Plan for Today (AP Physics 2)

3. Plan for Today (AP Physics 2) • Notes on Alpha, Beta, and Gamma Decay • HW: Finish half-life lab for Monday

4. a b- b+ g 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. Examples are: Alpha particles a b- particles (electrons) Gamma rays g b+ particles (positrons)

5. 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

6. Charge = +2e- = 3.2 x 10-19 C Mass = 4.001506 u Relatively low speeds ( 0.1c ) Not very penetrating The Alpha Particle An alpha particleais the nucleus of a helium atom consisting of two protons and two neutrons tightly bound.

7. Alpha decay results in the loss of two protons and two neutrons from the nucleus. The energy is carried away primarily by the K.E. of the alpha particle. 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. X is parent atom and Yis daughter atom

8. 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

9. 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

10. 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.

11. - Charge = e- = -1.6 x 10-19 C Mass = 0.00055 u - High speeds (near c) - - Very penetrating The Beta-minus Particle A beta-minus particleb-is simply an electron that has been expelled from the nucleus.

12. + Charge = +e- = 1.6 x 10-19 C Mass = 0.00055 u + High speeds (near c) + + Very penetrating The Positron A beta positive particleb+is essentially an electron with positive charge. The mass and speeds are similar.

13. - The energy is carried away primarily by the K.E. of the electron. 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 Yis daughter atom

14. The energy is carried away primarily by the K.E. of the positron. + 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 Yis daughter atom

15. 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

16. 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

17. 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

18. 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

19. 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

20. Pure - (Negatron) Emission

21. Beta (Negatron) Emission

22. Positron +Emission

23. Fate of the Positron

24. g g g g Charge = Zero (0) Mass = zero (0) Speed = c (3 x 108 m/s) Most penetrating radiation The Gamma Photon A gamma rayghas very high electromagnetic radiation carrying energy away from the nucleus.

25. 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

26. 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.

27. g g g g Charge = Zero (0) Mass = zero (0) Speed = c (3 x 108 m/s) Most penetrating radiation The Gamma Photon A gamma rayghas very high electromagnetic radiation carrying energy away from the nucleus.

28. 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

30. Uses of Radioactivity • Carbon Dating • Beta decay of 14C is used to date organic samples • The ratio of 14C to 12C 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

31. 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

32. 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

33. General Reaction: x + X  Y + y Nuclear Reactions It is possible to alter the structure of a nucleus by bombarding it with small particles. Such events are called nuclear reactions: For example, if an alpha particle bombards a nitrogen-14 nucleus it produces a hydrogen atom and oxygen-17:

34. Conservation of Charge: The total charge of a system can neither be increased nor decreased. Conservation Laws For any nuclear reaction, there are three conservation laws which must be obeyed: Conservation of Nucleons: The total number of nucleons in a reaction must be unchanged. Conservation of Mass Energy: The total mass-energy of a system must not change in a nuclear reaction.

35. 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)

36. 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.

37. Example 8:Calculate the energy released in the bombardment of lithium-7 with hydrogen-1. Substitution of these masses gives: Q =17.3 MeV Q = 0.018622 u(931.5 MeV/u) The positive Q means the reaction is exothermic.

38. Summary (Decay Particles) An alpha particleais the nucleus of a helium atom consisting of two protons and two tightly bound neutrons. A beta-minus particleb-is simply an electron that has been expelled from the nucleus. A beta positive particleb+is essentially an electron with positive charge. The mass and speeds are similar. A gamma rayghas very high electromagnetic radiation carrying energy away from the nucleus.

39. Alpha Decay: Beta-minus Decay: Beta-plus Decay: Summary (Cont.)

40. 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 mass-energy of a system must not change in a nuclear reaction. (Q-value = energy released) Summary (Cont.)