Radioactivity and Decay Processes

Radioactivity or Radioactive Decay

• Radioactivity is the process where an unstable atom's nucleus loses energy by emitting radiation.
• Unstable nuclei decay, emitting alpha, beta, and gamma radiation.
• Henri Becquerel discovered natural radioactivity in 1896.

Types of Radioactive Decays

• Atoms are either stable or unstable.
• Instability results from excess neutrons or protons.
• Radioactive atoms achieve stability by emitting nucleons (protons or neutrons) or other particles, or by releasing energy.
• Four main decay types: alpha, beta (negative and positive), gamma, and electron capture.

Alpha Decay

• An alpha particle is a highly energetic helium nucleus.
• It's emitted when the neutron-to-proton ratio is too low in an unstable atom.
• Alpha particles are positively charged, massive particles made of two protons and two neutrons.
• Atomic and mass numbers are conserved, resulting in a daughter atom with a different atomic number and mass number.
• Example: Uranium-238 decays to Thorium-234 plus an alpha particle.

Beta Decay

• Negative Beta Decay: A neutron transforms into a proton, electron (beta particle), and an antineutrino. This happens when the nucleus has extra neutrons.
• Positive Beta Decay: A proton converts to a neutron, positron (beta+ particle), and a neutrino. This occurs in nuclei with excess protons.

Gamma Decay

• Gamma rays are high-energy photons emitted from an excited nucleus.
• They have no mass or charge unlike alpha and beta particles.
• Gamma emission helps the nucleus to de-excite and reach a lower energy state.
• Example: Cobalt-60 undergoes beta-minus decay, then emits gamma photons to reach the ground state.

Electron Capture (EC)

• The nucleus captures an orbital electron.
• The electron combines with a proton, forming a neutron and emitting a neutrino.
• Similar to positive beta decay.

Radioactive Decay Law

• Radioactive nuclei decay randomly, independent of time.
• Decay follows an exponential law (Radioactive Decay Law).
• The number of radioactive nuclei decaying, dN, in a time dt is related to the total number of nuclei, N.
• λ (lambda) is the decay constant. It represents the probability of an atom decaying per unit time.
• The amount of decaying nuclei depends on the original number of nuclei and the time passed.

Activity

• Activity (A) is the rate of decay in a radioactive material.
• Measured in Curies (Ci) or Becquerels (Bq).
• 1 Ci = 3.7 x 10^10 Bq.
• 1 Bq = 1 disintegration per second.

Half-life

• Half-life is the time required for half of the radioactive nuclei to decay.
• It is related to the decay constant (λ).
• The figure shows the exponential decay of activity over time.
• After one, two, or n half-lives... (fraction of initial activity remains)

Average or Mean-Life (τ)

• Mean life (τ) is the average lifetime of all nuclei in a specific unstable atomic species.
• It's the sum of the lifetimes of all unstable nuclei divided by the initial number of unstable nuclei.
• Mean life is 1.443 times the half-life.