Diagnostic Radiography: Binding Energy and Ionisation

Lecture Introduction to Diagnostic and Therapeutic Radiography Students

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• Fill all seats, prioritizing those furthest from the entrance doors

Basics of Binding Energy and Ionisation of Atoms

• Lecture topic: Introduction to the Diagnostic and Therapeutic Radiography Professions
• Module code: HS1934
• Lecturer: Dr Benard Ohene-Botwe, Senior Lecturer, Diagnostic Radiography
• Contact details: [email protected], +44 (0)20 7040 4387

Objective

• Defining and highlighting nuclear binding energy, ionisation and the factors affecting ionisation energy
• Preparing students to understand ionisation in x-ray production and its interactions with matter

Atomic Structure

• Atom: The smallest part of a substance that cannot be broken down chemically
• Nucleus: Contains protons and neutrons
• Orbiting electrons: Negatively charged particles orbiting the nucleus

Atomic Mass

• Atomic mass (atomic weight): The mass of an atom
• Calculation of atomic mass: Equal to the sum of protons and neutrons in the nucleus (mass number)
• Mass of electrons are negligible compared to protons and neutrons
• Unit: Atomic mass unit (amu) or Dalton (Da or u)
• Conversion: 1 amu ≈ 1.66x10⁻²⁷ kg

Typical Atomic Masses (in amu)

• Proton: 1.00728

• Neutron: 1.00867

• Electron: 0.00055

• Protons and neutrons have nearly the same mass; neutrons are slightly heavier

• The electron's mass is significantly smaller compared to the protons and neutrons

• Watch the first few minutes of the provided video on Binding Energy

Atomic Mass Defect

• Mass defect: The difference between the actual mass of an atom and the sum of the masses of its components (protons, neutrons, and electrons).
• Example, Helium: The mass of a Helium atom is 4.00260 amu, but the mass of its components is 4.03298 amu; the difference is thus the mass defect.
• Significance: The "missing" mass equals the binding energy holding the atom together.

Converting Mass Defect into Binding Energy

• Einstein's mass-energy equivalence formula: E = Amc² (or E = Am x c²)
• E: Binding energy (in joules or MeV)
• Am: Mass defect (in kilograms or atomic mass units, u)
• c: Speed of light (3×10⁸ m/s)

Binding Energy

• Binding energy: The energy that holds the nucleus together
• Higher mass defect corresponds to a more tightly bound (more stable) atom
• The minimum energy needed to break atom into its components

Electron Binding Energy

• Electron binding energy: The energy required to remove an electron from an atom or ion in the gaseous state
• It is typically equal to or greater than the binding energy of the electron

Factors Affecting Ionisation Energy

• Distance from the nucleus: Electrons closer to the nucleus are more tightly bound and require more energy to remove. Larger atoms have electrons farther from the nucleus
• Higher nuclear charge: A higher number of protons in the nucleus creates a stronger positive charge attracting electrons meaning more energy is required to remove them
• Number of electrons: As electrons are removed, the positive charge of the nucleus increases, attracting remaining electrons more strongly, and requiring more energy to remove subsequent electrons
• Electron shielding: Inner electrons shield outer electrons from the full nuclear charge, weakening the attraction and requiring less energy to remove the outer electrons

Ionisation Process and Energy

• Ionisation: The process where an atom or molecule gains or loses electrons, becoming an ion
• Ionisation energy: The energy needed to remove an electron from an atom or an ion in the gaseous state
• Ionisation diagram: Illustrates the energy involved, with Proton, Neutron, Electron and Atom shown

Significance of Binding Energy in Radiography

• Nuclear binding energy: Key factor determining atomic stability; higher binding energies indicate greater stability. Lower binding energies correlate with increased susceptibility to radioactive decay
• Isotope properties: Binding energies of isotopes affect their stability and radioactive properties
• Radioisotope selection: Understanding binding energy facilitates selection of appropriate radioisotopes for nuclear medicine imaging techniques (PET and SPECT)
• Electron binding energy: Plays a crucial role in characteristic X-ray production
• Interaction with X and gamma rays: Binding energies determine how X-rays and gamma rays interact with matter