 On x-ray
exposure the latent image is formed by trapping electrons in the
screen, half of the energy disappears immediately in the form of
light, the remaining is trapped in metastable storage states in the
screen.
When exposed to X-rays, the europium
atoms in the phosphor crystalline lattice are ionized (converted
from 2+ to 3+), liberating a valence electron. These electrons are
raised to a higher energy state in the conduction band (see solid
and photoconduction for an explanation of conduction band). Once in
the conduction band, the electrons travel freely until they are
trapped in a so-called F-centre in a metastable state with an energy
level slightly below that of the conduction band, but higher than
that of the valence band. The number of trapped electrons is
proportional to the amount of X-rays absorbed locally. The trapped
electrons constitute the latent image. Due to thermal motion, the
electrons will slowly be liberated from the traps, and the latent
image should therefore be read without too much delay. At room
temperature, the image should, however, be readable up to 8 hours
after exposure.
The wide-latitude response of the storage phosphors means that a
high-quality image can be produced regardless of the relative amount
of exposure to the plate. The laser beam in the CR reader scans
across the imaging plate, the phosphors are excited and release the
energy they have stored. This energy is emitted from the plate as a
violet blue glow. The strength of this glow is directly proportional
to the amount of radiation absorbed. The phosphor glow is captured
in the scanner and converted into a digital image. Because visible
(“photo”) light excites (“stimulates”) the phosphors to glow (“luminesce”),
this process is known as photo stimulated luminescence and the
phosphors are often called photostimulable phosphors or storage
phosphors.
The laser scan does not extract all the energy stored in the
crystals so the storage phosphor plate must be erased. After the
plate is scanned it enters the eraser where it is flooded with
bright fluorescent light. This intense light removes any residual
energy remaining on the plate so that it can be used again. At the
end of the erase cycle, the operator removes the storage phosphor
plate from the eraser drawer and reloads it into a cassette. There
is no evidence that the storage phosphors’ exposure or erasure
response changes over time.
|
