1 / 32

Nuclear Physics and Radioactivity

Nuclear Physics and Radioactivity. Chapter 30. Structure and Properties of the Nucleus. Nucleus – center of the atom Discovered by Rutherford Proton (+) charge: +1.6E-19 C Mass = 1.6726E-27 kg Neutron Discovered by James Chadwick (1932) Neutral (no charge) Mass = 1.6749E-27 kg

kalona
Download Presentation

Nuclear Physics and Radioactivity

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Nuclear Physics and Radioactivity Chapter 30

  2. Structure and Properties of the Nucleus • Nucleus – center of the atom • Discovered by Rutherford • Proton • (+) charge: +1.6E-19 C • Mass = 1.6726E-27 kg • Neutron • Discovered by James Chadwick (1932) • Neutral (no charge) • Mass = 1.6749E-27 kg • Nucleons: protons and neutrons

  3. Structure and Properties of the Nucleus • Atomic number (A) • Number of protons • Determines the type of atom • For neutral atoms, the number of electrons is also equal to the atomic number • Mass number (Z) • Number of nucleons (protons and neutrons) • Number of neutrons = Z – A • Symbol • ZAX • 23892U

  4. Structure and Properties of the Nucleus • Isotopes • Same element with different numbers of neutrons • Same atomic number, but different mass numbers • Periodic table gives a weighted average of all the naturally occurring mass numbers (atomic mass) • Size of nucleus • Determined by Rutherford • Not an exact size due to wave-particle duality • r ≈ (1.2E-15m)(A1/3) • V = 4/3πr3 • V is proportional to A • However, we are not sure of the shape of the nucleus

  5. Example • Estimate the diameter of the following nuclei. • 11H • 4020Ca • 20882Pb • 23592U

  6. Structure and Properties of the Nucleus • Unified mass units (u) • Makes calculations easier • 126C: 12.000 000 u • Neutron = 1.008 665 u • Proton = 1.007 276 u • 11H: 1.007 825 u • Contains a proton and an electron • 1 u = 1.660 54E-27kg = 931.5 MeV/c2 • Nuclear spin quantum number, I • Both protons and neutrons are spin ½ • Whole integer if an even number of nucleons • Half integer if an odd number of nucleons

  7. Binding Energy • Total mass is always less than the mass of its parts • The missing mass is converted into energy • Binding energy = the total amount of energy required to break the nucleus into its protons and neutrons

  8. Example • Compare the mass of a 42He to the total mass of its constituent parts. What is the binding energy of the atom?

  9. Binding Energy • Binding energy per nucleon • Total binding energy of a nucleus divided by the number of nucleons (A) • 42He = 7.1 MeV • Drops as atoms get heavier • Easier to break apart nucleus • Allows for fission

  10. Example • Calculate the total binding energy and the binding energy per nucleon for 5626Fe, the most common stable isotope of iron.

  11. Example • What is the binding energy of the last neutron in 136C?

  12. Binding Energy • Nuclear forces • Strong nuclear force • Holds the protons together and electrically attracts neutral neutrons • Not a clear mathematical understanding yet • Works across a very short range ≈ 1/r7 • Stable nuclei have about the same number of neutrons and protons until the mass number gets to about 30 to 40; after that more neutrons are needed to counteract the increasing electrostatic repulsion of the protons • Weak nuclear force • Not clearly understood • Observed in some types of radioactive decay

  13. Radioactivity • Henri Becquerel (1896) • Studying phosphorescence • Uranium darkened a photographic plate without any external stimulus • Referred to as radioactivity • Marie (1867-1934) and Pierre (1859-1906) Curie • Isolated radium and polonium • Determined the radiation came from the nucleus

  14. Radioactivity • 3 types of radioactivity • Alpha rays (α) • Stopped by paper • Positively charged • Identified as a helium nucleus • Beta rays (β) • Stopped by a few mm of metal • Negatively charged • Electrons created by the nucleus (not part of the cloud) • Gamma rays (γ) • Passes through centimeters of lead • Electrically neutral • High energy photons (shorter wavelengths than x-rays)

  15. Alpha Decay • Parent nucleus • Original nucleus • Daughter nucleus • Nucleus of element after the decay • Different than parent nucleus • Transmutation: process of changing from one element to another • 22688Ra  22286Rn + 42He • 22688Ra: parent nucleus • 22286Rn: daughter nucleus • 42He: alpha particle

  16. Alpha Decay • Reason for alpha decay • Range of strong nuclear force is short • Acts only on close nucleons • Electrostatic force acts across the whole nucleus • For larger atoms, the nuclear force is unable to hold the atom together • Disintegration energy, or Q-value (Q) • Total energy released • Mass differences appears as KE of the recoiling daughter nucleus and the alpha particle • Mpc2 = MDc2 + mαc2 + Q • Q = KE • Q = Mpc2 – (MD + mα)c2 • If Q < 0, then the decay will not occur

  17. Example • Calculate the disintegration energy when 23292U (mass = 232.037 146 u) decays to 22890Th (228.028 731 u)with the emission of an alpha particle. (Assume the masses are for neutral atoms.)

