1Nuclear Physics• The nucleus• Nuclear stability• Radioactive decay• Standard model• Radiation doses• Medical physics2Nuclear medicineScintigraphyPositron emission tomographyBrachytherapyGamma knifeNeutron therapy3Nuclear structure• Atoms – 10-10m=0.1nm• Made of point like electrons and the nucleus (10-14m=10fm)• Nucleus is made of nucleons, which are protons and neutrons4NucleonsNucleons is the generic name for neutrons and protonsNumber of protons in a nucleus is Z (charge)Number of neutrons in a nucleus is NTotal atomic mass is N+Z5Elements and IsotopesAn element is determined by its chemical properties6IsotopesThe number of protons in an atom is the same as the number of electrons. But an element may have a different number of neutrons – this are known as isotopes7IsotopesThe isotopes of an element will have different atomic masses, e.g. oxygen occurs in nature with nucleons containing 8 protons and 8 neutrons 16O, and 8 protons and 10 neutrons 18O8IsotopesAlthough the chemical properties are the same for isotopes, the physical properties, such as boiling point, freezing point, rate of settling in a centrifuge, can be different.Isotopes can be separated out by physical processes.9Global temperature change and isotopes of oxygen18O is slightly more difficult to evaporate than 16O, so there is deficiency in the earth’s atmosphere of 18O.The ratio of the two isotopes is a measure of the atmospheric water vapor content over time – measured in ice cores from the Antarctic.10Atomic Mass Number and Atomic Mass• The atomic mass A, is the sum of the number of protons and neutrons A=Z+N.• The atomic mass unit, u, is defined as 1/12 the mass of 12C, u=1.6605x10-27kg• We can convert this mass to an energy2mcE 11Atomic Mass Number and Atomic Mass• From E=mc2, we get2221027/1/49.931/104924.1106605.11cGeVcMeVcJkgu12Nuclear stability•Back to the isotope chart, the N and Z are related.•The ratio N/Z changes from ≈1 to ≈1.5, along a line of stability•Nature likes to have more neutrons than protons•The red dots represent unstable isotopes which decay through radioactivity13Binding energy•Binding energy is the energy used in bonding the nucleons together.•The binding forces and bond energy is so great in a nucleus that we can measure it as a fraction of the mass of the system.•From Einstein’s special theory of relativity (1905) E=mc2, we can measure the mass which goes into forming the nucleon bonds14Binding energy of the Helium nucleus• We can calculate the different in mass between the constituent parts and the measured mass of the 4He nucleus• Δu=28.30MeV, or 0.75% of its mass.15Binding energyIn general, the binding energy, B, of a nucleus can be calculated from the measured atomic mass, matom, and the number of protons, Z and neutrons, N.Where mH, mnand matomare in units of the atomic mass unit, u.MeVmNmZmBatomnH49.931)( 16Binding energy per nucleon• We can look at the binding energy per nucleon for all the elements.• The most stable nucleus is Fe. • In a high energy environment (like a star) lighter nuclei will undergo fusion, heavier will undergo fission, until we’re left with iron.17Nucleon forceThe force which keeps neutrons and protons together is called the strong force and must • Be attractive• Have no effect on electrons• Be a short range force (not seen at atomic distances)• Be stronger than the electrostatic repulsion of protonsCalculations are difficult, (F=kx), and is described by Quantum Chromodynamics (QCD)18History of Radioactivity1894 - J.J.Thomson identified electrons1896 - Uranium salts were found to ruin photographic paper by Henri Becquerel.1898 - Investigated by Marie Curie who published data naming the rays radioactivity, and discovered the radioactive elements polonium and radium.1899-1903, Ernest Rutherford identified 3 different types of radiation – alpha, beta and gamma1911 – Rutherford published the nucleus/electron model1913 – J.J.Thomson identified isotopes19Three types of radiationIn a magnetic field, the 3 types of radiation act have different charge.20Ionizing properties of radiation• All 3 types of nuclear radiation can be detected by its ionization of atoms in its path.• A Geiger counter uses a high voltage E field to accelerate and amplify the electrons from ionization.• Damages cell tissue, free ions disrupt biochemical reactions, and breaks up DNA leading to tumors.21Ionizing properties of radiation• Irradiating stable isotopes does not make them radioactive • Used to sterilize food, US mail, hamburger patties and medical equipment • A large dose of radiation can cause long term biological effects in living tissue.22Nuclear decay• Radioactive isotopes decay over time.• The probability of the decay of an individual nucleus is constant, it does not depend on time.• The rate of decays will depend only on the number of nuclei left in the sampleNtN23Nuclear decay• The solution of this equation is an exponential• The symbol τ, is a time which represents the time it takes for the fraction of the nuclei to be reduced by 1/e=0.37• Note the probability of decay of an individual nucleus does not change, but the rate of the sample drops as fewer nuclei remain./0teNN24Half-life• The decay probability is often described as a half-life. • The half life is the time for which a sample (and so rate) will drop by a factor 2.693.02ln2122/1/0/02/1teNeNNtt25Half-lifeHalf lives range wildly, 238U has a half-life of 109years, 214Po has a half life of 160μs.The number of nuclei left is always reduced by a factor 2 after t1/2, and so some nuclei remain for practically ever.26Activity of a sample• The rate is proportional to the number of nuclei left.• The decay rate will change exponentially with time, with the same time constant, τ, and same half-life t1/2• Measured in becquerels, and curies1 becquerel = 1Bq = 1decay per sec1 curie = 1Ci = 3.7x1010Bq/0/0tteRReNRNtNR27Types of radiationThree types of radiation• Alpha – low penetration, high tissue ionization• Beta – medium penetration, medium ionization• Gamma – high penetration, low ionization• And others28Alpha radiation α• Nucleus ejects 2 protons and 2 neutrons (Helium nucleus)• Z →Z-2, N→N-2, A→A-4.• Alpha particles are easily stopped as they are heavy.• They
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