Medical Physics Lecture 10 (MD-PHS112) - International University of Africa

MEDICAL PHYSICS (MD-PHS112) International university of Africa Faculty of medicine(batch26)-2023&2024 Lecture 10 Radiation Outlines Some Properties of Nuclei Radioactivity The Decay Processes Natural Radioactivity Nuclear Magnetic Resonance and M...

MEDICAL PHYSICS (MD-PHS112) International university of Africa Faculty of medicine(batch26)-2023&2024 Lecture 10 Radiation Outlines Some Properties of Nuclei Radioactivity The Decay Processes Natural Radioactivity Nuclear Magnetic Resonance and Magnetic Resonance Imaging Radiation Exposure Radiation Damage Radiation Detectors Uses of Radiation X-ray Laser & MRI 2 3 4  Isotopic Notation Every atom of an element has the same number of protons. Atomic number (Z) Atoms of the same elements can have different numbers of neutrons, isotopes. Isotopes are identified by their mass number (A). Mass number = number of protons + neutrons 5  Isotopic Notation The number of neutrons is calculated by subtracting the atomic number from the mass number. When discussing nuclear properties, the nucleus is called nuclide. Each nuclide is identified by a symbol indicating the mass number and atomic number. 6 Important Atomic Symbols 7  Nuclear Equations We describe nuclear processes with nuclear equations. Atomic numbers and mass numbers are conserved. The sum of the atomic numbers on both sides must be equal. The sum of the mass numbers on both sides must be equal. 8 9 10  Mass Defect and Nuclear Binding Energy When a nucleus forms, some of the mass of the separate nucleons is converted into energy. The difference in mass between the separate nucleons and the combined nucleus is called the mass defect. The energy that is released when the nucleus forms is called the binding energy. 1 MeV = 1.602 × 10−13 J 1 amu of mass defect = 931.5 MeV The greater the binding energy per nucleon, the more stable the nucleus. Mass Defect and Binding Energy Mass Defect and Binding Energy Mass lost (m) = 4.03298 amu – 4.00260 amu Mass defect (m) = 0.03038 amu Binding energy (E) = 2.731 × 1013 J/mol 15 Radioactivity and Nuclear Physics Radioactivity is the emission of subatomic particles or high-energy electromagnetic radiation by the nuclei of certain atoms. Atoms that emit radiation are said to be radioactive. 16  Medicine and Other Applications Radioactivity has numerous applications, especially in medicine. Nuclear radiation is used for the diagnosis and treatment of medical conditions. Most radioactive emissions can pass through many types of matter (such as body tissue). Naturally occurring radioactivity is used to estimate the age of fossils, rocks, and ancient artifacts. Radioactivity led to the discovery of nuclear fission, used for electricity generation and nuclear weapons. 17 18  Types of Radioactive Decay Rutherford discovered three types of rays: – Alpha (a ) rays Have a charge of +2 and a mass of 4 amu What we now know to be helium nucleus – Beta (b) rays Have a charge of −1 c.u. and negligible mass Electron-like – Gamma (g) rays Form of light energy (not a particle like a and b) In addition, some unstable nuclei emit positrons. – Like a positively charged electron Some unstable nuclei will undergo electron capture. – A low energy electron is pulled into the nucleus. 19  Alpha Decay Occurs when an unstable nucleus emits a particle composed of two protons and two neutrons Most ionizing but least penetrating of the types of radioactivity Loss of an alpha particle means – the atomic number decreases by 2, and – the mass number decreases by 4. 20 21  Beta Decay Occurs when an unstable nucleus emits an electron About 10 times more penetrating than α but only about half the ionizing ability When an atom loses a β particle its – atomic number increases by 1, and – the mass number remains the same. 22 23 Gamma Emission Gamma (g) rays are high-energy photons of light. No loss of particles from the nucleus No change in the composition of the nucleus – Same atomic number and mass number Least ionizing but most penetrating Generally occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange 24 Penetrating Ability of Radioactive Rays a b g 0.01 mm 1 mm 100 mm Pieces of Lead 25 Positron Emission Positron has a charge of +1 and a negligible mass. – Antiparticle of electron Similar to beta particles in their ionizing and penetrating ability When an atom loses a positron from the nucleus, its – mass number remains the same, and – its atomic number decreases by 1. Positrons result from a proton changing into a neutron. 