Radiation and Nuclear Medicine Lecture Notes PDF

Biomedical Engineering – 4th Year Faculty of Engineering Sudan International University Radiation and Nuclear Medicine BOE4102 Atomic & Nuc...

Biomedical Engineering – 4th Year Faculty of Engineering Sudan International University Radiation and Nuclear Medicine BOE4102 Atomic & Nuclear Structure Lecture #02 Instructor: Dr. Emtithal Ahmed Doctor of Engineering Email: [email protected] 19/02/2023 Copyright by: Dr. Emtithal 1  Objective Ø To discuss about the structure of the atom including the Neutron, Proton and Electron Ø To learn about the Atomic Number, Atomic Mass, nuclear stability and radioactive or unstable nuclei 19/02/2023 2  Contents Ø Atom Ø Nucleus Ø Electron Binding Energy Ø Periodic Table of the Elements Ø Isotopes Ø Nuclear Stability 19/02/2023 3  Atom The atom is composed of: Ø positively charged (+) protons, Ø uncharged neutrons and Ø negatively charged (-) electrons 19/02/2023 4  Atom Thomson’s Model Rutherford’s Model 19/02/2023 5  Atom Bohr’s Model 19/02/2023 6  Nucleus Ø Protons and neutrons together form the nucleus of the atom. Ø The nucleus determines the identity of the element and its atomic mass. Ø Proton and neutrons have essentially the same mass but only the proton is charged while the neutron has no charge. 19/02/2023 7  Protons Ø Protons are positively charged particles found inside the nucleus of an atom. Each element has a unique atomic number (a unique number of protons). ØProton number never changes for any given element. For example, oxygen has an atomic number of 8 indicating that oxygen always has 8 protons. 19/02/2023 8  Neutrons Ø Neutrons are the other particle found in the nucleus of an atom. Unlike protons and electrons, however, neutrons carry no electrical charge and are thus "neutral." ØAtoms of a given element do not always contain the same number of neutrons. 19/02/2023 9  Electrons Ø Electrons are negatively charged particles that surround the nucleus in “orbits” similar to moons orbiting a planet. ØThe sharing or exchange of electrons between atoms forms chemical bonds which is how new molecules and compounds are formed. 19/02/2023 10  Atom Ex. Helium Shell proton N + - + - N electron neutron 19/02/2023 11  Electron Binding Energy (MeV) Ø Electrons exist in discrete “shells” around the nucleus (similar to planets around the sun) Ø Each shell represents a unique binding energy holding the electron to the nucleus Ø The shells are designated by letters (K, L, M, N …) where K, the shell closest to the nucleus, has the largest binding energy, so the K electron is the most tightly bound Ø Maximum number of electrons in each shell: 2 in K shell, 8 in L shell … Ø Einstein Introduced us to the equivalence of mass, m, and energy, E, at the atomic level using the following equation: E = m c2 Ø where c is the velocity of light.  Atom Mass Unit (amu) Where 1 amu is approximately equal to 1.6605 x 10-24 grams 19/02/2023 13  Atom Mass Unit (amu) Ø The atomic mass of the proton and the neutron is approximately: Proton = 1.6726 x 10-24 grams = 1.0073 amu Neutron = 1.6749 x 10-24 grams = 1.0087 amu Ø Thus, the neutron is just a little heavier than the proton. 19/02/2023 14  Atom Mass Unit (amu) Ø The difference in the mass of the neutron and the proton can be understood if we assume that the neutron is merely a proton combined with an electron forming a neutral particle slightly more massive than a proton alone. 19/02/2023 15  Atom Mass Unit (amu) Ø The atomic mass of the electron is approximately: Electron = 9.1094 x 10-28 grams = 0.00055 amu Ø Thus, the electron has a much smaller mass than either the proton or the neutron, 1837 times smaller or about 2000 times smaller. 19/02/2023 16  Summary of Atom Particle Symbol Mass (kg) Energy (MeV) Charge Proton p 1.672E-27 938.2 +1 Neutron n 1.675E-27 939.2 0 Electron e 0.911E-30 0.511 -1  Elements The number of protons in an atom dictate the element. For an uncharged atom, the number of 19/02/2023 electrons equals the number of protons. 18  10 Most Abundant Elements Element Symbol Protons Relative % of Earth’s Mass Oxygen O 8 46.6 Silicon Si 14 27.7 Aluminum Al 13 8.1 Iron Fe 26 5.0 Calcium Ca 20 3.6 Sodium Na 11 2.8 Potassium K 19 2.6 Magnesium Mg 12 2.1 Titanium Ti 22 0.4 Hydrogen H 1 0.1 19/02/2023 19  Periodic Table of the Elements Ø In 1869, Russian chemist Dmitri Mendeleev first described an arrangement of the chemical elements now known as the periodic table. Ø The periodic table displays all chemical elements systematically in order of increasing atomic number (the number of protons in the 19/02/2023 nucleus). 