The Imaging Plate is a flexible image sensor in which bunches of very small crystals (grain size: about 5 µm) of photo-stimulable phosphor of barium fluorobromide containing a trace amount of bivalent europium as a luminescence center, formulated as BaFBr: Eu²⁺, are uniformly coated on a polyester support film.   The composite structure of the Imaging Plate is shown in Figure 1.

Figure 1: Composite structure of the Imaging Plate

      Exposure of samples to the Imaging Plate is performed in a manner similar to that of photo-film.   The exposed Imaging Plate is scanned with a laser beam of red light while the plate is being conveyed with high accuracy in a phosphor reader as shown in Figure 2.

Figure 2: Principle of reading the radiation image from the Imaging Plate.
      The exposed Imaging Plate, while being conveyed, is scanned with a focused laser beam.   The PSL released upon the laser is collected into the photomultiplier tube (PMT) through the light collection guide and is converted to electric signals.

      Depending on the purpose, the reading density may be selected from 5 to 40 pixels/mm.   The reading sensitivity and sensitivity range can also be selected according to the purpose.   A bluish purple (400 nm) PSL, released upon laser excitation, is collected through the light collection guide to the photo-multiplier tube (PMT), and converted there to analog electric signals in chronological order.   Subsequently, these are converted to digital signals of 8 to 16 bits, again depending on the intended purpose.

      Image analysis and data processing are done on the CRT screen.   The processed image, if necessary, is printed either as a color or grayscale hard copy.   The image or data processings include those of image density/gradation, spatial frequency, operational reduction or addition between multiple numbers of images, and measurements of radiation dose, length or area.   Application calculation processing then becomes possible based on these data.   It is a particularly great advantage to quantify the image on the CRT as accurately as the scintillation counter method.   The Imaging Plate is reusable after erasing the residual latent image with uniformly irradiated visible light as shown in Figure 3.

Figure3: Process of recording, reading, erasing and reusing the radiation image by the Imaging Plate method.
        The exposed Imaging Plate is reusable after erasing the residual latent image with uniformly irradiated visible light.

      The BaFX: Eu²⁺ (X = Cl, Br or I) crystal is an ionic crystal having a tetragonal structure, and Ba is replaced with the Eu²⁺ ion to create a solid solution.   This crystal, when irradiated by radiation, for example, produces mainly two types of color centers in the crystal where an electron is trapped in an empty lattice of the F or X ion.   The color center actually produced mainly depends on the discrepancy between the stoichiometric composition of F and X.   The type of color center can be determined by comparing the theoretical value with the measured value from the electron spin resonance (ESR) spectrum by assuming an empty lattice for each anion.   Experiments carried out for the composition of X = Br reveal that the spectra of the PSL excitation process produced by visible rays after sufficient X-ray irradiation coincide well with the peak of the optical density, light current, ESR intensity and PSL intensity at the color center.   The relative change between the intensity of blue luminescence with the Eu²⁺ ion and that of red luminescence with the Eu³⁺ ion detected in a trace amount is also observed before and after the PSL excitation process.

      From these data, the luminescence mechanism of the BaFBr: Eu²⁺ photo-stimulable phosphor is interpreted as follows.   Part of the Eu²⁺ ions become Eu³⁺ ions through the primary excitation by X-rays, for example, with electrons being released into the conduction band.   These electrons are trapped into the Br ion empty lattices of the lattice defects, which are inherently present in the crystal, and color centers of the metastable state are formed.   When the PSL excitation light to be absorbed by the color center is irradiated, the trapped electrons are liberated again into the conduction band, or Eu³⁺ ions, becoming an excitation state of the Eu²⁺ ion, to release PSL.   The luminescence mechanism is schematically shown in Figure 4.

Figure 4: Imaging plate luminescence mechanism.
      An excited electron is trapped into the halogen ion empty lattice in the crystal, makes a color center having a metastable state, and emits the radiation energy.   Irradiation by the laser beam to be absorbed by the color center excites the electron again, and the recombination energy with the hole is transferred to the Eu ion, the luminescence center, resulting in luminescence.

      Although the PSL mechanism is unclear in some respects, research in the future shall well elucidate the mechanism.