Binding Energy in Radiography PDF

Welcome to HS1934 Lecture for Introduction to the Diagnostic and Therapeutic Radiography Students Please ensure that this is the session you are scheduled to attend. If not, please ask the lecturer so they can help redirect you. Thank you. This is expected to be a busy session today: Plea...

Welcome to HS1934 Lecture for Introduction to the Diagnostic and Therapeutic Radiography Students Please ensure that this is the session you are scheduled to attend. If not, please ask the lecturer so they can help redirect you. Thank you. This is expected to be a busy session today: Please fill up all the seats – do not leave spaces or put bags on seats Where possible, please fill up the room with seats furthest away from the entrance doors first to allow space for those who arrive later to come in and find a seat Basics of Binding Energy and Ionisation of atoms Introduction to the Diagnostic and Therapeutic Radiography Professions Module code HS1934 Dr Benard Ohene-Botwe Senior Lecturer Diagnostic Radiography [email protected] Tel: +44 (0)20 7040 4387 Objective This session will cover the definition and significance of nuclear binding energy, ionisation process and energy, factors affecting ionisation energy to prepare the students for ionisation in X-ray production and interactions with matter.  Atomic Structure An atom is the smallest part of a substance that cannot be broken down chemically.  Atomic Mass Atomic mass (also called atomic weight) is the mass of an atom. It can also be defined as the quantity of matter contained in an atom. It reflects the mass of an atom’s protons, neutrons, and electrons, although the electrons contribute very little due to their extremely small mass. Therefore, it is roughly equal to the number of protons plus the number of neutrons in the nucleus (the mass number)  Atomic Mass Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit called amu. The unit of atomic mass is the atomic mass unit (amu), also called dalton (Da or u). One atomic mass unit (amu) is defined as one-twelfth the mass of a carbon-12 atom. In terms of kilograms, the mass of 1 amu is ≈1.66×10−27 kg  Atomic Mass Typical masses of a proton, neutron, and electron expressed in atomic mass units (u) Protons and neutrons have nearly the same mass, with the neutron being slightly heavier. The electron’s mass is extremely small compared to protons and neutrons Watch the first few minutes of the video Atomic mass defect Atomic mass defect  Converting mass defect into binding energy To convert the mass defect (amu or u) into energy to determine the binding energy, we use Einstein's mass-energy equivalence formula: E = Δmc2 or E = Δm x c2 Where: 𝐸 is the binding energy (in joules or MeV), Δ𝑚 is the mass defect (in kilograms or atomic mass units, u), 𝑐 is the speed of light, 3×108 m/s.  The difference in mass is the mass defect, which corresponds to the binding energy that holds the nucleus together. A larger mass defect corresponding to a more tightly bound (more stable) atom. Hence binding energy can be defined as the energy that holds the atom together. It is also referred to as the minimum energy needed to disassemble an atom into individual parts. Electron binding energy is the measure of the energy with which an electron is bound to the nucleus or its atomic orbital.  Significance of binding energy in Radiography 1. The nuclear binding energy is a key factor in determining the stability of an atomic nucleus, with higher binding energies indicating greater stability and lower binding energies correlating with increased susceptibility to radioactive decay. This concept is particularly important when considering the nuclear binding energies of different isotopes, as they influence their stability and radioactive properties. Understanding these properties is crucial for selecting appropriate radioisotopes used in nuclear medicine imaging techniques such as PET and SPECT.  Significance of binding energy in Radiography 2. The concept of the binding energy of electrons within an atom plays a major role in the production of characteristic X-rays. 3. The concept of binding energy also determines how X-rays and gamma rays (γ-rays) interact with matter.  Ionisation process and energy Ionization is defined as the process by which an atom or molecule gains or loses electrons. When an atom undergoes ionisation, it becomes a charged atom or molecule called an ion. Atoms become positively charged cations when they lose electrons and negatively charged anions when they gain electrons Ionization energy is the amount of energy required to remove an electron from an atom or ion in the gaseous state. This energy is typically equal to or greater than the binding energy of the electron being removed, as binding energy refers to the energy that holds the electron in its orbital.  Factors affecting ionisation energy 1. The distance of electron from the nucleus Explanation: Electrons that are closer to the nucleus are more tightly bound to the atom (higher binding energy). This means they require more ionisation energy to remove them from the atom. The orbital shell closest to the nucleus is the K-shell. Next is L- shell then M, N,O P…. Larger atoms have electrons that are farther from the nucleus, resulting in a weaker electrostatic attraction between the nucleus and the outermost electrons. This makes it easier to remove an electron. Therefore, as atomic size increases, ionization energy generally decreases.  Factors affecting ionisation energy 2. Higher nuclear charge The higher the nuclear charge, the higher the ionisation energy Nuclear charge is a measure of how positive the nucleus is An atom with fewer electrons than protons will have a positive nuclear charge because protons are positively charged. An increase in the number of protons (nuclear charge) results in higher ionisation energy as the protons in the nucleus attract electrons more strongly. 3. Number of Electrons: For a given element, successive ionisation energies increase as more electrons are removed. Explanation: As electrons are removed, the positive charge of the nucleus increasingly attracts the remaining electrons, making it more difficult to remove subsequent electrons. Hence, higher ionisation energy would be required. 4. Electron shielding effect Electron shielding refers to the effect in which the attraction between an electron and the nucleus decreases in atoms with more than one electron, particularly when electrons are present in different shells. In a multi-electron atom, the inner electrons (those closer to the nucleus) "shield" the outer electrons from the full charge of the nucleus. This shielding weakens the attraction between the nucleus and the outermost electrons, as the inner electrons partially block the nuclear pull. Since the outer electrons experience less attraction to the nucleus, they are easier to remove. As a result, less ionisation energy is required to remove the outermost electrons. QUESTIONS School of Health & Psychological Sciences City St George’s, University of London Northampton Square London EC1V 0HB United Kingdom T: +44 (0)20 7040 5060 E: [email protected] www.city.ac.uk/department