Production and Characteristics of X- Rays.pdf

10 Production and Characteristics of X- Rays By: Abdullah Munqith Production and Characteristics of X- Rays Electromagnetic Radiation Electromagnetic radiation can be described in terms of a stream of photons, each traveling in a wave-like pattern at the speed of light. Each photon contains a certai...

10 Production and Characteristics of X- Rays By: Abdullah Munqith Production and Characteristics of X- Rays Electromagnetic Radiation Electromagnetic radiation can be described in terms of a stream of photons, each traveling in a wave-like pattern at the speed of light. Each photon contains a certain amount of energy. The different types of radiation are defined by the amount of energy found in the photons. Radio waves have photons with low energies while gamma-rays the most energetic of all. Electromagnetic Radiation Electromagnetic radiation can be expressed in terms of energy, wavelength, or frequency. Electromagnetic Radiation General Properties of all electromagnetic radiation Electromagnetic radiation can travel through empty space. They travel at speed of light (Speed of light : 2.99792458 x 108 m s-1) Consist of photons which are uncharged and nearly massless. They all have wavelike characteristics General Properties of all electromagnetic radiation λ=c/F where λ=wavelength. c=velocityof electromagnetic wave(3x108m/sec) F= Frequency General Properties of all electromagnetic radiation X - Ray: - It is an electromagnetic radiation with wavelength range (0.01 – 10 nm) Fundamental units (Review) Typical usage Current I ampere(A) Work/energy E joule(J) 1J = 1 kg*m2*s-2 Power P watt (W) 1W = 1 J/s kW Potential V volt(V) 1V = 1J/C kV mA Fundamental units (Review) Electron Volt (eV): e-accelerated through 1V attains an energy of 1eV. Used to describe energy of a single electron or photon or population of monoenergetic particles. X-rays X-rays are one of the main diagnostic tools in medicine since its discovery by Wilhelm Roentgen in 1895. Nowadays, X-rays with very high energy (around10 MeV) is used for therapeutic applications X-rays are produced when high energetic electrons interact with matter. The kinetic energy of the electrons is converted into electromagnetic energy by atomic interactions X-ray production Two types of electron interactions are responsible for x-rays production. 1. 2. Bremsstrahlung mechanism Characteristic mechanism Bremsstrahlung “braking radiation” Bremsstrahlung braking radiation Bremsstrahlung “braking radiation” Source Acceleration Deceleration Electrons Bremsstrahlung “braking radiation” Bremsstrahlung “braking radiation” Bremsstrahlung “braking radiation” Bremsstrahlung “braking radiation” Bremsstrahlung “braking radiation” Bremsstrahlung “braking radiation” Bremsstrahlung “braking radiation” Bremsstrahlung “braking radiation” Bremsstrahlung “braking radiation” Bremsstrahlung “braking radiation” Bremsstrahlung “braking radiation” Bremsstrahlung “braking radiation” An electron interaction with nucleus to produce Bremsstrahlung Bremsstrahlung “braking radiation” Bremsstrahlung spectrum created from 90 keV electrons Bremsstrahlung “braking radiation” Incoming electron is both energetic (high kinetic energy) and charged (negative). This electron comes within the proximity of a positively charged nucleus in the target. Columbic forces attract and decelerate the electron, causing a significant loss of kinetic energy and a change in the electron's trajectory. Bremsstrahlung “braking radiation” An x-ray photon with energy equal to the kinetic energy lost by the electron is produced (conservation of energy). This radiation is termed Bremsstrahlung “braking radiation” The X-ray energy depends on the interaction distance between the electron and the nucleus. A direct collision of an electron with the target nucleus results in loss of all of the electron’s kinetic energy —> the highest x-ray energy is produced ( very low probability) Bremsstrahlung “braking radiation” Characteristic Radiation Energetic electrons can also interact with other electrons occupying orbital shells, which results in X-ray photons (Characteristic Radiation) Characteristic Radiation Characteristic Radiation Generation of a Characteristic X-ray in a target atom occurs in the following sequence: 1- The incident electron interacts with the K-shell electron via a repulsive electrical force 2- The K-shell electron is removed (only if the energy of the incident electron is greater than the K-shell binding energy), leaving a vacancy in the K-shell. 3- An electron from L- shell (or from a different shell) fills the vacancy. 4- A characteristic X-ray photon is emitted with an energy equal to the difference between the binding energies of the two shells. Characteristic Radiation Resultant X-ray energy is dependent on the difference in energy levels between electron orbits For a given target material (i.e. tungsten), the orbital energy levels are constant. Therefore, the emitted X-rays have discrete energies that are characteristic of the element (i.e. the characteristic x-ray photons can identify the target material). X-ray production Not all of the lost kinetic energy is converted to photon energy Vast majority of energy is converted to thermal energy (heat)