Lecture 22 Radioactivity Nucleus Isotopes Alpha, Beta & Gamma radiation Decay equations Conservation laws Radioactivity Discovery 1896 – Antoine Henri Becquerel 1852-1908, discovered nuclear radiation. (Shared Nobel Prize in Physics, 1903) Observed that a photographic plate was darken by invisible penetrating rays emitted from pitch blend (mineral containing uranium) Energetic rays: • had no apparent source No energy needed to create the rays? – Violates the law of conservation of energy!!!! 1905 – Einstein Energy can be created by the destruction of a small amount of mass: E = mc2 Law of conservation of energy modified to conservation of energy + mass Radioactivity 1898 – Marie and Pierre Curie extracted new and highly radioactive elements Polonium and Radium from pitch-blend. Both Shared Nobel Prize in Physics, 1903 With Henri Becquerel Other elements including Uranium were later found to be radioactive Certain elements had nuclei that were unstable and would “decay” causing emission of penetrating, highly energetic “rays” Radioactivity Independent of Chemical State Radiation independent of chemical state of radioactive element chemical reaction •nucleus unchanged •only orbital electrons participate Radioactivity Nothing to do with orbital electrons! unaffected by chemical, physical conditions Radioactivity disintegration or decay of an unstable nucleus. 3 distinct types of radiation discovered: Named, α, β and g The Nucleus Atomic Structure Rutherford’s experiment 1911 Alpha particles directed at a very thin film of gold foil Most passed through the foil with no deflection Indicated that the atom is mostly empty space A few particles were scattered at very large angles Indicated that the nucleus is a concentrated mass within the atom Electrons do not deflect alpha particles •to small and light Conclusion: inside electron orbits is mostly empty space with an dense nucleus at its center Radioactivity Nucleus Atom _ Neutrons Nucleons { _ Protons _ _ + Approximate diameters • Atom10-10 m _ • Nucleus 10-14 to 10-15 m _ Nucleus has most of the mass • Density of about 1017 kgm-3 Extremely large forces in the nucleus Nuclear force (Attractive) between nucleons • proton and protons, • neutrons and neutrons (short range • neutrons and protons force) Coulomb repulsive force •between protons Nuclear force > Coulomb repulsive force . result stable nuclei • Responsible for large energy associated with nuclear radiation • High energy in nuclear power Radioactivity Nucleus Atom _ Neutrons nucleons { _ Protons _ _ + _ _ Z is the atomic number (number of protons in the nucleus) Z = 1 Hydrogen Z = 2 helium Z = 3 Lithium etc Mass number A = Z + N where N is number of neutrons Many combinations of nucleons are possible – only some are stable Unstable combinations result in nuclear decay to a stable nucleus Radioactivity Nuclear force (Attractive) •between nucleons Coulomb repulsive force •between protons Nucleus not stable if number of protons is large relative to number of neutrons Extra neutrons mitigate the effect of the repulsive forces between the protons Stable Nuclei Large nuclei stable only if they contain more neutrons than protons Radioactivity Nuclear notation Element whose symbol is X can be denoted protons +neutrons A Z X protons Examples 238 is called Uranium 238. It has 92 protons and (238-92) 92 U = 146 neutrons 235 is called Uranium 235. It has 92 U 92 protons and (235-92) = 143 neutrons Radioactivity Examples 1 1 H a hydrogen nucleus (or just a proton) 4 a helium nucleus (or an alpha particle) 2 He Z is often not written (i.e. 235U) Notation can be used for particles other than nuclei Examples 1 A neutron is denoted by n 235 0 92 U An electron or beta particle denoted by 0 1 Isotopes Nuclei with the same charge but different masses are called isotopes of the element Same number of protons but Isotopes different number of neutrons Different isotopes • Same element (same chemical properties) • Same number of protons • Different nuclear properties Examples 238 235 92U 92U 1 2 3 1 H 1 H 1 H 12 6 C - most abundant in nature 14 - used in radioactive dating 6 C Radioactivity Radioactive decay Alpha, beta, and gamma radiation may be emitted Can be distinguished experimentally Beam of radiation containing all three types passed through a