1. Nuclear Physics Physics 12

2. Protons, Neutrons and Electrons  The atom is composed of three subatomic particles: Particle Charge (in C) Symbol Mass (in kg) Electron-1.602x10 -19 e-e- 9.109 56x10 -31 Proton1.602x10 -19 p+p+ 1.672 614x10 -27 Neutron0n0n0 1.674 920x10 -27

3. Atomic Nucleus  Atom described using: X – atomic symbol A – atomic mass number (nucleon number) Z – atomic number  Number of protons and electrons = Z  Number of neutrons = A - Z

4. Strong Nuclear Force  The electrostatic forces inside a nucleus would rip it apart if there was not another force  By the end of the 1930’s physicists had determined that nucleons attract each other  This is the strongest force in the known universe

5. Stability and the Nucleus  Although the Strong Nuclear Force is strong enough to hold a small nucleus together, as the size of the nucleus becomes larger, the electrostatic forces begin to become more important  As a result, if we consider various nuclei based on their Atomic Number and Neutron Number we get the following result:

6. Stability and the Nucleus Each black dot represents a stable nucleus, with the number of neutrons shown on the vertical axis and the number of protons on the horizontal axis

7. Nuclides and Isotopes  Nuclides are different combinations of nucleons  Isotopes occur when an element (specific Atomic Number) has different numbers of neutrons (different Atomic Mass Numbers)  For example, there are three common isotopes of hydrogen:

8. Nuclides and Isotopes

9. Nuclear Binding Energy  It takes 13.6 eV to separate an electron from a hydrogen atom  However, it takes more than 20 MeV to separate a neutron from a helium- 4 atom  The energy to separate all the nucleons in a nucleus is called the binding energy

10. Larger nuclei are held together a little less tightly than those in the middle of the Periodic Table

11. Mass Defect  If you were able to apply the 20 MeV required to separate a neutron from helium-4, what would happen to it?  This is dealt with using Einstein’s Special Theory of Relativity and the fact that mass and energy are equivalent  E = mc 2  The mass of helium-4 (2p, 2n) is smaller than that of helium-3 (2p, 1n) and a neutron  The energy that was added to remove the neutron was converted into mass  The difference between the mass of a nuclide and the sum of the masses of its constituents is called mass defect

12. Atomic Mass Unit (u)  When dealing with nucleons, it is often more useful to deal with mass in unified atomic mass units (u) instead of kilograms Particle Mass (in kg) Mass (in u) Electron9.109 56x10 -31 0.000 549 Proton1.672 614x10 -27 1.007 276 Neutron1.674 920x10 -27 1.008 665

13. Binding Energy Example  Determine the binding energy in electron volts and joules for an iron- 56 nucleus given that the nuclear mass is 55.9206u

14. Binding Energy Example  Determine the binding energy in electron volts and joules for an iron- 56 nucleus given that the nuclear mass is 55.9206u

15. Binding Energy Example  We would expect the binding energy per nucleon to be about 8MeV:

16. Radioactive Isotopes  In discussing the nucleus, we looked at a plot of stable nuclei  It is also possible to have a nucleus that is not stable (meaning that it will fall apart)  An unstable nucleus will decay following a few very specific processes  We call this decay radioactivity and classify it into one of three types

17. Radioactive Isotopes

18. Alpha Decay  An alpha particle ( α ) is a helium nucleus (two protons and two neutrons)  A nucleus that emits an alpha particle will lose the two protons and two neutrons  Large nuclei will emit alpha particles  They do not penetrate matter well and a sheet of paper or 5cm of air will stop most  They can free electrons from atoms, meaning they are a form of ionizing radiation

19. Alpha Decay

20. Beta Decay  When a nucleus emits a beta particle ( β ), it appears to lose an electron or positron from within the nucleus  There are two types of beta decay (β - and β + )  Beta particles can penetrate matter to a greater extent than alpha particles; they can penetrate about 0.1mm of lead or 10m of air  They are also a form of ionizing radiation but less damaging than alpha particles

21. Beta Decay (β - )  In this type of beta decay, a neutron becomes a proton and a β - particle (high energy electron) is emitted  In addition an antineutrino ( ) is emitted (antimatter) along with the beta minus particle  The nucleus’s atomic number increases by one while the atomic mass number remains the same

22. Beta Decay (β - )

23. Beta Decay (β + )  In this type of beta decay, a proton becomes a neutron and a β + particle (high energy positron or antielectron) is emitted  In addition a neutrino ( ) is emitted along with the beta plus particle  The nucleus’s atomic number decreases by one while the atomic mass number remains the same

24. Beta Decay (β + )

25. Gamma Decay ( γ )  When a nucleus goes through alpha or beta decay, the daughter nucleus is often left in an excited state  In order to reduce the energy of the nucleus, it will go through gamma decay (high energy photon) to return to the ground state  Gamma radiation can pass through 10cm of lead or 2km of air  It is the most damaging of all due to the energy of the gamma particle

26. Gamma Decay

27. Decay Series  When a large nucleus decays by alpha and beta radiation, the daughter nucleus will be more stable than the original nucleus  However, the daughter nucleus may still be unstable and will itself go through alpha or beta radiation  This leads to a decay series

29. Rate of Radioactive Decay  It is impossible to predict when a specific nucleus will decay  You can describe the probability of decay  The concept of half life is used with radioactive decay: the time required for half of the sample to decay  Using the half life equation, it is possible to determine how much of a sample would remain after a given period of time

30. Half Life  N  sample remaining  N 0  original sample  Δt  elapsed time  T  half life

31. Half Life