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D2R1 (Dimension 2 revision 1) is the most recent development of CdTe based X-ray detectors within a series of highly successful imaging spectrometers CALISTE. The detector consists of a CdTe crystal which is directly connected to a low-noise readout ASIC by a flip-chip bonding process. The reduced stray capacitance in combination with an adapted ASIC design results in a superior energy resolution of 584 eV FWHM at 60 keV.
The 16x16 pixel array with a 300um pixel pitch constitutes a 4.8x4.8 mm^2 detector surface on a 750um thick crystal. Such fine-pitched hard X-ray detectors show not only an improved spatial resolution but also an improved spectral resolution at soft and medium energies. A slightly diminishing spectral resolution is only observed for energies that are large enough to increase the split ratio significantly.
X-ray polarimetry based on incoherent scattering also benefits from the improved spectral and spatial resolution. Furthermore, the sensitivity for polarimetric measurements that uses only a single detector unit is greatly enhanced because of an increased efficiency for detecting Compton scattered events: within smaller pixel structures, the position of the incoherent scattering and the position of the scattered photon absorption are less likely within the same pixel and can be therefore detected individually.
After a description of the new ASIC concept we are presenting laboratory measurements that were realized with several different detector modules in order to verify their spectral and spatial properties.
The home made ASIC of D2R1 is based on a Charge Sensitive Amplifier (CSA) in combination with a Multi Correlated Double Sampling method: the continuous sampled outputs of the CSA are averaged on -chip before and after an event detection. The difference of these two values represent the signal height of the detected event.
The ASIC exhibit very good performance and the Equivalent Noise Charge is as low as 29 electors rms, making them perfectly suitable to read semiconductor detectors of any kind and any bias polarity.
In order to investigate the spectral and spatial properties the focus of the data analysis is put on the event split ratio and its dependence with energy. The determination of the virtual pixel size for single events, i.e. the region within a pixel that results in a single event detection, is key for a proper understanding of the evolution of the spectral and spatial resolution with energy. While split events decrease the spectral performance because of added noise contributions of multiple readout channels, they increase the spatial resolution by allowing a center-of-mass calculation with a sub-pixel resolution. The virtual pixel size for single, double, triple, and quadruple events are estimated with an analytical model which is verified by measurements at different energies (5.6 keV, 13.9 keV, 60 keV, 122 keV and 245 keV). Finally, the polarimetric performance of D2R1 is examined via detailed simulations.
The wide accessible energy range between 2-250 keV and the fast timing capabilities complete D2R1 to suite a variety of different applications. Excellent spatial, spectral, and timing capabilities in the medium and hard X-ray range are key parameters for future X-ray missions. All these properties are well combined within the D2R1 concept.
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