Physics of Radiography X-ray production and Spectra

By the end of the session you should be able to: 1.Describe heat producing electron collisions 2.Describe x-ray producing collisions 3.Identify breaking radiation spectra 4.Identify characteristic spectra 5.Understand how different target atoms appear 6.Identify differences seen with different accelerating voltages

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E h x f h (Planck’s constant) h = x 10 Js -34 What energy would result from radiation with a frequency of 1Hz? What energy would result from radiation with a frequency of 1000Hz (sometimes written as 1x10 3 Hz) Energy here measured in Joules (J) Frequency measured in Hertz (Hz)

E h x f h (Planck’s constant) h = x 10 Js -34 Energy here measured in Joules (J) Frequency measured in Hertz (Hz) What energy would a typical radio wave photon have for absolute radio 1215kHz ( kHz or x 10 6 Hz)? What photon energy would a typical X-ray have if using a frequency of 1.5x10 18 Hz? ( Hz)

E h x f h (Planck’s constant) h = x 10 Js -34 Energy units in the atomic system are given in eV (electron volt) 1electron volt = 1.6 x Joules e.g. If I was given energy in Joules I would divide the energy amount by 1.6x and the answer would be the energy in electron volts. Look back at your previous answers and convert to electron volts

Low energy photons are most common produced by small deflections; these do not contribute to the useful x-ray beam so are filtered. Large deflections are least likely and the max photon energy is proportional to the voltage applied across the x-ray tube.

Heat producing collisions: Incoming electrons collide with and displaces outer shell tungsten electron Incoming electrons deflected by tungsten electrons 99% of incoming electron kinetic energy goes into heat producing electrons. These collisions are most common as there are many outer shell tungsten electrons available to interact with, this is why a good method for removing heat is required Each incoming electron can undergo a number of heat producing collisions X-ray producing collisions: Incoming electrons pass close to the nucleus and are slowed down, this deceleration causes a large loss of energy which is emitted in the form of x-rays  Since a wide range (spectrum) of energies is possible it is called the continuous spectrum of bremsstrahlung radiation Characteristic x-rays are emitted at specific energies for each element – each type of atom has electron with different energies. When an inner shell electron is excited (displaced to an outer shell) or ionised (displaced from the atom), the energy lost as electrons drops down into the vacant space is emitted in the form of x-rays. Spectra produced are characteristic of the atom and lines are produced depending on the shell from which the electron was emitted

High filament current = more electrons = higher intensity High tube voltage = greater electron kinetic energy = greater quality/penetrating power