Physics of RadiographyX-ray production and Spectra
By the end of the session you should be able to:
1. Describe heat producing electron collisions2. Describe x-ray producing collisions3. Identify breaking radiation spectra4. Identify characteristic spectra5. Understand how different target atoms appear6. Identify differences seen with different accelerating
voltages
s
Eh x f
h (Planck’s constant) h = 6.626 x 10 Js
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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 1x103Hz)
Energy here measured in Joules (J)Frequency measured in Hertz (Hz)
Eh x f
h (Planck’s constant) h = 6.626 x 10 Js
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Energy here measured in Joules (J)Frequency measured in Hertz (Hz)
What energy would a typical radio wave photon have for absolute radio 1215kHz (1215000 kHz or 1.215 x 106 Hz)?
What photon energy would a typical X-ray have if using a frequency of 1.5x1018 Hz? (1500000000000000000Hz)
Eh x f
h (Planck’s constant) h = 6.626 x 10 Js
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Energy units in the atomic system are given in eV (electron volt)
1electron volt = 1.6 x 10 -19 Joules
e.g. If I was given energy in Joules I would divide the energy amount by 1.6x10-19 and the answer would be the energy in electron volts.
Look back at your previous answers and convert to electron volts
100,000 eVelectron fromfilament
8000 eVphoton
20,000 eVphoton 10,000 eV
photontargetnucleus
3000 eVphoton
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 intensityHigh tube voltage = greater electron kinetic energy = greater quality/penetrating power
