NUCLEAR PHYSICS

Atomic Models

Thomson's Model (Plum Pudding Model)

Proposed by J.J. Thomson in 1911, this model suggested that the atom is like a plum pudding. Electrons (plums) are embedded in a sphere of positive charge (pudding), balancing the atom's overall neutrality. Thomson’s model was later replaced because it didn’t explain the results of Rutherford’s gold foil experiment, which led to the discovery of the nucleus. The inability to explain the existence of a concentrated mass at the center of the atom led to the development of more accurate models.

Rutherford's Model (Nuclear Model)

In Rutherford's gold foil experiment, alpha particles were fired at thin gold foil. Most alpha particles passed straight through, suggesting that the atom is mostly empty space. A few were deflected at small angles, and some even bounced back, indicating a concentrated positive charge at the center, which was identified as the nucleus. This led to the proposal that:

• The atom is mostly empty space.

• The nucleus is small, dense, and positively charged.

• Electrons revolve around this nucleus in defined orbits.Rutherford’s discovery laid the foundation for modern atomic theory, later refined by Bohr's model, which introduced quantized orbits for electrons, shaping our understanding of atomic structure.

Nuclear Radiation

Types of Radiation

Alpha (α) Radiation

Alpha particles consist of 2 protons and 2 neutrons (a helium nucleus), with a 2+ charge.

• Penetration: Alpha particles have low penetration due to their larger mass and slower speed. They can be stopped by 3 cm of air or a sheet of paper.

• Ionization: They are highly ionizing due to their large mass and strong positive charge. As they interact with materials, they lose energy quickly, causing significant ionization in a short distance.

• Deflection in Magnetic Field: Alpha particles are deflected eastward due to their positive charge and relatively larger mass compared to beta particles.

• Applications: Due to their strong ionizing power, alpha radiation is used in smoke detectors, where it interacts with air particles to detect changes in the environment.

Beta (β) Radiation

Beta particles are high-energy electrons formed when a neutron in the nucleus decays into a proton and a beta particle (electron).

• Penetration: Beta particles have moderate penetration and can be stopped by 3 mm of aluminum foil.

• Ionization: They are moderately ionizing because they interact less than alpha particles but still affect the material before losing energy.

• Deflection in Magnetic Field: Beta particles are deflected at large angles, moving to the eastward due to their negative charge and smaller mass compared to alpha particles.

• Applications: Beta radiation is used in radiation therapy to treat certain types of cancer because it can penetrate deeper into tissues compared to alpha particles.

Gamma (γ) Radiation

Gamma rays are high-energy photons (light particles) emitted when the nucleus of an atom transitions to a lower energy state after alpha or beta decay.

• Penetration: Gamma rays have high penetration and can pass through most materials, requiring 30 cm of lead or 6 feet of concrete to be stopped.

• Ionization: They are low ionizing because they interact minimally with the material.

• Deflection in Magnetic Field: Gamma rays are unaffected by magnetic fields because they are neutral and have no mass.

• Applications: Gamma radiation is used in medical imaging, such as PET scans, and radiotherapy for treating cancer, due to its ability to penetrate deep into tissues without causing significant damage to surrounding cells.

Ionizing Ability, Penetration, and Magnetic Behavior

• Ionization: Describes how strongly a particle can interact with atoms, knocking electrons loose and creating ions. Alpha particles, with their large mass and charge, are strongly ionizing, while gamma rays, being neutral, are weakly ionizing.

• Penetration: Describes how far a particle can travel through a material before being stopped. Gamma rays, being massless, have high penetration, while alpha particles, with their larger mass, have low penetration.

• Magnetic Behavior: Charged particles (alpha and beta) are deflected by magnetic fields, but gamma rays, being neutral, are not affected. The differing ionizing abilities and penetration powers of these radiations make them suited for various uses in medicine, industry, and research. For example, alpha radiation is ideal for smoke detectors due to its strong ionization, while gamma radiation is used for medical treatment and imaging because it can pass through the body without damaging tissues as much as alpha or beta radiation.

Half-Life

Half-life is the time required for half of the atoms in a sample of a radioactive isotope to decay. This process occurs at a constant rate, although individual decays are random.

• Real-world application: Half-life is crucial in techniques like carbon dating, which is used to determine the age of ancient artifacts and fossils by measuring the amount of carbon-14 left in them. The known half-life of carbon-14 (about 5730 years) allows scientists to calculate the age of organic materials with remarkable precision.

• Use in Nuclear Medicine: Half-life is essential in radiotherapy, where isotopes like Iodine-131 are used to treat thyroid cancer. The half-life of Iodine-131 is about 8 days, meaning it stays active just long enough to be effective in treatment while minimizing exposure to healthy tissues.

Radioactive Decay

Radioactive decay is the spontaneous breakdown of an unstable atomic nucleus, releasing radiation (alpha, beta, or gamma) in the process. This occurs to achieve a more stable atomic configuration. The stability of an isotope depends on the neutron-to-proton ratio in the nucleus. Isotopes with too many or too few neutrons compared to protons are unstable and undergo radioactive decay.

• Real-world application: Radioactive decay is used in nuclear power plants where the decay of isotopes like Uranium-235 provides a steady source of energy. The decay of radioactive elements is also crucial in nuclear medicine for diagnostic imaging and treatment.

Discovery of the Neutron

The neutron was discovered by James Chadwick in 1932 after bombarding boron with alpha particles. This experiment revealed an uncharged particle with a similar mass to a proton. The discovery of the neutron explained the extra mass in the nucleus, as protons alone could not account for the total mass of an atom. The neutron helps stabilize the nucleus by reducing the repulsive forces between protons, which all have the same positive charge. Without neutrons, the nucleus would be unstable due to the repulsion between protons.

Conclusion: Connections and Real-World Applications

Rutherford’s nuclear model and the subsequent discovery of the nucleus revolutionized atomic theory by introducing the idea that most of an atom’s mass is concentrated in a tiny, dense nucleus, with electrons moving around it. The properties of alpha, beta, and gamma radiation determine their usefulness in various industries —from medical imaging and cancer treatment (gamma radiation) to smoke detectors (alpha radiation) and radiation therapy (beta radiation). Half-life and radioactive decay play a significant role in nuclear energy, radiotherapy, and archaeological dating, demonstrating the relevance of these concepts beyond the classroom.