5.3 Gamma decay and internal conversion

Gamma decay and internal conversion are crucial processes in nuclear physics, explaining how excited nuclei release energy. These mechanisms showcase the interplay between nuclear and atomic physics, revealing the complex nature of radioactive decay.

Understanding these processes is essential for grasping the full picture of nuclear decay. They complement alpha and beta decay, providing insights into nuclear structure, energy levels, and the various ways atoms can shed excess energy.

Nuclear Excited States and Deexcitation

Nuclear Energy Levels and Transitions

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• Nuclear excited states represent higher energy configurations of nucleons within an atomic nucleus
• Excited nuclei possess excess energy above their ground state
• Energy levels in nuclei are quantized, similar to electron energy levels in atoms
• Transitions between nuclear energy levels involve emission or absorption of energy
• Isomeric transition occurs when a nucleus in a metastable state transitions to a lower energy state
  • Metastable states have longer lifetimes compared to typical excited states (ranging from nanoseconds to years)
  • Isomers are denoted with an "m" after the mass number (Technetium-99m)
• Nuclear deexcitation involves the release of excess energy as the nucleus returns to its ground state
  • Can occur through various processes (gamma emission, internal conversion, pair production)
  • Energy conservation dictates that the energy released equals the difference between initial and final nuclear states

Mechanisms of Nuclear Deexcitation

• Gamma decay serves as a primary mechanism for nuclear deexcitation
  • Involves emission of high-energy photons (gamma rays)
  • Does not change the number of protons or neutrons in the nucleus
• Internal conversion offers an alternative deexcitation pathway
  • Excess nuclear energy transfers directly to an atomic electron
  • Results in ejection of an electron from the atom without gamma ray emission
• Pair production can occur for high-energy transitions (> 1.022 MeV)
  • Creates an electron-positron pair from the excess energy
  • Requires sufficient energy to overcome the rest mass of two electrons

Gamma Decay

Characteristics of Gamma Radiation

• Gamma rays consist of high-energy photons emitted during nuclear transitions
• Typical gamma ray energies range from tens of keV to several MeV
• Gamma radiation exhibits electromagnetic wave properties
  • No rest mass or electric charge
  • Travels at the speed of light in vacuum
• Highly penetrating compared to alpha and beta radiation
  • Requires dense materials (lead, concrete) for effective shielding
• Gamma decay does not alter the atomic number or mass number of the nucleus
  • Nucleus remains the same element with the same number of nucleons

Nuclear Transition Types and Selection Rules

• Electric transitions involve changes in the charge distribution of the nucleus
  • Characterized by electric multipole moments (dipole, quadrupole, etc.)
• Magnetic transitions result from changes in the current distribution within the nucleus
  • Described by magnetic multipole moments
• Multipolarity refers to the angular momentum carried away by the emitted gamma ray
  • Denoted as E1, E2, M1, M2, etc. (E for electric, M for magnetic, number for order)
• Selection rules govern allowed transitions based on angular momentum and parity changes
  • ΔJ (change in angular momentum) must be less than or equal to the multipolarity
  • Parity change determines whether electric or magnetic transition occurs
• Transition probabilities decrease with increasing multipolarity
  • E1 and M1 transitions are generally faster than higher-order transitions

Internal Conversion

Mechanism and Characteristics of Internal Conversion

• Internal conversion involves direct transfer of nuclear excitation energy to an atomic electron
• Competes with gamma decay, especially for low-energy transitions and high atomic numbers
• Does not involve the emission or absorption of a photon
• Probability of internal conversion increases with:
  • Increasing atomic number (Z)
  • Decreasing transition energy
  • Increasing multipolarity of the transition
• Internal conversion coefficient (α) quantifies the ratio of internal conversion to gamma decay
  • α = (rate of internal conversion) / (rate of gamma emission)
  • Can be measured experimentally or calculated theoretically

Conversion Electrons and Atomic Reorganization

• Conversion electrons are atomic electrons ejected during internal conversion
• Kinetic energy of conversion electron equals the nuclear transition energy minus the electron binding energy
• Conversion electrons have discrete energies characteristic of the nuclear transition and electron shell
• Ejection of inner-shell electrons leads to atomic reorganization
  • Outer electrons fill the vacancy left by the conversion electron
  • Results in emission of characteristic X-rays or Auger electrons
• Conversion electron spectroscopy provides valuable information about nuclear structure
  • Allows determination of transition energies, multipolarities, and nuclear spins
• Internal conversion impacts the overall decay scheme of radioactive nuclei
  • Affects apparent half-lives and branching ratios in decay chains