CHEM3440Lec15F06 - CHEM*3440 Chemical Instrumentation Topic...

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Unformatted text preview: CHEM*3440 Chemical Instrumentation Topic 15 Radiochemical Methods Nuclear Stability Atomic nuclei consist of protons and neutrons held together by the strong nuclear force, in balance with the electromagnetic and weak forces. Most nuclei are unstable and decay by various emission pathways. A narrow proportion of neutrons:protons leads to a stable nucleus. All others (most) are unstable. The degree of stability is measured by the nuclear lifetime. Observed variation in lifetimes throughout the periodic table spans more than 25 orders of magnitude (from s to 10 18 years). All elements have at least 1 unstable (radioactive) nucleus. Technetium (Tc) is the lightest element to have no known stable nuclei. Radioactive Decay Products Possible products ejected from a decaying nucleus: Symbol Charge Mass ! +2 4 "--1 1/1840 " + +1 1/1840 # Decay Processes Alpha Decay ! Beta Decay: Electron Emission or Positron Emission "- or ! + Electron Capture EC Isomeric Transition: Gamma Ray Emission # Internal Conversion: X-Ray Emission X Neutron Emission n Spontaneous Fission SF Alpha Decay 1 Nucleus ejects an alpha particle ( 4 He nucleus). More common process found amongst the heavier nuclei (6 th row elements and above) Ejected alpha particle carries most kinetic energy (some recoil left in daughter nucleus). Energy is difference in binding energy of two nuclei and is unique to a speciFc parent nucleus decay process. 90 234 Th + 2 4 He (E kinetic = 4.196 MeV) 3 92 238 U 4 90 234 Th + 2 4 He (E kinetic = 4.1 49 MeV) + c (E = h o = 47 keV ) 2 alpha decay pathways for the U-238 nucleus Alpha Decay 2 Alpha particles are charged and massive; they interact very strongly with matter. Ultimately, they capture two electrons to become a He atom. This strong interaction makes them easy to detect. They ionize material they pass through in the process of losing their energy. Strong interaction also gives them low penetrating power; stopped by a few inches of air. Not usually a problem to human health outside the body, but is a problem when it lodges in the lung. (This is why radon is a health hazard.) Beta Decay 1 Nucleus ejects a positron or an electron. Kinetic energy varies continuously from 0 up to a maximum. Can only happen if a third particle participates: a neutrino. 6 14 C " 7 14 N + b- + o 30 65 Zn " 29 65 Cu + b + + o The Frst reaction has a neutron become a proton with the ejection of an electron. The second reaction has a proton become a neutron with the ejection of a positron. Difference in nuclear binding energies are carried away by the kinetic energy of the leptons (electrons and neutrinos and their anti-particles). Beta Decay 2 Smaller masses involved mean that the emitted particles interact more weakly with matter than do " particles. Neutrinos can pass through the entire earth without interaction....
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CHEM3440Lec15F06 - CHEM*3440 Chemical Instrumentation Topic...

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