20  Periodic Table of the Elements 19/02/2023 21  Sample Element - Zirconium Electron Shell Configuration: 40 Zr Zirconium K 1s-2 2 91.2 L 2s-2 2p-6 8 M 3s-2 3p-6 3d-10 18 N 4s-2 4p-6 4d- 2 10 O 5s-2 2 10 + 18 + 12 = 40 K L M N O 19/02/2023 22  Sample Element - Zirconium Name: Zirconium Symbol: Zr Atomic Number: 40 Atomic Mass: 91.224 amu Melting Point: 1852.0 °C Boiling Point: 4377.0 °C No. of Protons/Electrons: 40 No. of Neutrons: 51 Classification: Transition Metal Phase at Room Temperature: Solid Density @ 293 K: 6.49 g/cm3 Color: Grayish Date of Discovery: 1789 19/02/2023 Discoverer: Martin Klaproth 23  Isotopes Ø Atoms of an element that have a different number of neutrons in the nucleus are called isotopes of each other. isotope notation Xy = element symbol typically written as: A Xy A = atomic mass (neutron + protons) Z Z = atomic number (protons) 19/02/2023 24  Isotopes The number of protons and electrons remain the same. But the number of neutrons varies. 19/02/2023 25  Isotopes equal number of protons and neutrons Ø There are many isotopes. Most have more neutrons than protons. Some are stable but most are unstable(radioactiv e) 19/02/2023 26  Atomic Structure 4 He Atomic mass The number of protons and neutrons in an atom Atomic number The number of protons in an atom 2 number of electrons = number of protons 19/02/2023 27  Atomic Structure 19/02/2023 28  Atomic Structure 19/02/2023 29  Atomic Structure ATOMIC NUMBER (Z) = number of protons in nucleus MASS NUMBER (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons ISOTOPS are atoms of the same element (X) with different numbers of neutrons in the nucleus Mass Number A Atomic Number Z X Element Symbol 1 2 3 1 H 1 H (D) 1 H (T) 235 238 92 U 92 U 19/02/2023 30  Two Isotopes of Sodium 19/02/2023 31  Nuclear Stability Ø A stable or non-radioactive nuclide is one whose atoms do not decay Ø If one plots the stable nuclei, an interesting pattern emerges (shown in next slide) Ø The graph in the next slide shows a plot of neutron number N vs atomic number Z for the stable nuclei 19/02/2023 32  The Line of Stability N>Z 19/02/2023 33  Nuclear Stability Ø For the heaviest stable nuclei, N is about 1.5 times Z Ø The presence of the extra neutrons overcomes the positively charged protons’ tendency to repel each other and disrupt the nucleus Ø The nucleus is held together by a poorly understood force, the Nuclear Force 19/02/2023 34  Nuclear Stability Ø The nuclear force is an extremely short-range force Ø It acts over a maximum distance of about two proton diameters Ø The nuclear force is responsible for the binding energy that holds the nucleus together 19/02/2023 35  Unstable Nuclei Ø Nuclei which do not fall on the line of stability tend to be unstable or “radioactive” Ø They are called “radionuclides” Ø A few radionuclides do fall on the line of stability but their rate of decay is so slow that for all practical purposes they are stable 19/02/2023 36  Unstable Nuclei Ø Radionuclides undergo a process called radioactive transformation or disintegration Ø In this process, the nucleus emits particles to adjust its neutron (N) to proton (Z) ratio Ø This change in the N to Z ratio tends to move the radionuclide toward the line of stability 19/02/2023 37  Some Common Radionuclides Ø Naturally occurring 235U and 238U Ø 60Co, 137Cs, 90Sr found in nuclear power plants Ø 192Ir used in radiography Ø 99mTc used in nuclear medicine Ø 131I used in treatment of thyroid conditions 19/02/2023 38  Summary Ø Atoms are composed of -protons – positively charged particles -neutrons – neutral particles -electrons – negatively charged particles Ø Protons and neutrons are located in the nucleus. Electrons are found in orbitals surrounding the nucleus. Ø Every different atom has a characteristic number of protons in the nucleus. atomic number = number of protons Ø Atoms with the same atomic number have the same chemical properties and belong to the same element. Ø Each proton and neutron has a mass of approximately 1 dalton. Ø The sum of protons and neutrons is the atom’s atomic mass. Ø Isotopes – atoms of the same element that have different atomic mass numbers due to different numbers of neutrons.  Summary 19/02/2023 40  Ø Radioactive Decay 19/02/2023 41