CHAPTER 30

1. CHAPTER 30 Nuclear Physics (Introduction, Radioactivity and Decay)

2. RUTHERFORD’S EXPERIMENT Send alpha particles through a target foil and detect the deflection. Most got through with very small deflection as expected by Rutherford, but some had large deflection and bounced right back.

3. WHAT RUTHERFORD THOUGHT… In those days, people thought the mass of an atom is spread evenly over its entire volume, so nothing in an atom could cause the large deflections. “It was almost as incredible as if you had fired a bullet at a piece of tissue paper and it came back and hit you.” Clearly something was wrong with the old picture of atoms.

4. BOLD PROPOSAL: NUCLEUS! The mass of an atom concentrates mostly in a very small region at the center. Nucleus is small but massive, so they can cause very big deflection. An atom is “mostly empty”.

5. How small is the nucleus Diameter of an atom: ~ 10-10m Diameter of a nucleus: ~ 1fm = 10-15m A nucleus is smaller by a factor of 105. Analogy: The length of an American football field: ~100m If the football field is the atom, a nucleus would have a diameter of ~100m/105 = 1mm Imagine the mass of the entire football stadium is located in this tiny region, the density must be very high.

6. INSIDE A NUCLEUS Protons and neutrons bind together. Proton: Charge +e Neutron: Charge 0 So this neutron walks into a bar, orders a pint of lager and begins to open his wallet when the barman says, "For you, no charge!".

7. ELECTRONS FLY AROUND THE NUCLEUS The size of the nucleus is greatly exaggerated in the picture.

8. SYMBOLISM • X is the chemical symbol of the element • Example: • Mass number is 56 • Atomic number is 26 • Contains 26 protons • Contains 30 (56-26) neutrons • The Z may be omitted since the element can be used to determine Z

9. EXAMPLES OF NUCLEI Notation: Z = Atomic number = Proton number N = Neutron number A = Mass number = Z + N

10. CLASSIFICATION OF ATOMS The proton number Z determines the number of electrons, which determines the chemical properties. We use the chemical properties to classify atoms long before we knew about the nucleus. Therefore Z decides what element it is.

11. ISOTOPES For a particular element (a particular Z), N can vary. These different versions of the element are called isotopes. Example: Different versions of H and He

12. ATOMIC MASS UNIT

13. SOME MORE MASSES IN VARIOUS UNITS

14. SIZE OF A NUCLEUS

15. DENSITY OF A NUCLEUS

16. NUCLEAR FORCE Strong force (QCD: Quantum Chromodynamics) Very strong, overcome electric repulsion in the nucleus.

17. RADIOACTIVITY • Radioactivity is the spontaneous emission of radiation • Discovered by Becquerel in 1896 • Many experiments were conducted by Becquerel and the Curies • Experiments suggested that radioactivity was the result of the decay, or disintegration, of unstable nuclei

18. RADIOACTIVITY – TYPES • Three types of radiation can be emitted • Alpha particles • The particles are 4He nuclei • Beta particles • The particles are either electrons or positrons • A positron is the antiparticle of the electron • It is similar to the electron except its charge is +e • Gamma rays • The “rays” are high energy photons

19. DISTINGUISHING TYPES OF RADIATION • The gamma particles carry no charge • The alpha particles are deflected upward • The beta particles are deflected downward • A positron would be deflected upward, but would follow a different trajectory than the a due to its mass

20. PENETRATING ABILITY OF PARTICLES • Alpha particles • Barely penetrate a piece of paper • Beta particles • Can penetrate a few mm of aluminum • Gamma rays • Can penetrate several cm of lead

21. RADIOACTIVE DECAY

22. Units Curie: 1 Ci = 3.7×1010dacay/s Becquerel: 1 Bq = 1 dacay/s

23. HALF-LIFE T1/2 When will the N decrease to half the initial value?

24. USING HALF-LIFE The number of nuclei that remain after time t:

25. EXAMPLE How much of the original nuclei remains after two half-lives?

26. EXAMPLE Moon rock: NAr / NK =10.3 Suppose all Ar atoms are produced from K, find the age of the rock.

27. ALPHA DECAY Alpha particle = Helium nucleus 4He Quantum tunneling. Alpha particle is trapped inside a potential well and tunnels out.

28. ALPHA DECAY – EXAMPLE • Decay of 226 Ra • If the parent is at rest before the decay, the total kinetic energy of the products is 4.87 MeV

29. BETA DECAY • Symbolically • ν 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

30. BETA DECAY This is governed by weak interactions.

31. BETA DECAY – EXAMPLES

32. NEUTRINO Neutrino has almost no interactions with matter. Billions of neutrinos pass through our bodies every second. Most neutrinos pass through the entire planet without a single interaction. Very difficult to detect. Almost massless (~ 0.01 eV to 10eV). May account for part of the “missing mass” of the universe.

33. Super-Kamiokande 50 000 tons (5×107 kg) of ultra pure water. Volume of water: 50 000 m3 (roughly 10 Olympic size swimming pool)

34. CARBON DATING • Beta decay of C-14 is used in dating organic materials • The process depends on the ratio of C-14 to C-12 in the atmosphere which is relatively constant • When an organism dies, the ratio decreases as a result of the beta decay of the C-14

35. RADIOACTIVE DATING

36. GAMMA DECAY • Gamma rays are given off when an excited nucleus decays to a lower energy state • The decay occurs by emitting a high-energy photon • The X* indicates a nucleus in an excited state

37. GAMMA DECAY – EXAMPLE • Example of a decay sequence • The first decay is a beta emission • The second step is a gamma emission • Gamma emission doesn’t change Z, N, or A • The emitted photon has an energy of hf equal to ΔE between the two nuclear energy levels

38. SUMMARY OF DECAYS