Phosphor X-ray Detectors
The physics of
X-ray conversion for
phosphors is essentially the same as
described for scintillators. Also, the key functional elements of any
phosphor-based detector are the same as for a scintillator. For astronomy,
the similarities end there. While large area single crystals of NaI and CsI
are valued for their stopping power in hard X-ray and
gamma-ray
astronomy, thin layers of phosphors are used to do high
resolution,
soft X-ray imaging. In fact, theoretically, phosphors can have the highest
spatial resolution of all photon counting X-ray imagers -- limited to a few
times
the phosphor grain size (about a micron) in films only a few grain layers
thick. This extraordinary resolution has yet to be achieved in any
practical sense.
Decay constants vary enormously with phosphor composition. A commonly used
material, P43 (the name for Gd2 O2 S(Tb)), has a light
decay time of a few hundred microseconds -- which severely limits its maximum
counting rate.
X-ray energy conversion efficiencies are comparable with those of NaI,
about 10 - 15 The same is true of
energy
resolution.
The X-ray absorption efficiency is extremely dependent on what kind of
phosphor you have and how thick it is. At 6 keV, P43 exceeds 85absorption
efficiency.
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