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Lecture Chapter3 Class New 2020 Chapter 3: Radioactivity Beta Emission Processes • Three favors of beta decay • Energy spectrum of beta particles through beta decay • Other processes involving beta emission, internal conversion, photoelectric effect and Auger electrons. • Major health hazards related to beta emission 101 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Understanding the Radiation from Cs-137 Decay scheme: What will happen to the excited Ba- 137 nucleus? 10% Internal conversion, conversion, Internal http://faithandsurvival.com/ wp- content/uploads/2012/04/f ukushima-cesium-137- spread.jpg 102 NPRE 441, Principles of Radiation Protection Typical Decay Products from Unstable Radioisotopes Alpha decay Beta decay Beta decay Beta-plus decay Orbital electron capture (E.C.) Alpha particles Daughter nuclei Beta particles Positrons Daughter nuclei Annihilation Bremsstrahlung gamma rays X-rays Excited Daughter Nuclei Gamma ray emission Internal conversion (I.C.) IC electrons Excited atoms Gamma rays Auger electrons Characteristic Bremsstrahlung X-rays X-rays 103 Chapter 7: Radiation Dosimetry Beta Radiation – Submersion Dose from Kr-85 An example. Cember, pp. 231. The Fukushima nuclear disaster is estimated to have released between 20- 200 megacuries of Krypton 85 from three melted down reactors 104 NPRE 441, Principles of Radiation Protection, Spring 2020 Chapter 3: Radioactivity Beta Emission • Beta particle is an ordinary electron. Many atomic and nuclear processes result in the emission of beta particles. • One of the most common source of beta particles is the beta decay of nuclides, in which Beta decay A A 0 Z X → Z+1Y + −1 β +υ Beta-plus decay A A 0 Z X → Z−1Y + 1 β +υ Electron capture A − A Z X + e → Z−1Y +υ 105 Chapter 3: Radioactivity Energy Release of Beta Decay The energy release in a beta decay is given as Q = M p - (M d + M e ) • The energy release is once again given by the conversion of a fraction of the mass into energy. Note that atomic electron bonding energy is neglected. • For a beta decay to be possible, the energy release has to be positive. 106 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Atomic Mass Unit The atomic mass unit (symbol: u) or is the standard unit that is used for indicating mass on an atomic or molecular scale (atomic mass). One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically equivalent to 1 g/mol. It is defined as one twelfth of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state, and has a value of 1.660538921×10−27 kg. 107 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Energy Release of Beta Decay An example The corresponding energy release is given by Q = Mp – Md - Me =0.001837 AMU or equivalently Q = 1.71 MeV Similar to the case of alpha decay, the energy shared by the recoil nucleus is Me/(Mp+Me) ´ Q ?? … So the electron generated will be mono-energetic ?? 108 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Typical Energy Spectrum of Beta Particles The energy release is shared by all three daughter products. Due to the relatively large mass of the daughter nucleus, it attains only a small fraction of the energy. Therefore, the kinetic energy of the beta particle is E - » Q - E b n 109 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Examples for Beta Decay • Beta emissions are normally associated with complicated decay schemes and the emission of other particles such as gamma rays. • There exist the so called “pure beta emitters”, such as 3H, 14C, 32P and 90Sr, which have no accompanying gamma rays. 110 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Positron Emission • A positron is the anti-particle of electrons, which carries the same mass as an electron but is positively charged. • Positrons are normally generated by those nuclides having a relatively low neutron- to-proton ratio. • An typical example of positron emitter is 22 22 0 11 Na®10 Ne+1b +u 111 NPRE 441, Principles of Radiation Protection Z Chart of the Nuclides (177, 117) The nuclides are the possible nuclei of atoms. Z determines the chemistry, because the neutral atom with the nuclide as its nucleus has Z electrons. Half-life N Source: http://www.nndc.bnl.gov/chart/reZoom.jsp?newZoom=5 112 Chapter 3: Radioactivity Energy Release Through Positron Decay The energy release Q associated with the positron emission process is given by Q » M p - M d - M e - M e+ = M p - (M d + 2M e ) where the atomic electron binding energy is ignored. 