ABSTRACT
Currently, most digital radiation detectors for security applications are based on integrating the X-ray quanta (photons) emitted from the X-ray tube for each frame. This technique is vulnerable to noise due to variations in the magnitude of the electric charge generated per X-ray photon. Higher energy photons deposit more charge in the detector than lower energy photons so that in a quantum integrating detector, the higher energy photons receive greater weight. This effect is undesirable in many detection applications because the higher part of the energy spectrum provides lower differential attenuation between materials, and hence, these energies yield images of low contrast.
Direct conversion X-ray quantum counting detectors solve the noise problem associated with photon weighting by providing better weighting of information from X-ray quanta with different energies. In an X-ray quantum counting system, all photons detected with energies above a certain predetermined threshold are assigned the same weight. Adding the energy windowing capability to the system (i.e., counting photons within a specified energy range) theoretically eliminates the noise associated with photon weighting and decreases the required X-ray dosage by up to 40% compared to integrating systems.
Direct conversion detectors are also essential to XRD detection and imaging applications where precise information about the energy of incoming photons is crucial. This chapter reviews key challenges that are present in spectroscopic and photon-counting electronics for XRD. Energy resolution, charge-sharing correction, high flux capability, and pileup effects affect both the count rate linearity and the spectral response. One way to counteract that problem is to use smaller pixels, but smaller pixel will lead to more charge sharing. In this respect, the various chips offer an interesting combination of relatively small pixels and still very good energy resolution.
The readout integrated circuits (ROIC) design issues will be discuss in detail in this chapter. The ROIC family can be divided into spectroscopy and photon-counting chips. The photon-counting chips usually operate in a synchronous manner while the spectroscopic chips can be both asynchronous and synchronous. Although over 100 ROIC have been designed and used, it is no doubt that new designs will enter the marketplace in the next 5–10 years and are likely to be based on very advanced CMOS chip fabrication technologies.
