The Atomic Nucleus and Radioactivity - Detailed Notes

Objectives

  • Discovery of radioactivity
  • Definition of radioactivity
  • Types of radioactive decay
  • Origins and reasons for radioactivity in elements
  • Natural vs. artificial transmutation
  • Concepts of radioactive decay and half-life
  • Radiation detectors and measuring techniques
  • Effects of radioactivity on human health

Discovery of Radioactivity

  • X-ray Discovery

  • Wilhem Roentgen discovered X-rays (1895) via cathode rays striking glass tubes.

  • X-rays are high-energy electromagnetic waves produced from electronic transitions in atoms.

  • They penetrate soft tissue, creating images of bones.

  • Henri Becquerel's Experiment (1896)

  • Investigated spontaneous emissions similar to X-rays.

  • Used a black paper-wrapped photographic plate to detect emitted rays from various elements, discovering uranium emitted radiation.

  • Identified additional radioactive elements: thorium, actinium, polonium, and radium (via Marie and Pierre Curie).

Understanding Radioactivity

  • Definition of Radioactivity

  • Radioactivity is the spontaneous decay of an unstable atomic nucleus, emitting radiation (alpha, beta, gamma).

  • Types of Radiation Decay

  • Alpha (α) decay: Emit helium nuclei (2 protons, 2 neutrons).

    • Atoms reduce both atomic mass (A) and atomic number (Z) by 4 and 2 respectively.

  • Beta (β) decay: Neutrons transform into protons, emitting electrons.

    • A remains constant, Z increases by 1.

  • Gamma (γ) radiation: High-frequency electromagnetic wave emitted after alpha/beta decay without changing A or Z.

Radiation Properties

  • Penetration and Ionization Power

  • Alpha: High ionization, low penetration (stopped by paper).

  • Beta: Medium ionization, medium penetration (stopped by aluminum).

  • Gamma: Low ionization, high penetration (stopped by concrete/lead).

  • Deflection in Fields

  • Adjust their paths in electric and magnetic fields: alpha particles deflect towards negative, beta towards positive, gamma remain unaffected.

Sources of Radioactivity

  • Natural:
  • Cosmic rays from space, terrestrial radiation from soil elements (e.g., uranium), radon in air, and radioactive food/drink.
  • Artificial:
  • Man-made sources from industry, research, and medical applications.

Measuring Radioactivity

  • Conventional Units
  • Rem/Sievert (Sv): Measure biological risk of radiation.
    • 1 Sv = 100 rem.
  • Rad/Gray (Gy): Measure energy absorbed by tissue.
    • 1 Gy = 100 rad.
  • Curie (Ci)/Becquerel (Bq): Measure disintegration rates.
    • 1 Ci = 37 billion Bq.

Effects of Radioactivity

  • Health Risks
  • Ionizing radiation damages cells, leading to cancer, radiation sickness, or cellular mutations.
  • The most significant exposure is from natural sources (75%) vs. artificial (25%).

Radioactive Dating

  • Radiometric Techniques: Utilize known half-lives of isotopes (e.g., carbon-14) for dating organic materials.
  • Mathematics of Decay: Nf = (½)^n Ni; where Nf is final quantity, Ni is initial quantity, and n is the number of half-lives elapsed.

Transmutation of Elements

  • Natural Transmutation: Involves decay to achieve stability via alpha, beta emissions.
  • Artificial Transmutation: Changing an element into another through particle bombardment.

Summary on Stability

  • Nuclear Stability:
  • Generally, atoms with more than 82 protons are unstable due to increased repulsive forces between protons.
  • Stable isotopes often have a balanced neutron-proton ratio (1:1 for lighter elements, higher for heavier elements).