Imaging and Detection

Abstract

Excellent X-ray detection properties and the unique nature of avalanche breakdown observed in amorphous selenium (a-Se) have enabled its use for several important applications such as in flat panel X-ray image (FPXI) detectors for medical and industrial applications and high resolution HARP (High gain Avalanche Rushing amorphous Photoconductor) TV pickup tubes. Stabilized amorphous selenium (doped and alloyed a-Se) meets the essential requirements for these applications and has been commercialized. Advances in photolithography and electronic microfabrication techniques have enabled the development of large area X-ray detectors with integrated readout mechanisms based on arrays of thin-film transistors. Unlike older, charged-coupled device (CCD)-based detectors that require optical coupling and image demagnification (discussed later), thin-film transistor-based, flat panel systems are constructed such that the pixel charge collection and readout electronics for each pixel are immediately adjacent to the site of the X-ray interactions. Digital detectors offer several advantages when compared to conventional analog detectors. FPXIs based on a-Se, operating at the ideal limit of high detective quantum efficiency (DQE) and are particularly suitable for mammography, chest radiology, angiography, fluoroscopy, and computed tomography. An all solid-state version of the HARP has been recently demonstrated with excellent avalanche gain which is expected to lead to novel imaging device applications that would be quantum noise limited. This chapter reviews the important characteristics of a-Se, its applications, and competing technologies for imaging and detection applications.