Download Chapter 3 - RadTherapy

Chapter 3
The Production of X-rays
X-rays have several basic properties:
 Are unaffected by gravity (they have no mass)
 Are unaffected by electric fields
 Are unaffected by magnetic fields
 Travel in straight lines
 Are exponentially attenuated by matter
 Cannot be focused
Collision Interactions: produce characteristic radiation
 Elastic: momentum and kinetic energy are conserved (brakes off)
 Inelastic: momentum is conserved, but kinetic energy is not. (brakes on)
Excitation: an increase in electron energy that raises an electron to a higher
energy level.
Ionization: an increase in energy great enough to remove an electron from the
atom; the basis for chemical changes and therefore biological effects.
Ion pair: negative electron and positively charged atom
Incoming electron that creates ion pair is deflected with less energy than it had
before the collision.
Characteristic Radiation: the potential energy difference between where the
outer orbital electron that moves starts and where it ends is given off as a
photon.
 Large: x-ray photon
 Small: light photon
The exact energy of the emitted photon is dependant on the atoms (electron shell
energy spacings of the target material- characteristic radiation/ characteristic xrays) that initially lost the electron.
Energy spectrum: a graph displaying the number of photons in small energy
intervals versus photon energy.
 Characteristic radiation from excited or ionized atoms produces a series of
photons with discrete energies (multiple lines)
 Bremsstruhlung: produce a continuous spectrum
Radiative Interactions: produce bremsstrahlung radiation
Bremsstrahlung: when an electron interacts with the positive charge of an
atomic nucleus, the nucleus’s electric field exerts force on the electron and
causes it to change direction or velocity this change causes a loss in kinetic
energy x-ray photon.
Photon produced with an energy equal to the energy loss of the electron.
In bremsstrahlung production the maximum energy of the x-ray equals the kinetic
energy of the incoming electrons.
The closer the electron comes to the nucleus, the stronger the electrostatic
attraction and the more the change of the electrons velocity.
Filters: used in medical x-ray beams to change their spectral characteristics,
reduce the fraction of low-energy x-rays that contribute unnecessary radiation to
the patient. Filters are chosen in order to deposit maximum energy in the
diseased tissues.
Hardening: to increase the average energy of photons.
Orthovoltage range: 150-400 kVp
Thoraeus filter: tin, copper and aluminum (example of orthovoltage beam filter)
Megavoltage beams use a transmission filter that serves to remove low energy
photons.
Beam direction depends on the energy of the incoming electrons.
 Low energies: most x-rays produced are emitted from the target at almost
right angles to the initial electron, have a great deal of angular spread.
 High energies: x-rays emitted in a more forward direction with less angular
spread, as energy increases- beam gets narrower and more intense in
forward direction.
Targets are made of high atomic number (Z) materials, which produce x-rays
when hit by high energy electrons.
Reflection/reflectance target: used so that the x-rays come off the target
unimpeded in directions useful for imaging.
Thick target: all incoming electrons are stopped in the target.
Electron kinetic energytargetelectromagnetic photon energy
Transmission target: photon energy comes off the target in the same direction
as the electrons enter.
In diagnostic tubes 99% energy transformed to heat.
An estimate of the fraction of the electron energy converted to photon energy:
f = 3.5 x 10-4 ZE
Z = atomic number of target
E = maximum energy of electrons in MeV
The minimum requirements for x-ray production are high speed electrons, means
to decelerate them rapidly (high voltage source, source of electrons and an x-ray
target)
The forms of energy involved in the production of an x-ray beam:
 Electrical energy: voltage
 Electrical potential energy: electrons at cathode
 Kinetic energy: high speed electrons before striking target
 Electromagnetic energy: photons produced in target
 Heat: in target and tube housing- due to loss of kinetic energy