CHEM 115 - Exam 1 - 24.1: Modes of Radioactive Decay

Alpha Decay

• loss of an alpha particle (they are identical to helium-4 nuclei)

• alpha decay is the most common means for a heavy, unstable nucleus to become more stable

• every element beyond bismuth, Z=83, exhibits alpha decay

Beta Decay

• loss of a beta particle - a negatively charged particle identified as a high-speed electron

• beta decay results in a daughter nuclide with the same mass but with one more proton than in the parent nuclide - an atom with the next higher atomic number is formed

Positron (β⁺) Emission

• positron: antiparticle of the electron 

• occurs through a process in which a proton in the nucleus is converted into a neutron, which remains in the nucleus, and a positron, which is expelled

• the daughter has the same mass but has one less proton than the parent nuclide

• proton → neutron + positron

Electron Capture

• occurs when the nucleus draws in an electron from a low atomic energy level 

• the electron combines with a proton in the nucleus to produce a neutron

• the orbital vacancy is quickly filled by an electron that moves down from a higher energy level, and that process continues through still higher energy levels, with x-ray photons and neutrinos carrying off the energy difference in each step

• even though the processes are different, EC has the same net effect as a positron emission

Gamma Emission

• involves the radiation of very high-energy photons called gamma rays from an excited nucleus

• the excited nucleus lowers its energy by emitting high-energy, short wavelength photons

• many nuclear processes leave the nucleus in an excited state, so gamma emission accompanies many other (mostly beta) types of decay

• several gamma photons of different energies can be emitted from an excited nucleus as it returns to the ground state

• because gamma rays have no mass or charge, gamma emission does not change mass number (A) or charge of the particle (Z)