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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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