Wk 1 Radiation Physics PDF

Nature of radiation RADIATION – – Transmission of energy through space & matter – Either particulate or electromagnetic v 22  Particulate radiation Composed of nucleus/subatomic particles – high velocity E. g. – alpha...

Nature of radiation RADIATION – – Transmission of energy through space & matter – Either particulate or electromagnetic v 22  Particulate radiation Composed of nucleus/subatomic particles – high velocity E. g. – alpha rays, beta rays, cathode rays 23 ALPHA RAYS From decay of radioactive elements (Radium, Radon) Give up energy quickly penetrate few microns of body tissue v 24  Beta rays & cathode rays – High energy electrons – Smaller and lighter than α particles – Depth of penetration – 1.5 cm in tissue – Used in radiation therapy – Cathode rays are man-made v 25  Electromagnetic radiation Movement of energy through space – Electric + magnetic fields E. g.- gamma rays, x-rays, UV rays, visible light, infrared rays, microwaves, radio waves 26 Characteristics of X-Rays Invisible and undetectable by the senses No mass or weight or charge Travel at speed of light Travel in short-wavelength, high-frequency waves Travel in straight line and can be deflected or scattered v  They are Absorbed by matter Cause ionization and changes in living cells Can cause certain substances to fluoresce Can produce image on photographic film  Oil prevents overheating by absorbing the heat created by the production of x-rays. The tubehead seal, made of leaded glass or aluminum, keeps the oil in the tubehead and acts as a filter for the x-ray beam. The x-ray tube is where x-rays are produced Position indicator device The PID is the open-ended lead-lined cylinder that extends from the opening of the metal housing The collimator limits the size of the x-ray beam to a circular 2-inch opening. Towards the x ray tubehead, a small opening is present with an aluminum filter  The aluminum filter is 0.5 to 2.5 mm thick sheet located inside the PID in the path of the x-ray beam It filters out the non-penetrating, longer- wavelength x rays. X ray tube The x-ray tube is the heart of the x-ray– generating system. It is made of glass and is about 6 inches long and 1 inch in diameter. Air has been removed from the tube to create a vacuum. This vacuum environment allows electrons to flow with minimum resistance between the electrodes (cathode and anode)  Anode – Positive electrode made of tungsten target embedded in copper – The tungsten target called as focal spot - converts the bombarding electrons into x-ray photons. – Copper- a good conductor of heat – dissipates the heat produced Cathode – Negative electrode made of tungsten filament mounted on molybdenum cup (called as focusing cup)  Filament in cathode – Source of electrons – Placed inside focusing cup – Made up of TUNGSTEN wire -2mm diameter – Forms electron cloud by process of thermionic emission v 40 Extension arm folds up and can be swiveled from side to side. Never leave the extension arm in an extended position when the machine is not in use, because the weight of the tubehead can cause it to become loose. If the tubehead is loose, it can drift (slip out of position) after it is positioned for an exposure. This movement can cause a cone cut (central ray positioned off the film)  Types of X-Rays Produced – Primary radiation - Bremsstrahlung – “braking radiation” or General radiation – Characteristic radiation General (braking) radiation is produced when an electron – hits the nucleus of a tungsten atom or when an electron passes very close to the nucleus of a tungsten atom  Primary radiation - Kinetic energy of Bremsstrahlung – “braking electrons converted to x radiation” or General ray energy radiation v  Characteristic radiation is produced when a high-speed electron dislodges an inner-shell electron from the tungsten atom Accounts for a very small part of x-rays produced in the dental x-ray machine. It occurs only at 70 kVp and above because the binding energy of the K-shell electron is approximately 70 keV.