Lecture 5: Natural Radioactivity

Natural Radioactivity

Introduction to Natural Radioactivity

  • Nucleogenesis produces nuclides which can be either stable or unstable.

  • Unstable nuclei decay through various mechanisms involving the release of high kinetic energy particles or gamma (γ) radiation.

  • High-energy products of these processes are referred to as radioactivity.

Major Radioactive Decay Mechanisms

  1. Alpha (α) Decay

    • General Equation Representation:
      {}^{212}{83}Bi ightarrow {}^{4}{2}He + {}^{208}_{81}Tl

    • Definition: α decay is the process in which an α particle, basically a helium nucleus, is emitted from a parent nucleus.

    • Key Features of α Particle:

      • Mass = 4

      • Charge = 2+

    • This decay is exemplified by uranium-238:

      • {}^{238}{92}U ightarrow {}^{4}{2}He + {}^{234}_{90}Th

      • In this reaction, uranium transmutates into thorium, emitting an α particle.

    • Characteristics of α Particle:

      • Symbol: {}^{4}_{2}He

      • Composition: 2 protons and 2 neutrons (total mass number of 4).

    • Another example of α decay is thorium-230: {}^{230}_{90}Th .

  2. Beta (β) Decay

    • Definition: β (or β-) is an electron emitted from the nucleus as a result of nuclear decay where one neutron is converted into a proton to balance charge.

    • Example General Equation:
      {}^{12}{5}B ightarrow {}^{12}{6}C + {}^{0}_{-1}e

    • Key Features:

      • β particle is essentially an electron that balances nuclear charge post decay.

  3. Positron Emission (β+)

    • Definition: A positron (β+) is emitted from the nucleus when a proton is converted into a neutron.

    • General Equation Example:
      {}^{12}{7}N ightarrow {}^{12}{6}C + {}^{0}_{+1}e

    • Antiparticle Interaction: Positron often collides with an electron leading to annihilation:
      e^{+} + e^{-}
      ightarrow ext{γ-rays}

  4. Electron Capture

    • Definition: This involves a nucleus capturing an electron leading to a decrease in atomic number, often followed by emissions of x-rays.

    • Characteristics: X-rays are produced as other electrons transition to fill the vacancy left by the captured electron.

    • Note: X-rays are not typically classified as radioactivity, though they can cause radiation damage.

Nuclear Equation Considerations

  • To ensure proper atomic equations when inserting electrons:

    • Assigned mass number for an electron is zero (0).

    • Assigned atomic number for an electron is negative one (-1).

    • Representation of a beta particle (electron):
      {}^{0}_{-1}e

    • Example of Beta Decay:
      {}^{234}{90}Th ightarrow {}^{0}{-1}e + {}^{234}_{91}Pa

Gamma Radiation

  • Gamma rays are frequently emitted during nuclear reactions.

  • Example during U-238 decay:

    • {}^{238}{92}U ightarrow {}^{4}{2}He + {}^{234}{90}Th + 2 imes {}^{0}{0} ext{γ}

    • Additional gamma rays of various energies accompany the decay process.

Nuclear Stability

  • Factors affecting nuclear stability include:

    1. Size of the nucleus

    2. Composition of the nucleus (proton/neutron ratio) or N/P ratio.

  • Observations on Stable Nuclides:

    • All known stable nuclides exist in a zone of stability.

    • Zone is approximately where the N:Z ratio is close to 1, bending towards more neutrons as nucleus size increases.

  • Stability Rule:

    • Unstable isotopes decay towards the zone of stability and stabilize below {}^{209}_{83}Bi .

  • Periodic Table Observation:

    • The periodic table shows a band of stable isotopes including radioactive elements from lanthanides and actinides.

Graphical Representation of Decaying Isotopes

  • Radioactive decay sequences illustrated graphically show atomic number vs neutron number:

    • Alpha Decay: Decrease of two protons (Z) and two neutrons (N).

    • Beta Decay: Decrease of one neutron and increase of one proton.

    • Isotopes (same Z, different N) are depicted along vertical lines in the graph.

  • Example: Uranium-238 generates a family of daughter isotopes via decay series.

Radio Nuclides in Nature

  • Definition of Radio Nuclides:

    • Radioactive isotopes are present in the environment as a result of natural nuclear reactions or anthropogenic contamination.

1. Primary Radio Nuclides

  • Definition: Long-lived radio nuclides formed through nucleogenesis referred to as primary or primordial radio nuclides.

  • Characteristics:

    • About 20 primary radio nuclides exist, challenging to detect due to:

      • Long half-lives (T > 10^{10} years).

      • Isotopic abundance potentially below 1%.

      • Low energy emissions of alpha and beta particles.

      • Gamma photons being highly converted.

  • Implications:

    • They are often treated as stable nuclides in elemental isotopic compositions.

    • Example: Potential existence of primary 244Pu.

2. Isotopic Composition and Atomic Weights of Primary Radio Nuclides

  • Consequence of long-lived primary radio nuclides: progressive nucleogenesis of stable elements originated at the Big Bang.

  • Formation timeline:

    • Hydrogen as the oldest element formed.

    • Most elements formed approximately 5 imes 10^{9} years ago.

  • Continuous decay of parent nuclides alters isotopic composition and atomic weights over time.