  18. Alpha Decay • Why alpha particles? • Strongly bound • Mass is significantly less than four separate nucleons • Otherwise, it would violate the conservation of energy • Applications • Smoke detectors • Contains 24195Am • Radiation ionizes air molecules creating a current • Amount of radiation given off is too small to cause damage to humans

  19. Beta Decay • Transmutation occurs • 146C  147N + e- + neutrino • Conservation of nucleons • Conservation of charge • Neutron changes into a proton and a beta particle (electron) is created • e- = beta particle • NOT an orbital electron • Created within the nucleus • Function of the weak nuclear force • Neutrino • No charge • Small or zero mass

  20. Example • How much energy is released when 146C decays to 147N by beta emission?

  21. Beta Decay • Lost KE • Experiment showed that most beta particles did not have calculated KE • Momentum was also not conserved • Enrico Fermi (1901-1954) • Hypothesized particle: neutrino; “little neutral one” • Carries lost energy and momentum • No charge • Very small mass (<0.6 eV/c2) • Spin = ½(h/2π) • Symbol: ν (nu) • 146C  147N + e- + ν

  22. Beta Decay • Why beta decay? • Nucleus has too many neutrons vs. protons: unstable • Above line in graph • β+ decay? • Nucleus has too few neutrons vs. protons: unstable • Below line in graph • Emits a positron: positive charged electron • 1910Ne  199F + e+ + ν

  23. Beta Decay • Electron Capture • Nucleus absorbs one if its innermost electrons • Usually from the K shell (K capture) • Conservation of charge • Electron disappears • Proton becomes a neutron • 74Be + e- 73Li + ν

  24. Gamma Decay • High energy photons • Nucleus can be in an excited state • Gamma ray is emitted when a nucleus jumps to a lower state • Energy levels are much further apart (keV to MeV) • Higher states caused by collisions with other particles • * = excited state • X-ray vs. gamma ray • X-ray is used if the photon is produced by an electron-atom reaction • Gamma ray is used if the photon is produced in a nuclear process 125B β- (9.0 MeV) β- (13.4 MeV) 126C* γ (4.4 MeV) 126C

  25. Half Life and Rate of Decay • Radioactive decay • Not all of the atoms decay at the same time • Radioactive decay law • N = Noe-λt • N: number of remaining nuclei after time, t • No: initial number of nuclei at time, t = 0 • λ: decay constant; different for each isotope • Activity • Rate of decay • ΔN/Δt = λN • N = original number of nuclei • (ΔN/Δt) = (ΔN/Δt)oe-λt

  26. Half Life and Rate of Decay • Half life • Time it takes ½ of a given sample to decay • Carbon 14 • Half life = 5730 years • Used to determine the age of a once living object • T½ = 0.693/λ • T½ = half life (seconds) • λ = decay constant

  27. Example • The isotope 146C has a half-life of 5730 years. If at some time a sample contains 1E22 carbon-14 nuclei, what is the activity of the sample?

  28. Example • A laboratory has 1.49 μg of pure 137N, which has a half-life of 600 seconds. • How many nuclei are present initially? • What is the activity initially? • What is the activity after 1 hour? • After approximately how long will the activity drop to less than one per second (1 s-1)

  29. Decay Series • Decay series • Multiple decays of a parent isotope • Diagonal lines = alpha decays • Horizontal lines = beta decay • Explains why elements that should have disappeared from decay since the universe was formed are still around • Decay of heavier elements with longer half-life

  30. Radioactive Dating • Determines the age on an object • Living plants absorb CO2 • Small fraction is carbon-14 • n + 147N  146C + p+ • Nitrogen produces carbon • Plant is alive • Uses CO2 to build or replace tissue • Carbon-14 to carbon-12 ratio stays relatively constant • Plant dies • No new CO2 is brought into the plant • Carbon-14 decays into carbon-12 • The ratio of carbon-14 to carbon-12 decreases • Knowing the half-life of carbon-14 and the ratio of carbon-14 to carbon-12 allows us to estimate how long ago the plant died

  31. Example • The mass of carbon in an animal bone fragment found in an archeological site is 200 g. If the bone registers an activity of 16 decays/second, what is its age?

  32. Radioactive Dating • Limitations • After 60,000 years, the amount of carbon-14 drops below measureable amounts • Other isotopes can be used • Uranium-238 can measure the age of rocks to 4E9 (4 billion) years.

More Related