Plan for Today AP Physics 2 Notes on
- Slides: 40
Plan for Today (AP Physics 2) • Notes on Alpha, Beta, and Gamma Decay • HW: Finish half-life lab for Monday
Radioactivity As the heavier atoms become more unstable, particles and photons are emitted from the nucleus and it is said to be radioactive. All elements with A > 82 are radioactive. a bb+ g Examples are: Alpha particles a b- particles (electrons) Gamma rays g b+ particles (positrons)
Decay – General Rules • When one element changes into another element, the process is called spontaneous decay or transmutation • The sum of the mass numbers, A, must be the same on both sides of the equation • The sum of the atomic numbers, Z, must be the same on both sides of the equation • Conservation of mass-energy and conservation of momentum must hold
The Alpha Particle An alpha particle a is the nucleus of a helium atom consisting of two protons and two neutrons tightly bound. Charge = +2 e- = 3. 2 x 10 -19 C Mass = 4. 001506 u Relatively low speeds ( 0. 1 c ) Not very penetrating
Radioactive Decay As discussed, when the ratio of N/Z gets very large, the nucleus becomes unstable and often particles and/or photons are emitted. Alpha decay results in the loss of two protons and two neutrons from the nucleus. X is parent atom and Y is daughter atom The energy is carried away primarily by the K. E. of the alpha particle.
Alpha Decay • When a nucleus emits an alpha particle it loses two protons and two neutrons – N decreases by 2 – Z decreases by 2 – A decreases by 4 • Symbolically – X is called the parent nucleus – Y is called the daughter nucleus
Alpha Decay – Example • Decay of 226 Ra • Half life for this decay is 1600 years • Excess mass is converted into kinetic energy • Momentum of the two particles is equal and opposite
Example 5: Write the reaction that occurs when radium-226 decays by alpha emission. From tables, we find Z and A for nuclides. The daughter atom: Z = 86, A = 222 Radium-226 decays into radon-222.
The Beta-minus Particle A beta-minus particle b- is simply an electron that has been expelled from the nucleus. - Charge = e- = -1. 6 x 10 -19 C Mass = 0. 00055 u - High speeds (near c) - Very penetrating
The Positron A beta positive particle b+ is essentially an electron with positive charge. The mass and speeds are similar. + + Charge = +e- = 1. 6 x 10 -19 C Mass = 0. 00055 u + High speeds (near c) + Very penetrating
Beta-minus Decay Beta-minus b- decay results when a neutron decays into a proton and an electron. Thus, the Z-number increases by one. X is parent atom and Y is daughter atom The energy is carried away primarily by the K. E. of the electron. -
Beta-plus Decay Beta-plus b+ decay results when a proton decays into a neutron and a positron. Thus, the Z-number decreases by one. X is parent atom and Y is daughter atom The energy is carried away primarily by the K. E. of the positron. +
Beta Decay • During beta decay, the daughter nucleus has the same number of nucleons as the parent, but the atomic number is changed by one • Symbolically
Beta Decay, cont • The emission of the electron is from the nucleus – The nucleus contains protons and neutrons – The process occurs when a neutron is transformed into a proton and an electron – Energy must be conserved
Beta Decay – Electron Energy • The energy released in the decay process should almost all go to kinetic energy of the electron (KEmax) • Experiments showed that few electrons had this amount of kinetic energy
Neutrino • To account for this “missing” energy, in 1930 Pauli proposed the existence of another particle • Enrico Fermi later named this particle the neutrino • Properties of the neutrino – Zero electrical charge – Mass much smaller than the electron, probably not zero – Spin of ½ – Very weak interaction with matter
Beta Decay – Completed • Symbolically – is the symbol for the neutrino – is the symbol for the antineutrino • To summarize, in beta decay, the following pairs of particles are emitted – An electron and an antineutrino – A positron and a neutrino
Pure - (Negatron) Emission
Beta (Negatron) Emission
Positron + Emission
Fate of the Positron
The Gamma Photon A gamma ray g has very high electromagnetic radiation carrying energy away from the nucleus. g Charge = Zero (0) g Mass = zero (0) g Speed = c (3 x 108 m/s) g Most penetrating radiation
Gamma Decay • Gamma rays are given off when an excited nucleus “falls” to a lower energy state – Similar to the process of electron “jumps” to lower energy states and giving off photons – The photons are called gamma rays, very high energy relative to light • The excited nuclear states result from “jumps” made by a proton or neutron • The excited nuclear states may be the result of violent collision or more likely of an alpha or beta emission
Gamma Decay • Nuclear transition from an excited state to a lower energy state • Nuclear excited state can be created by particle collision or as a result of nuclear decay.