113 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Orbital Electron Capture In electron capture (EC), one of the atomic electrons is captured by the nucleus and unites with an proton to form an neutron with the emission of a neutron A - A Z X + e ®Z -1Y +u 0 1 1 -1e+1H ®0 n +u • For neutron deficient atoms to attain stability through positron emission, it must exceed the weight of the daughter by at least two electron masses. If this condition can not be satisfied, the neutron deficiency can be overcome by the EC process. • For example, 114 NPRE 441, Principles of Radiation Protection Positron Decay and Orbital Electron Capture are Competing Processes 1 p® 1n+0e +u Positron decay A A 0 1 0 1 Z X → Z−1Y + 1e +υ 0 1 1 Elec. capture A - A -1e+1p®0 n +u Z X + e ®Z -1Y +u Alpha decay 210 206 4 84 Po® 82 Pb+2 He Beta decay 115 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Energy Release Through Orbital Electron Capture For positron decay to be possible, we need Q M M M M + 0, = p - d - e - e > so M M M M + M 2M p > d + e + e = d + e M p and M d are the atomic masses of the parent and daughter atoms For Electron Capture to occur, Q = M p - M d -f > 0 so that M p > M d +f where f is the binding energy of the orbital electron 116 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Orbital Electron Capture and Positron Decay • Electron capture and positron decay are normally competing processes through which a neutron deficient nucleus may attain an increased stability. • Both the emission of a positron and the capture of an electron, a neutrino is always emitted in order to conserve energy. • In positron decay, the neutrino carries the difference between the energy release and the energy of the resultant positron. In electron capture, however, the neutrino must be mono- energetic. 117 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Positron Annihilation following Positron Decay 118 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Examples for Beta Decay • Beta emissions are normally associated with complicated decay schemes and the emission of other particles such as gamma rays. • There exist the so called “pure beta emitters”, such as 3H, 14C, 32P and 90Sr, which have no accompanying gamma rays. 119 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Orbital Electron Capture and Positron Decay (Revisited) • Electron capture and positron decay are normally competing processes through which a neutron deficient nucleus may attain an increased stability. • Both the emission of a positron and the capture of an electron, a neutrino is always emitted in order to conserve energy. • In positron decay, the neutrino carries the difference between the energy release and the energy of the resultant positron. In electron capture, however, the neutrino must be mono- energetic. 120 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity An Example 121 NPRE 441, Principles of Radiation Protection Typical Decay Products from Unstable Radioisotopes Alpha decay Beta decay Beta decay Beta-plus decay Orbital electron capture (E.C.) Alpha particles Daughter nuclei Beta particles Positrons Daughter nuclei Bremsstrahlung Annihilation X-rays gamma rays Excited Daughter Nuclei 122 Chapter 3: Radioactivity Gamma Ray Emission following Beta Decay 123 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Excited Nucleus vs Excited Atom 124 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Internal Conversion An excited nucleus De-excite through the emission of a The excitation energy is transferred directly gamma ray to an orbital electron, causing it to be è Gamma Ray Emission ejected from the atom è Internal Conversion Conversion electron with an energy E - E E b = ex - b N IC Coefficient (or Branching Ratio) = g Ne 125 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Gamma Ray Emission • Gamma rays are emitted from nuclei following the transition between different nuclear states. • Gamma rays are emitted with discrete energy. A gamma ray spectrum is characteristic to the particular radionuclide that are present. 126 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Metastable Nuclear States and Gamma Ray Emission 127 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Important Gamma Ray Emitter: Tc-99m Decay scheme: • Tc-99m accounts for >90% of imaging studies in nuclear medicine and therefore subject to extensive dosimetry study. • Half-life: ~6h; gamma energy: 140keV, both ideal for imaging applications. • Tc-99m is obtained from the decay of the molybdenum isotope 99Mo. 128 NPRE 441, Principles of Radiation Protection Chapter 3: Radioactivity Internal Conversion • Conversion electrons can originate from several different electron shells within the atom, a single excited state generally leads to several groups of electrons with different energies. • The only practical laboratory scale source of mono-energetic electron groups in high keV to MeV energy range.
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