3.3 Gamma decay and internal conversion

Gamma decay and internal conversion are two ways excited atomic nuclei release energy. Gamma decay emits high-energy photons, while internal conversion transfers energy directly to an electron, ejecting it from the atom.

These processes help nuclei reach stable ground states after radioactive decay. Understanding them is crucial for grasping how atoms shed excess energy and achieve stability in various nuclear reactions.

Gamma Decay

Emission of High-Energy Photons

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• Gamma decay involves the emission of high-energy photons called gamma rays from an atomic nucleus
• Gamma rays are a form of electromagnetic radiation with the highest energy and shortest wavelength in the electromagnetic spectrum
• During gamma decay, the atomic nucleus transitions from an excited nuclear state to a lower energy state
• The energy difference between the initial and final nuclear states determines the energy of the emitted gamma ray

Nuclear Excited States and Isomeric Transitions

• Atomic nuclei can exist in various excited states with higher energy levels than the ground state
• These excited states are often populated following other radioactive decay processes such as alpha or beta decay
• Nuclei in excited states are unstable and will eventually transition to lower energy states through the emission of gamma rays
• Isomeric transitions are a specific type of gamma decay where the initial and final nuclear states have the same spin and parity (angular momentum and symmetry properties)

Internal Conversion

Conversion Electrons and Competing Process

• Internal conversion is a competing process to gamma decay, where the excitation energy of the nucleus is directly transferred to an atomic electron
• Instead of emitting a gamma ray, the nucleus ejects an electron from one of the inner atomic shells (K, L, M, etc.)
• The ejected electron is called a conversion electron and has a specific energy equal to the difference between the nuclear transition energy and the binding energy of the electron shell

Multipolarity and Selection Rules

• The probability of internal conversion depends on the multipolarity of the nuclear transition, which describes the angular momentum and parity change between the initial and final nuclear states
• Electric monopole (E0) transitions can only occur through internal conversion, as they do not involve a change in angular momentum or parity
• Higher-order multipole transitions (dipole, quadrupole, etc.) are less likely to undergo internal conversion compared to lower-order transitions
• Selection rules based on conservation of angular momentum and parity govern the allowed transitions and determine the multipolarity of the conversion electrons