The image plate phosphor.



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The image plate phosphor.

The image plates themselves are those produced by the Fuji Photo Film Co. Ltd. and typically consist of a flexible sheet of plastic coated with a polycrystalline layer of in an organic binder. A final polyethylene terephthalate layer serves as a protective coating. The ability of this phosphor to act as an image storage device relies on the presence of the dopant, the presence of naturally occuring point defects in the crystal lattice and a phenomena known as Photostimulable Luminescence or PSL.

The most basic mechanism proposed for PSL was proposed by Takahashi et al. [105] and will be described briefly below. The energy level diagram for the mechanism is shown in Fig. gif.

When X-rays are incident on the phosphor they may further ionise the cations to . The fast electrons so produced populate the conduction band from where they may either drop back to recombine with cations or become trapped in colour centres.

  
Figure: Energy level diagram showing the excitation and de-excitation of electrons via PSL into centres in the crystal lattice of (taken from Seggern et al. [101]).

The colour centres of interest in PSL are known as centres and are caused by the absence of halogen anions from their designated position in the lattice. The resulting -centres are long lifetime traps into which electrons having enough energy to enter the conduction band may fall. Once an electron becomes trapped in an centre it may stay in that meta-stable state until it is again excited into the conduction band by exposure to visible light from where it may either fall back into an centre or recombine with cations with the emission of blue light at around .

The important factors which make this phosphor suitable as an image storage device are

  1. the number of centres populated by electrons from the excitation process is proportional to the X-ray dose received.
  2. the intensity of the luminescent light under PSL is proportional to the number of F-centres containing trapped electrons.
  3. the F-centres created by X-irradiation are generally long lived with a time constant of the order of a few hours at normal operating temperatures.



next up previous contents index
Next: Image readout and Up: Image plate detector. Previous: Image plate detector.



Gwyndaf Evans
Fri Oct 7 15:42:16 MET 1994