Nuclear Physics

1. Nuclear Physics • Most of the great success of the technology of the world around us results from our ability to manipulate atoms and their electrons. • An atom has another important component, the nucleus. • What can we accomplish by manipulating nuclei?

3. Nuclear Structure Slide 30-12

4. Structure and Properties of the Nucleus The Nucleus is the (tiny) central part of an atom. The Nucleus is made of protons and neutrons Proton has positive charge: Neutron is electrically neutral:

5. Structure and Properties of the Nucleus Neutrons and protons are collectively called nucleons. The different nuclei are referred to as nuclides. Number of protons: atomic number, Z Number of nucleons: atomic mass number, A Neutron number: N = A - Z

6. Structure and Properties of the Nucleus A and Z are sufficient to specify a nuclide. Nuclides are symbolized as follows: X is the chemical symbol for the element; it contains the same information as Z but in a more easily recognizable form.

7. Structure and Properties of the Nucleus Nuclei with the same Z – so they are the same element – but different N are called isotopes. For many elements, several different isotopes exist in nature. Different isotopes of the same element have the same atomic number but different mass numbers.

9. Checking Understanding • How many neutrons are in the following isotope? (The isotope may be uncommon or unstable.) • 8 • 7 • 6 • 5 • 4 Slide 30-13

10. Answer • How many neutrons are in the following isotope? (The isotope may be uncommon or unstable.) • 8 • 7 • 6 • 5 • 4 Slide 30-14

11. Structure and Properties of the Nucleus Masses of atoms are measured with reference to the carbon-12 atom, which is assigned a mass of exactly 12u. A u is a unified atomic mass unit.

12. Structure and Properties of the Nucleus From the following table, you can see that the electron is considerably less massive than a nucleon. E= mc2 or m = E/c2

13. Binding Energy and Nuclear Forces The total mass of a stable nucleus is always less than the sum of the masses of its separate protons and neutrons. Where has the mass gone?

14. Binding Energy and Nuclear Forces It has become energy, such as radiation or kinetic energy, released during the formation of the nucleus. This difference between the total mass of the constituents and the mass of the nucleus is called the total binding energy of the nucleus.

15. Binding Energy of a Helium Nucleus Slide 30-27

16. Binding Energy Slide 30-26

17. Binding Energy Slide 30-26

18. Binding Energy and Nuclear Forces To compare how tightly bound different nuclei are, we divide the binding energy by A to get the binding energy per nucleon.

19. Curve of Binding Energy • Light nuclei can become more stable through fusion. • Heavy nuclei can become more stable through fission. • All nuclei larger than a certain size spontaneously fission. Slide 30-28

20. Binding Energy and Nuclear Forces The force that binds the nucleons together is called the strong nuclear force. It is a very strong, but short-range, force. It is essentially zero if the nucleons are more than about 10-15 m apart. The Coulomb force is long-range; this is why extra neutrons are needed for stability in high-Z nuclei.

21. Nuclear Forces Slide 30-29

22. Binding Energy and Nuclear Forces The higher the binding energy per nucleon, the more stable the nucleus. More massive nuclei require extra neutrons to overcome the Coulomb repulsion of the protons in order to be stable.

23. Binding Energy and Nuclear Forces Nuclei that are unstable decay; many such decays are governed by another force called the weak nuclear force.

24. Radioactivity Towards the end of the 19th century, minerals were found that would darken a photographic plate even in the absence of light. This phenomenon is now called radioactivity. Marie and Pierre Curie isolated two new elements that were highly radioactive; they are now called polonium and radium.

25. Radioactivity Alpha and beta rays are bent in opposite directions in a magnetic field, while gamma rays are not bent at all.

26. Alpha Decay Example of alpha decay: Radium-226 will alpha-decay to radon-222

27. Beta Decay Slide 30-34

29. Half Life • Radioactive decay is random. We can only know what the probability of decay is. • Different nuclei have different probabilities of decay. • Half life is the time you would have to wait for half the nuclei to decay.

30. Raise your Hands Put your hands down if your birthday occurs • In the first 6 months (Jan-June) of the year • July, August, September • Between Oct. 1 and Nov. 15 • After Dec. 8 • Nov.15-30 • HAPPY BIRTHDAY!

31. Half Life Slide 30-44

32. Half Life N0 is the initial number of nuclei t1/2 is the half life If t = t1/2 N, the number of nuclei left, will be ½ N0 If t = 2t1/2 N, the number of nuclei left, will be ¼ N0 Slide 30-44

33. Example Problem: Decay Times The Chernobyl nuclear reactor accident in the Soviet Union in 1986 released a large plume of radioactive isotopes into the atmosphere. Of particular health concern was the short-lived (half life: 8.0 days) isotope 131I, which, when ingested, is concentrated in and damages the thyroid gland. This isotope was deposited on plants that were eaten by cows, which then gave milk with dangerous levels of 131I. This milk couldn’t be used for drinking, but it could be used to make cheese, which can be stored until radiation levels have decreased. How long would a sample of cheese need to be stored until the number of radioactive atoms decreased to 3% of the initial value? Slide 30-46