Carpentieri C, Bisogni M G, Del Guerra A, Delogu P, Fantacci M E, Fogli J, Marchi A, Marzulli V, Rosso V, Stefanini A, Tofani A
Physics Department, University of Pisa and INFN Sezione di Pisa, Largo Pontecorvo 3, 56127 Pisa, Italy.
Radiat Prot Dosimetry. 2008;129(1-3):119-22. doi: 10.1093/rpd/ncn147. Epub 2008 May 16.
Recent advances in semiconductor pixel detectors and read-out electronics allowed to build the first prototypes of single photon-counting imaging systems that represent the last frontier of digital radiography. Among the advantages with respect to commercially available digital imaging systems, there are direct conversion of photon energy into electrical charge and the effective rejection of electronic noise by means of a thresholding process. These features allow the photon-counting systems to achieve high imaging performances in terms of spatial and contrast resolution. Moreover, the now available deep integration techniques allow the reduction of the pixel size and the improvement of the functionality of the single cell and the read-out speed so as to cope with the high fluxes found in diagnostic radiology. In particular, the single photon-counting system presented in this paper is based on a 300-microm thick silicon pixel detector bump-bonded to the Medipix2 read-out chip to form an assembly of 256 x 256 square pixels at a pitch of 55 microm. Each cell comprises a low-noise preamplifier, two pulse height discriminators and a 14-bit counter. The maximum counting rate per pixel is 1 MHz. The chip can operate in two modalities: it records the events with energy above a threshold (single mode) or between two energy thresholds (window mode). Exploiting this latter feature, a possible application of such a system as a fast spectrometer is presented to study the energy spectrum of diagnostic beams produced by X-ray tubes.
半导体像素探测器和读出电子学的最新进展使得能够构建单光子计数成像系统的首批原型，这些系统代表了数字射线照相的最新前沿。相对于市售数字成像系统的优势包括：光子能量直接转换为电荷，以及通过阈值处理有效抑制电子噪声。这些特性使光子计数系统在空间和对比度分辨率方面能够实现高成像性能。此外，目前可用的深度集成技术能够减小像素尺寸，提高单个单元的功能和读出速度，以应对诊断放射学中发现的高通量。特别是，本文介绍的单光子计数系统基于一个300微米厚的硅像素探测器，通过凸点键合连接到Medipix2读出芯片，形成一个间距为55微米的256×256方形像素阵列。每个单元包括一个低噪声前置放大器、两个脉冲高度鉴别器和一个14位计数器。每个像素的最大计数率为1兆赫兹。该芯片可以在两种模式下运行：记录能量高于阈值的事件（单模式）或记录两个能量阈值之间的事件（窗口模式）。利用后一个特性，展示了这种系统作为快速光谱仪的一种可能应用，用于研究X射线管产生的诊断束的能谱。
