Chapter 1: Basic Radiation Physics PDF

# Chapter 1: Basic Radiation Physics ## Atomic Structure - Matter is composed of elements, such as hydrogen, oxygen, carbon, etc. - An atom is the basic component of each element. All atoms of a given element are identical. - An atom consists of a positively charged nucleus orbited by negatively...

# Chapter 1: Basic Radiation Physics ## Atomic Structure - Matter is composed of elements, such as hydrogen, oxygen, carbon, etc. - An atom is the basic component of each element. All atoms of a given element are identical. - An atom consists of a positively charged nucleus orbited by negatively charged electrons. - The nucleus is composed primarily of protons (positively charged) and neutrons (no charge, similar mass to protons). - The number of protons (atomic number, Z) determines the element's chemical properties and its position on the periodic table. - The total number of protons and neutrons (mass number, A) approximates the atomic weight. - The number of electrons equals the number of protons, making the atom electrically neutral. - An atom can be visualized as a solar system, with the sun representing the nucleus and planets representing the electrons. - An atom is incredibly small, with a diameter of the order of 10^-8 cm and a nucleus diameter of 10^-12 - 10^-13 cm. - The mass of an atom is concentrated in the nucleus, due to the negligible mass of electrons. - The mass of a proton is 1.6723 x 10^-24 g and the mass of a neutron is 1.67747 x 10^-24 g. - There are 92 naturally occurring elements, with each containing a progressively increasing number of protons, neutrons, and electrons. - Hydrogen is the simplest element, with one proton and one electron. ## Isotopes - Isotopes of an element have the same atomic number (number of protons) but different mass numbers (number of neutrons). - Isotopes of an element are chemically identical. - Example: isotopes of hydrogen include ^1_1H, ^2_1H, and ^3_1H. ## Radioactivity - The stability of an element depends on the ratio of neutrons to protons in the nucleus. - For lighter elements, excluding hydrogen, the neutron-to-proton ratio is generally 1:1. - Heavier elements tend to have a neutron-to-proton ratio greater than 1, resulting in instability and radioactive decay. - Radioactive decay occurs when an unstable nucleus releases energy in the form of radiation to achieve stability. - This phenomenon was discovered by Henry Becquerel in 1896, who observed that uranium emitted invisible radiant energy. - Madame Curie later coined the term "radioactivity" to describe this phenomenon. - Radioactivity is spontaneous and unaffected by physical or chemical factors. - Heavier elements are naturally radioactive, but lighter elements can be made radioactive by bombarding them with charged particles or neutrons. - Radioisotopes are isotopes of an element that exhibit radioactive properties. ## Specific Activity - Specific activity refers to the number of becquerels (or curies) per unit mass (or volume) of radioactive material. - It provides a measure of the activity of a sample per unit mass or volume. - The specific activity of pure elemental radium is 1 curie per gram or 3.7 x 10^10 Bq/gm. ## Modes of Radioactive Disintegration - Radioactive decay can occur through several modes, each involving the emission of specific particles or energy. - Alpha particles: - Consist of 2 protons and 2 neutrons (identical to a helium nucleus). - Emitted by higher atomic number elements. - Daughter nucleus has an atomic number 2 less and a mass number 4 less than the parent nucleus. - Alpha emission may be followed by gamma emission. - Alpha particles have a mass about four times that of a hydrogen nucleus and carry two units of positive charge. - They cause ionization in matter, but are not very penetrating and can be stopped by a thin sheet of paper. - Beta particles (β-): - Are electrons with negative charge. - A neutron decays into a proton, emitting an electron and an antineutrino. - The daughter nucleus has the same mass number but a different atomic number than the parent nucleus. - Beta particles have a range of a few meters in air, depending on their energy. - They cause ionization, but are less intense than alpha particles. - Beta particles are comparatively easily absorbed in matter, with penetrating power dependent on their energy and the atomic number of the absorber. - Positrons (β+): - Are electrons with positive charge. - They are emitted by the nucleus, with the decay of a proton into a neutron. - The daughter nucleus has the same mass number but a different atomic number than the parent nucleus. - Positron emission is not as common as β- emission. - Electron Capture: - An alternative to positron emission. - A proton captures an electron from an inner orbit, transforming into a neutron. - Daughter nucleus has an atomic number one less than the parent nucleus. - X-rays are emitted as an electron from a higher orbit falls into the inner orbit. - Gamma Rays: - Electromagnetic