An indirect flat-imager with programmable avalanche gain and field emitter array (FEA) readout is being
investigated for low-dose x-ray imaging with high resolution. It is made by optically coupling a structured
x-ray scintillator CsI (Tl) to an amorphous selenium (a-Se) avalanche photoconductor called HARP
(high-gain avalanche rushing photoconductor). The charge image created by HARP is read out by
electron beams generated by the FEA. The proposed detector is called SAPHIRE (Scintillator Avalanche
Photoconductor with HIgh Resolution Emitter readout). The avalanche gain of HARP depends on both a-Se thickness and applied electric field ESe. At ESe of > 80 V/μm, the avalanche gain can enhance the signal
at low dose (e.g. fluoroscopy) and make the detector x-ray quantum noise limited down to a single x-ray
photon. At high exposure (e.g. radiography), the avalanche gain can be turned off by decreasing ESe to <
70 V/μm. In this paper the imaging characteristics of the FEA readout method, including the spatial
resolution and noise, were investigated experimentally using a prototype optical HARP-FEA image
sensor. The potential x-ray imaging performance of SAPHIRE, especially the aspect of programmable
gain to ensure wide dynamic range and x-ray quantum noise limited performance at the lowest exposure
in fluoroscopy, was investigated.
An indirect flat-panel imager (FPI) with avalanche gain is being investigated for low-dose x-ray imaging. It is made by optically coupling a structured x-ray scintillator CsI(Tl) to an amorphous selenium (a-Se) avalanche photoconductor called HARP. The final electronic image can be read out using either an array of thin film transistors (TFT) or field emitters (FE). The advantage of the proposed detector is its programmable gain, which can be turned on during low dose fluoroscopy to overcome electronic noise, and turned off during high dose radiography to avoid pixel saturation. This paper investigates the important design considerations for HARP such as avalanche gain, which depends on both the thickness dSe and the applied electric field ESe. To determine the optimal design parameter and operational conditions for HARP, we measured the ESe dependence of both avalanche gain and optical quantum efficiency of an 8 μm HARP layer. The results were applied to a physical model of HARP as well as a linear cascaded model of the FPI to determine the following x-ray imaging properties in both the avalanche and non-avalanche modes as a function of ESe: (1) total gain (which is the product of avalanche gain and optical quantum efficiency); (2) linearity; (3) dynamic range; and (4) gain non-uniformity resulting from thickness non-uniformity. Our results showed that a HARP layer thickness of 8 μm can provide adequate avalanche gain and sufficient dynamic range for x-ray imaging applications to permit quantum limited operation over the range of exposures needed for radiography and fluoroscopy.
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