The Gamma Photon A gamma ray g has very high electromagnetic radiation carrying energy away from the nucleus. g Charge = Zero (0) g Mass = zero (0) g Speed = c (3 x 108 m/s) g Most penetrating radiation
Gamma Decay – Example • Example of a decay sequence – The first decay is a beta emission – The second step is a gamma emission – The C* indicates the Carbon nucleus is in an excited state – Gamma emission doesn’t change either A or Z
Uses of Radioactivity • Carbon Dating – Beta decay of 14 C is used to date organic samples – The ratio of 14 C to 12 C is used • Smoke detectors – Ionization type smoke detectors use a radioactive source to ionize the air in a chamber – A voltage and current are maintained – When smoke enters the chamber, the current is decreased and the alarm sounds
More Uses of Radioactivity • Radon pollution – Radon is an inert, gaseous element associated with the decay of radium – It is present in uranium mines and in certain types of rocks, bricks, etc that may be used in home building – May also come from the ground itself
Natural Radioactivity • Classification of nuclei – Unstable nuclei found in nature • Give rise to natural radioactivity – Nuclei produced in the laboratory through nuclear reactions • Exhibit artificial radioactivity • Three series of natural radioactivity exist – Uranium – Actinium – Thorium
Nuclear Reactions It is possible to alter the structure of a nucleus by bombarding it with small particles. Such events are called nuclear reactions: General Reaction: x + X Y + y For example, if an alpha particle bombards a nitrogen-14 nucleus it produces a hydrogen atom and oxygen-17:
Conservation Laws For any nuclear reaction, there are three conservation laws which must be obeyed: Conservation of Charge: The total charge of a system can neither be increased nor decreased. Conservation of Nucleons: The total number of nucleons in a reaction must be unchanged. Conservation of Mass Energy: The total massenergy of a system must not change in a nuclear reaction.
Example 7: Use conservation criteria to determine the unknown element in the following nuclear reaction: Charge before = +1 + 3 = +4 Charge after = +2 + Z = +4 Z=4– 2=2 (Helium has Z = 2) Nucleons before = 1 + 7 = 8 Nucleons after = 4 + A = 8 (Thus, A = 4)
Conservation of Mass-Energy There is always mass-energy associated with any nuclear reaction. The energy released or absorbed is called the Q-value and can be found if the atomic masses are known before and after. Q is the energy released in the reaction. If Q is positive, it is exothermic. If Q is negative, it is endothermic.
Example 8: Calculate the energy released in the bombardment of lithium-7 with hydrogen-1. Substitution of these masses gives: Q = 0. 018622 u(931. 5 Me. V/u) Q =17. 3 Me. V The positive Q means the reaction is exothermic.
Summary (Decay Particles) An alpha particle a is the nucleus of a helium atom consisting of two protons and two tightly bound neutrons. A beta-minus particle b- is simply an electron that has been expelled from the nucleus. A beta positive particle b+ is essentially an electron with positive charge. The mass and speeds are similar. A gamma ray g has very high electromagnetic radiation carrying energy away from the nucleus.
Summary (Cont. ) Alpha Decay: Beta-minus Decay: Beta-plus Decay:
Summary (Cont. ) Nuclear Reaction: x + X Y + y + Q Conservation of Charge: The total charge of a system can neither be increased nor decreased. Conservation of Nucleons: The total number of nucleons in a reaction must be unchanged. Conservation of Mass Energy: The total massenergy of a system must not change in a nuclear reaction. (Q-value = energy released)
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