Direct Conversion CdZnTe and CdTe Detectors for Digital Mammography

Hybrid CdZnTe and CdTe pixel detector arrays with 50 50 micron pixel sizes that convert x-rays directly into charge signals are under development at NOVA for applications to digital mammography. CdZnTe and CdTe have superior x-ray quantum efficiency compared to either emulsion-based film, phosphor-based detectors or other low-Z, solid-state detectors such as silicon. In this paper, latest results from thin (0.15 to 0.2 mm) CdZnTe and CdTe detectors will be presented in terms of modulation transfer function (MTF), detective quantum efficiency (DQE), and phantom images. Single-crystal CdZnTe detectors yield better results in DQE as well as phantom images, compared to the polycrystalline CdZnTe detectors. This is due to the nonuniformities in the polycrystal that degrade the charge transport properties. Because of the charge-coupling limitation of the readout ASIC that was originally designed for Si detectors, the detector is biased to collect holes from the front side. This charge collection mode limits the CdZnTe detector performance. Their DQE measurements yield 25% and 65% for the poly-crystal and single-crystal CdZnTe detectors respectively. Poly-crystal CdTe test detectors were also hybridized to the same type charge readout chip. Since CdTe has much longer hole-propagation lengths compared to CdZnTe, it shows better performance in the hole-collecting mode. However, severe polarization effect degrades performance of the present device. Excellent images were also obtained from the CdTe detectors. Future work to redesign the readout ASIC and, thus, improve the detector performance is discussed. Application to industrial imaging such as nondestructive evaluation (NDE) and nondestructive inspection (NDI) is a natural extension.