Photostimulable phosphors

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Photostimulable phosphors (PSP) are materials that store absorbed energy within excited electrons and release it in the form of light on exposure to laser energy.

The process can be broken up as follows :

  • an x-ray or gamma photon interacts with the PSP and releases high energy secondary electrons through photoelectric effect or Compton scattering 
  • these secondary electrons excite surrounding electrons from their valence bands to their conduction bands through inelastic collisions. This continues until the secondary electron has lost all its kinetic energy
  • electrons excited to the conduction band fall into empty electron traps within the forbidden zone, which are created by impurities in the material
  • in order to return to the valence band, the electrons require excitation energy to overcome the trap
  • in PSP the energy required to return to overcome the trap is high enough that they require laser light energy to do so
  • the electron from the conduction band then returns to the valence band and due to the energy difference must lose energy in the form of an emitted photon.
    • Application

    PSPs are used to record and reproduce a latent x-ray image by absorbing the radiation, then releasing the stored energy as light photons when stimulated by a HeNe laser. The emitted photons are detected by a photomultiplier tube, and an electronic signal is produced which is converted to a digital image for viewing on PACS .

    Structure

    PSP materials are crystal lattices, which give near uniform characteristics to electron bands, and impurities, which alter the electron bands to induce electron traps in the forbidden zone. One of the materials of choice is barium fluorobromide doped with europium (2+ valence), however many others exist with their own unique properties.

    The material itself needs certain properties to satisfy a number of criteria in order for it to be useful in image acquisition:

    • release the stored energy when exposed to a wavelength produced by common lasers
    • release the stored energy in a photon wavelength readily absorbed by common photomultiplier tubes
    • retain the latent image without significant signal loss over time due to phosphorescence