26 27 Electron Capture Occurs when an inner orbital electron is pulled into the nucleus No particle emission, but atom changes – Same result as positron emission When a proton combines with the electron to make a neutron, its – mass number stays the same, and – its atomic number decreases by 1. 28 29 What Causes Radioactivity? The particles in the nucleus are held together by a very strong attractive force found only in the nucleus called the strong force. Acts over only very short distances Protons and neutrons are called nucleons. The neutrons play an important role in stabilizing the nucleus, since they add to the strong force but don’t repel each other like the protons do. 30 N/Z Ratio The ratio of neutrons : protons is an important measure of the stability of the nucleus. If the N/Z ratio is too high, neutrons are converted to protons via b decay. If the N/Z ratio is too low, protons are converted to neutrons via positron emission or electron capture. Or via a decay, though not as efficiently 31 Valley of Stability For Z = 1 ⇒ 20, stable N/Z ≈ 1. For Z = 20 ⇒ 40, stable N/Z approaches 1.25. For Z = 40 ⇒ 80, stable N/Z approaches 1.5. For Z > 83, there are no stable nuclei. 32  Decay Series Atoms with Z > 83 are radioactive. In nature, often one radioactive nuclide changes into another radioactive nuclide. All of the radioactive nuclides are produced one after the other until a stable nuclide is reached. This process is called a decay series. 33 U-238 Decay Series 34  Detecting Radioactivity Particles emitted by radioactive nuclei have a lot of energy and, therefore, can be readily detected. Thermoluminescent dosimeters contain crystals of salts that are excited by the ionizing radiation. When the crystals are heated, the excited electrons relax to their ground state, emitting light. The amount of light emitted is proportional to the radiation exposure.  Detecting Radioactivity Radioactive rays cause air to become ionized. A Geiger-Müller counter works by counting electrons generated when Ar gas atoms are ionized by radioactive rays.  Detecting Radioactivity Radioactive rays cause certain chemicals to give off a flash of light when they strike the chemical. A scintillation counter is able to count the number of flashes per minute.  Kinetics of Radioactive Decay The rate of change in the amount of radioactivity is constant and is different for each radioactive “isotope.” Each radionuclide has a particular length of time it requires to lose half its radioactivity—a constant half- life. – Constant half-life follows first-order kinetics. Unlike chemical reactions, the rate of radioactive change is not affected by temperature. 38 Kinetics of Radioactive Decay Rate = kN N = number of radioactive nuclei The shorter the half-life, the more nuclei decay every second; therefore, we say the sample is “hotter.” 39 40 41 42 43 Radiocarbon Dating All things that are or were once alive contain carbon. Three isotopes of carbon exist in nature, one of which, C-14, is radioactive. – C-14 radioactive with half-life = 5730 years Atmospheric chemistry keeps producing C-14 at nearly the same rate it decays.  Radiocarbon Dating While still living, 14C/12C is constant because the organism replenishes its supply of carbon. – CO2 in the air is the ultimate source of all C in an organism. Once the organism dies, the 14C/12C ratio decreases. The half-life of 14C is 5715 years. By measuring the 14C/12C ratio in a once-living artifact and comparing it to the 14C/12C ratio in a living organism, we can tell how long ago the organism was alive. The limit for this technique is 50,000 years old. – About 9 half-lives, after which radioactivity from 14C will be below background radiation  Radiometric Dating Radiocarbon dating can measure only relatively young objecst,