Indirect flat-panel detector with avalanche gain: design and operation of the avalanche photoconductor

Abstract

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 d Se and the applied electric field E Se . To determine the optimal design parameter and operational conditions for HARP, we measured the E Se 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 E Se : (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.

Development of solid‐state avalanche amorphous selenium for medical imaging

“…1(a)], which is called SHARP-AMFPI, uses a thin HARP layer (e.g., 15 µm) to detect the optical photons generated from a high resolution xray scintillator and amplify the signal. 2,4,5 The direct detection method [ Fig. 1(b)], which is called field shaping multi-well avalanche detector (SWAD), 6 creates a separate avalanche region inside the wells while maintaining nonavalanche fields in the bulk, where the majority of x-ray interactions occur.…”

Stability, dark current, noise, and performance optimization of selenium detectors

“…Addressing these issues are essential for further advancing this technology to a new horizon. In the area of in-efficiency of charge generation in comparison to other direct conversion materials, several researchers now are exploring schemes of signal amplifications (ref 6,7,8,9) by charge avalanche or by in-pixel amplification. Other issues such as dark current, detector stability, residual image removal, and thermal effects are discussed in this paper.…”

SWAD: inherent photon counting performance of amorphous selenium multi-well avalanche detector

“…To overcome the low conversion gain limitation of a-Se we have proposed novel direct and indirect conversion detectors which utilize a layer of avalanche a-Se referred to as high gain avalanche rushing photoconductor (HARP) [12][13][14][15] . HARP amplifies the photogenerated charge before the signal is readout in the TFT array.…”