Issues of Ohmic Contacts in Human Medical Photon-Counting CT Detectors

Human medical computed tomography (CT) detectors based on photon-counting have to support high photon event rates and at the same time must not polarize. CZT/CdTe has become the most promising direct conversion material for this purpose and is in use on photon-counting CT prototype scanners at several CT manufacturers. An important design choice is the type of electrode contacts used. In the past, CdTe with Schottky contacts usually resulted in severe polarization already at low flux, while with ohmic contacts this was not observed giving rise to the view that ohmic contacts could help to reduce polarization. In our investigations toward CZT for high flux CT detectors, we found however that, despite a baseline restorer eliminating slowly changing additional currents efficiently, highly optimized CZT with ohmic contacts exhibited an unacceptable violation of the Poisson figure of merit “variance≤mean”, according to which the variance of the observed number of counts (OC) does not exceed the average of the OC (with the variance equaling the average at low flux). This violation is not observed with (semi)-blocking contacts. Based on a simple model for the mechanism of photoconduction known from the literature, two extreme cases are discussed. They both explain why the injection current causes additional noise in the number of observed counts due to the Poisson arrivals, which also modulate the injection current. For the second extreme case, the power spectral density of the injection current is estimated.