radiation similar to X-rays and visible light. - Emitted when the nucleus is in an excited state after emitting an alpha or beta particle. - Highly penetrating, with penetrating power increasing with energy. - No charge. - Ionize matter indirectly. ## Radioactive Decay - Radioactive decay is a statistical process, meaning it is impossible to predict when any individual atom will decay. - The rate of decay is constant, with the number of radioactive atoms decreasing exponentially over time. - This decay can be described by the equation: A = A₀e^(-λt) - A₀ is the initial activity. - A is the activity after time t. - λ is the decay constant. - Half-life (t1/2): - The time required for half the radioactive atoms to decay. - Expressed as t1/2 = 0.693/λ. - Varies for different isotopes, ranging from fractions of a second to thousands of years. - Used to identify isotopes and understand their radioactive properties. ## Effective Half-life - The effective half-life (Teff) considers both the physical half-life (Tp) of radioactive decay and the biological half-life (TB) of elimination from a system. - The equation for effective half-life is: 1/Teff = 1/Tp + 1/TB. ## Successive Radioactive Transformations - Decay Chains - A decay chain occurs when a parent nucleus decays into a daughter nucleus, which may also be radioactive. - The daughter nucleus then decays into another daughter nucleus, and so on, forming a chain. - Each decay in the chain has its own characteristic decay constant (λi). - Radioactive equilibrium occurs when the rate of production of a daughter nucleus equals the rate of decay of that daughter nucleus. - Two types of radioactive equilibrium: - Secular radioactive equilibrium: - Occurs when the parent nucleus has a very long half-life. - Each daughter nucleus decays at a rate determined by the long half-life of the parent nucleus. - Examples: uranium series, thorium series. - Transient radioactive equilibrium: - Occurs when the parent nucleus has a longer half-life than the daughter nucleus. - Both the parent and daughter nuclei decay at approximately the same rate. - Example: 99Mo-99mTc generator. ## Production of X-rays - X-rays are produced when high-energy electrons, accelerated by an electric field, interact with a target material. - Two main mechanisms of energy loss for electrons: - Collisional energy loss: Occurs when electrons interact with atomic electrons, causing ionization or excitation of the atom. - Radiative energy loss: Occurs when electrons interact with the electric field of atomic nuclei, producing electromagnetic radiation (bremsstrahlung). - Bremsstrahlung: - Continuous X-rays produced by the deceleration of electrons as they interact with atomic nuclei. - The energy of bremsstrahlung is proportional to the square of the atomic number of the target material and inversely proportional to the square of the mass of the electron. - Major component of X-ray production. - Contributes significantly to the x-ray output from an X-ray tube. - Characteristic x-rays: - Produced when an electron knocks out an inner shell electron from a target atom. - Energy is specific to the target atom and its electron configuration. - Less contributing to the overall x-ray spectrum than bremsstrahlung, but provide valuable information about the target material. ## High-Energy X-ray Production - Linear accelerators (LINACs) are used to accelerate electrons to high energies for cargo inspection and other applications. - Electrons are accelerated using radiofrequency (RF) fields or microwaves within a vacuum chamber. - High-energy electrons interact with a target material, such as tungsten, to produce X-rays capable of penetrating dense materials. ## Artificially Produced Radioisotopes - Radioisotopes are produced artificially by bombarding target materials with neutrons, charged particles, or photons. - Bombarding particles are generated by nuclear reactors or high-energy accelerators. ## Reactor-Produced Radioisotopes - Nuclear reactors are a primary source of radioisotopes. - Neutron activation: Target material is bombarded with neutrons within a reactor, causing the target nuclei to capture neutrons and become radioactive. - Fission products: Fission reactions in nuclear reactors produce various radioactive isotopes as fragments of the split nucleus. ## Accelerator-Produced Radioisotopes - Accelerators generate high-energy charged particles, such as protons or alpha particles, which are used to bombard target materials and induce nuclear reactions. - This method is essential for producing radioisotopes that cannot be effectively produced in reactors or through other methods. ## Neutron Sources - Neutron sources are essential for research, medical applications, and other fields. - Types of neutron sources: - Alpha-neutron sources: Use alpha decay to eject neutrons. - Gamma-neutron sources: Use gamma rays to eject neutrons. - Accelerator neutron sources: Utilize charged particles in accelerators to bombard targets and generate neutrons.