1. Nuclear Physics Properties of Nuclei Binding Energy Radioactivity

2. Nuclear Components Nucleus contains nucleons: protons and neutrons Atomic number Z = number of protons Neutron number N = number of neutrons Mass number A = number of nucleons = Z + N Each element has unique Z value Isotopes of element have same Z, but different N and A values Notation:

3. Nucleus Charge and Mass ParticleChargeMass ( kg )Mass ( u )Mass ( MeV/c 2 ) Proton +e+e1.672 6 E−271.007 276938.28 Neutron 01.675 0 E−271.008 665939.57 Electron −e−e9.109 E−315.486 E−40.511 Unified mass unit, u, defined using Carbon 12 Mass of 1 atom of 12 C ≡ 12 u

4. Nuclei Sizes Scattering experiments determine size Measured in femtometers (aka fermis) All nuclei have nearly the same density Fig. 29.2, p. 959

5. Nuclear Stability An attractive nuclear force must balance the repulsive electric force Called the strong nuclear force Neutrons and protons affected by the strong nuclear force 260 stable nuclei If Z > 83, not stable Fig. 29.3, p. 960

6. Binding Energy Total energy of nucleus is less than combined energy of individual nucleons Difference is called the binding energy (aka mass defect) Energy required to separate nucleus into its constituents Fig. 29.4, p. 961 Binding Energy vs. Mass Number

7. Radioactivity Unstable nuclei decay to more stable nuclei Can emit 3 types of radiation in the process Fig. 29.5, p. 962 A positron ( e + ) is the antiparticle of the electron ( e − )

8. Decay Constant and Half-Life Decay rate (aka activity) is number of decays per second λ is the decay constant Unit is Curie ( Ci ) or Becquerel ( Bq ) Decay is exponential Half-life is time it takes for half of the sample to decay Fig. 29.6, p. 919

9. Alpha Decay Unstable nucleus emits  particle (i.e., a helium nucleus) spontaneously Mass of parent is greater than mass of daughter plus  particle Most of KE carried away by  particle Fig 29.7, p. 966

10. Beta Decay Involves conversion of proton to neutron or vice-versa Involves the weak nuclear force KE carried away by electron/antineutrino or positron/neutrino pair Neutrinos: q = 0, m < 1 eV/c 2, spin ½, very weak interaction with matter Fig. 29.8a, p. 968

11. Gamma ( γ ) Decay Following radioactive decay, nucleus may be left in an excited state Undergoes nuclear de-excitation: protons/neutrons move to lower energy level Nucleus emits high energy photons ( γ rays) No change in A or Z results

12. Radioactive Carbon Dating Cosmic rays create 14 C from 14 N Ratio of 14 C/ 12 C remains constant ( 1.3×10 – 12 ) in atmosphere Living organisms have same ratio Dead organisms do not (no longer absorb C from atmosphere) T ½ of 14 C = 5730 yr We measure decay rates, R

13. Radioactive Carbon Dating Cosmic rays create 14 C from 14 N Constant ratio of 14 C/ 12 C ( 1.3×10 –12 ) in atmosphere Living organisms have same ratio Dead organisms do not (no longer absorb C ) T ½ of 14 C = 5730 yr Measure decay rates, R

14. Natural Radioactivity Three series of naturally occurring radioactivity 232 Th more plentiful than 238 U or 235 U Nuclear power plants use enriched uranium Other series artificially produced Thorium Series Fig. 29.10, p. 971

15. Nuclear Reactions Accelerators can generate particle energies up to 1 TeV Bombard a nucleus with energetic particles Nucleus captures the particle Result is fission or fusion Atomic and mass numbers ( Z and A ) must remain balanced Mass difference before and after reaction determines Q value –Exothermic: Q > 0 –Endothermic: Q < 0 Endothermic requires incoming particle to have KE min

16. Fusion and Fission

17. Interaction of Radiation with Matter Radioactive emissions can ionize atoms Problems occur when these ions (e.g., OH −, H + ) react chemically with other ions Genetic damage affects reproductive cells Somatic damage affects other cells (lesions, cataracts, cancer, fibrosis, etc.)

18. Quantifying Radioactivity QuantityDefinitionSI unitCommon Unit Activity # nuclei that decay per sec 1 Bq ≡ 1 decay/s 1 Ci = 3.70×10 10 Bq Exposure (defined for X and γ rays only) Ionization per kg 1 R ≡ amount of radiation that produces 2.58×10 −4 C/kg Roentgen (R) Absorbed Dose ( D ) Energy absorbed per kg 1 Gray (Gy) ≡ 1 J/kg 1 rad = 10 −2 Gy Relative Biological Effectiveness ( RBE ) How much more damage is done compared to X or γ rays of equivalent energy (unitless). Dose Equivalent ( H ) Damage expected 1 Sv ≡ 1 RBE×Gy 1 rem = 10 −2 Sv

19. RBE Factors Radiation TypeRBE Factor X and γ rays1.0 β particles1.0−1.7 α particles10−20 Slow n4−54−5 Fast n and p10 Heavy ions20 Table 29.3, p. 974

20. Sources of Ionizing Radiation From Touger, Introductory Physics, Table 28-4, p. 817

21. Typical Dose Equivalents From Touger, Introductory Physics, Table 28-4, p. 817

22. Exercise Is the dose equivalent greater if you are exposed to a 100 mrad dose of α particles or a 300 mrad dose of β particles? α particles: β particles: α particles are more effective at delivering a dose, but do not penetrate as far as β particles