1. Nuclear Physics and Radioactivity AP Physics Chapter 30

2. Nuclear Physics and Radioactivity 30.1 Structure and properties of the Nucleus

3. 30.1 Structure and Property of the Nucleus Nucleus is composed of two particles Proton – positive charge Neutron – neutral Together they are called nucleons 30.1

4. 30.1 Structure and Property of the Nucleus To present this information we use the symbol form Z – number of protons (atomic number) A – atomic mass (not average) The number of Neutrons (N) is Sometime written without the Z, as that information is redundant 30.1

5. 30.1 Structure and Property of the Nucleus Isotopes – the same element, but different numbers of neutrons or mass number These isotopes would be Not all isotopes are equally common C-12 is 98.9% C-13 is 1.1% Called the Natural Abundance 30.1

6. 30.1 Structure and Property of the Nucleus Masses of atoms are determined using a mass spectrometer The mass is given in unified atomic mass units (u) Carbon – 12 is given the mass of 12.000000u 30.1

7. 30.1 Structure and Property of the Nucleus Masses are often given in electron volts This is derived from Einstein’s equation Using the mass of a proton And placing into Einstein’s equation 30.1

8. Nuclear Physics and Radioactivity 30.2 Binding Energy and Nuclear Forces

9. The total mass of a stable nucleus is always less than the sum of the masses of its separate protons and neutrons The difference is mass is the binding energy So for example the mass of Helium 4 is 4.002603u 30.2

10. 30.2 Binding Energy and Nuclear Forces This is the energy needed to break apart the nucleus To be a stable nucleus, the mass must be less than the parts The binding energy per nucleon is the total binding energy divided by A 30.2

11. 30.2 Binding Energy and Nuclear Forces Strong Nuclear Force – attractive force between all nucleons Drops to essentially zero if the distance between the nucleons is greater than 10 -15 m Occur by the exchange of a particle called a meson Weak Nuclear Force – very weak, show in types of radioactive decay 30.2

12. 30.2 Binding Energy and Nuclear Forces 30.2

13. Nuclear Physics and Radioactivity 30.3 Radioactivity

14. Henri Becquerel (1896) uranium darkens photographic plates Radioactive decay – unstable nuclei fall apart with the emission of radiation 30.3

15. 30.3 Radioactivity Rays can be classified into three catagories 1.Alpha (  ) – barely penetrates paper 2.Beta (  ) – penetrates up to 3mm of aluminium 3.Gamma (  ) – penetrates several cm of lead 30.3

16. Nuclear Physics and Radioactivity 30.4 Alpha Decay

17. An alpha particle is a helium nucleus When an atom undergoes alpha decay it loses 2 protons and 2 Neutrons Reactions are written 30.4

18. 30.4 Alpha Decay Parent nucleus – the original Daughter nucleus – nucleus of new atom Transmutation – change of one element into another Basic form for alpha decay is The alpha particle is ejected because it has a very large binding energy and is difficult to break apart 30.4

19. Nuclear Physics and Radioactivity 30.5 Beta Decay

20. Beta particle (  - ) – electron Also produces an antineutrino Antineutrino – has no charge and almost no mass The result of the decay is that a neutron becomes a proton 30.5

21. 30.5 Beta Decay For Carbon – 14 decay Or the general form which would be The electron does not come form the electron cloud, but from the decay of a neutron into a proton It is identical to any other electron 30.5

22. 30.5 Beta Decay Unstable isotopes with too few neutrons compared to their number of protons decay by emitting a positron Positron – same mass as an electron, positive charge This is an example of an antiparticle (antimatter) The decay pattern is 30.5

23. Nuclear Physics and Radioactivity 30.6 Gamma Decay

24. Gamma Ray – photon of EMR A nucleus can be in an excited state like an electron When it drops down it emits a  ray Much larger than for electrons For a given decay the  ray has the same energy 30.6

25. 30.6 Gamma Decay The nucleus may enter an excited state by Violent collision with another particle The particle after a decay is often in an excited state The equation can be written 30.6

26. Nuclear Physics and Radioactivity 30.7 Conservation of Nucleon Number

27. In radioactive decay all conservation laws are true 1.Energy 2.Linear Momentum 3.Angular Momentum 4.Electric Charge Law of Conservation of Nucleon Number – the number of nucleons (protons or neutrons) remains the same, although they may change type 30.7

28. Nuclear Physics and Radioactivity 30.8 Half-Life and Decay Rate

29. 30.8 Half-Life and Rate of Decay Individual radioactive nuclei in a random process Based on probability we can approximate the number of nuclei in a sample that will decay Where is the decay constant 30.8

30. 30.8 Half-Life and Rate of Decay The greater the decay constant, the greater the rate of decay The more radioactive it is The equation can be solved for N using calculus and we get Where N 0 is the initial number of nuclei present N is the number remaining after time t The number of decays per unit time is called the activity or rate of decay 30.8

31. 30.8 Half-Life and Rate of Decay Half-Life – the time it takes for half the original amount of parent isotope to decay (T ½ ) 30.8

32. Nuclear Physics and Radioactivity 30.9 Calculations Involving Decay Rates and Half-Life

33. 30.9 Calc. Involving Decay Rates and Half-Life Carbon-14 has a half-life of 5730 yr. What is the activity of a sample that contains 10 22 nuclei? 1 decay/s is called a becquerel (Bq) 30.9

34. 30.9 Calc. Involving Decay Rates and Half-Life 1.49mg of Nitrogen-13 has a half life of 600s. a.How many nuclei are present? b.What is the initial activity? 30.9

35. 30.9 Calc. Involving Decay Rates and Half-Life 1.49mg of Nitrogen-13 has a half life of 600s. c.What is the activity after 3600s? 6 half lives If this had not been a perfect half life we would have used 30.9

36. Nuclear Physics and Radioactivity 30.10 Decay Series

37. Decay Series – a successive set of decay 30.10