strong magnetic field Beam separates into three distinct parts • Undeflected beam • 2 beams deflected in different directions Source g (,,g) Strong Magnetic Field Bin Radioactivity α particles, α radiation Characteristics 2 neutrons and 2 protons (Helium nucleus 4 2 He +ve charge twice that of electron mass of 7000 times that of an electron Most of the energy carried by alpha radiation is in the form of kinetic energy Typical decay equation AA44 ZZX22 Y He Parent X → Daughter Y + α Radioactivity Example: Decay— particle emitted 238 234 4 92U 90 Th 2 He energy • Daughter nucleus (Thorium) has 2 less protons . Z = 92 – 2 = 90 Th • Daughter nucleus has lost atomic mass of 4 . A = 238 – 4 = 234 • Energy is always released in a nuclear reaction Energy of atom (mass) less than individual parts Radioactivity Beta Decay Emission of an electron Created at the time of decay •Not one of the orbital electrons •Not existing in the nucleus prior to decay Created and ejected from nucleus Neutron splits to form an electron and a proton Beta particle (electron) •Charge (-1.6 * 10-19 C) •mass (9.11 * 10-31 kg) Energy carried by beta radiation is kinetic •Moves much faster than alpha particle • at greater than half speed of light Radioactivity Decay notation Can be used for neutrons and electrons AAX Y 0 e ZZ11A Z X 1 1 0 0n 1 p 1 e 14 14 0 6C 7 N 1 antineutrino Atomic mass stays the same Number of protons increases As if one neutron has changed to a proton Antineutrino created in and ejected from the nucleus (all decay) • Mass-less particle? • Travels at the speed of light ? • No effect to biological tissues • So penetrating that it deposits no energy Radioactivity Gamma Decay Excited nucleus returns to non-excited state by releasing gamma radiation Emission of a high frequency (wave) photon Gamma rays: only generated in the nucleus No Charge No Mass Move at the speed of light Like all electromagnetic waves (photons) No change to the identity of the nucleus 40 40 * indicates excited state 20Ca 20 Ca g Something must excite the nucleus • Often preceded by another type of decay where nucleus is left in an excited state 60 60 0 27Co 28Ni* + -1 + antineutrino Followed by 60 60 28Ni* 28Ni + gg 1 + 2 Radioactivity Gamma Decay • EM radiation. Very high energy • Uncharged • Source is often excited nuclear state occurring after alpha and beta decay. • Excited state may remain for some time. Metastable state Source gamma rays associated with nucleus, X-rays associated with outer electrons Energy Ranges 50keV < Gamma rays < 40MeV 15 keV < Diagnostic X-rays < 150 keV 2 eV < Visible Light Photons < 4 eV Radioactivity Nuclear equations must balance Conservation laws of physics must be satisfied 226 222 4 88Ra 86 Rn 2 He g Conserved: •Total number of nucleons (A) (protons + neutrons) •mass + energy In all nuclear decays small quantity of mass destroyed E = mc2 Nuclear reactions Laws of Conservation observed in all nuclear decays Conservation •charge •total number of nucleons •Conservation of mass plus energy Energy produced in nuclear decay is the result of a small amount of mass being destroyed E = mc2 Write the decay equation for the following: 239 94 Pu decaying by emission 239 235 4 94Pu 92U + 2 He + energy Nuclear reactions Unit of energy is the Joule But unit of energy used in atomic and nuclear physics is the Electron volt (eV) Definition The electron volt is equal to the amount of energy gained by an electron as it accelerates through a potential difference of one volt Energy (Joules) = qV Charge on an electron = 1.6 x10-19 C eV = 1.6 x10-19 C x 1volt =1.6 x 10-19 Joules 1eV = 1.6 x 10-19J 15 keV < Diagnostic X-rays < 150 keV 2 eV < Visible Light Photons < 4 eV Radioactivity Range Distance radiation can travel in a given material before dissipating all of its energy Depends on the material Greater electron density stops radiation most effectively Range depends on radiation type Equal energies and same material •Alpha radiation - Smallest range •Beta radiation - Middle range •Gamma radiation - Largest range Radiation interacting with electrons in the material results in energy dissipation Summary of Radiation Range Type Mass kg Structure Charge (in air) Damage Alpha 6.6 x10-27 4 +2e mm High () 2 He Beta 9.11 x 10-31 electron -e cm Med () Gamma 0 EM 0 m Low (g) .