Alpha decay happens when a nucleus emits an alpha particle (helium with two neutrons). This decay is written · a particles turned out to be the same as Helium as: nuclei, having two protons and two neutrons · b particles turned out to be electrons, the same particle as found in the cathode ray tube experiments. · g rays turned out to be electromagnetic radiation, like light but with much greater energy (higher frequency) Slide 15 / 33 Slide 16 / 33 Beta Decay Gamma Decay Beta decay happens when a nucleus emits a beta particle Gamma decay happens when a nucleus in an excited state (an electron or positron). This decay is written as: emits a Gamma particle (a high energy photon). This decay is written as: Slide 17 / 33 Slide 18 / 33 Conservation of Nucleon Number Half Life and Rate of Decay In addition to the other conservation laws, there is the law A macroscopic sample of any radioactive substance of conservation of nucleon number. consists of a great number of nuclei. These nuclei do not decay at one time. This law states that the total number of nucleons (A) remains constant in any process. However, one type can Actually, the decay is random and the decay of one nuclei change into the other type. has nothing to do with the decay of any other nuclei. Then number of decays during a short time period is proportional to the number of nuclei as well as the time period. # N = - # N # t Where # is the decay constant.
Slide 19 / 33 Slide 20 / 33 Half Life and Rate of Decay Nuclear Reactions and Transmutation of Elements The rate of decay is usually given by its half life rather than its decay constant. A nuclear reaction takes place when a nucleus (or particle) collides with another nucleus (or particle). A half life of an isotope is defined as the amount of time it takes for half of the original amount of the isotope to decay. This process is called transmutation if the original nucleus is transformed into a new nucleus. A half life is given by: For Example: Slide 21 / 33 Slide 22 / 33 Nuclear Reactions and Nuclear Reactions and Transmutation of Elements Transmutation of Elements Since energy is conserved, Q is equal to the change in kinetic energy: Energy and momentum must be conserved in nuclear reactions. Q = KE b + KE Y - KE a - KE X General Reaction: If Q is positive, the products have more kinetic energy (energy a + X Y + b is released in the reaction). The reaction is exothermic, and will occur no matter how small the initial kinetic energy is. The reaction energy, or Q-value, is the sum of the initial masses minus the sum of the final masses, multiplied by c 2 : If Q is negative, the reactants have more kinetic energy (energy is absorbed in the reaction). The reaction is Q = (M a + M X - M b - M Y ) c 2 endothermic and there is a minimum kinetic energy that must be available before the reaction can occur. Threshold energy is the minimum energy necessary for the reaction to occur. Slide 23 / 33 Slide 24 / 33 Nuclear Reactions and Nuclear Fission and Nuclear Reactors Transmutation of Elements After absorbing a neutron, a U-235 nucleus will split into two parts. Neutrons are very effective in This can be visualized as a kind of nuclear reactions. liquid drop. They have no charge, so they As the nucleus splits, neutrons are are not repelled by the released. nucleus. A typical reaction is: Scientists were able to create transuranic elements by neutron bombardment. Although others may occur.
Slide 25 / 33 Slide 26 / 33 Nuclear Fission and Nuclear Reactors Nuclear Fission and Nuclear Reactors The energy release in a The chain reaction needs to be self sustaining in fission reaction is quite order to create a nuclear reactor. The reaction large. The smaller nuclei must continue indefinitely in a controlled are stable with fewer manner. neutrons, so multiple neutrons emerge from each fission. The neutrons can be used to induce fission in surrounding nuclei, causing a chain reaction. Slide 27 / 33 Slide 28 / 33 Nuclear Fission and Nuclear Reactors Nuclear Fission and Nuclear Reactors Control rods, usually made from cadmium or boron, Neutrons that escape absorb neutrons and are used for fine control of the from the uranium do not reaction. They keep the reaction just barely critical. contribute to fission. There is a critical mass below which a chain reaction will not occur because too many neutrons escape. Slide 29 / 33 Slide 30 / 33 Nuclear Fusion Nuclear Fission and Nuclear Reactors The lightest nuclei can fuse to form heavier nuclei, releasing Atomic bombs use fission as well. The core is deliberately energy in the process. An example is the sequence of fusion designed to undergo a massive uncontrolled chain reaction. processes that change hydrogen into helium in the Sun, as This releases huge amounts of energy. shown below.
Slide 31 / 33 Slide 32 / 33 Nuclear Fusion Nuclear Fusion The net effect is to transform four protons into a helium nucleus plus two positrons, two neutrinos and two gamma There are three fusion reactions that are being considered rays. for power reactors: More massive stars can fuse elements as heavy as iron in their cores. These reactions use relatively common fuels (deuterium or tritium) and release much more energy than fission does. Slide 33 / 33 Nuclear Fusion A successful fusion reactor has not yet been achieved. Fusion (or thermonuclear) bombs have been built.
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