DECTRIS X-ray detector systems operate in “single-photon counting” mode and are based on the newly developed hybrid-pixel technology. The main difference to existing detectors is that the X-ray is directly transformed into electric charge (Fig. 1) and processed in the CMOS readout chips. This absolutely new design shows no dark current or readout noise and leads to a high dynamic range of 1,000,000 (20 bits) per image, short readout times of less than 3 ms, a high framing rate of over 200 images per second and an excellent point-spreadfunction. The quantum efficiency of the 0.32 mm thick silicon sensor is optimal for experiments in the energy range from 3 to 15 keV, however the detectors can be used for energies of up to 40 keV. The counting rate is above 2x10⁶ pixels/s to handle the high flux of modern synchrotron light sources.

Principle

A DECTRIS hybrid-pixel detector is composed of a silicon sensor, which is a two-dimensional array of pn-diodes usually processed in high-resistivity silicon, connected to an array of readout channels designed with advanced CMOS technology (Fig. 2). Each readout channel is connected to its corresponding detecting element through a microscopic indium ball, with a typical diameter of 18 um. This process is called microbump-bonding and was refined at the Paul Scherrer Institut (PSI).

The great advantage of this approach is that standard technologies are used for the silicon sensor and the CMOS readout chips, which guarantuees highest quality. Both of them can be optimized separately, as the best silicon substrates for X-ray detection and for high-speed/high-quality electronics are very different. Moreover, the small size of the pixel and of the interconnection results in a very low capacitance, which has the beneficial effect of reducing the noise and power consumption of the pixel readout electronics.

A hybrid pixel which features single-photon counting comprises a preamplifier, which amplifies the charge generated in the sensor by the incoming X-ray, and a discriminator, which produces a digital signal if the incoming charge exceeds a predefined threshold. The discriminator feeds a 20 bit counter, which then leads to a completely digital storage and noiseless readout of the number of detected X-rays per pixel.

The fundamental unit of the DECTRIS detectors, the module, consists of a single fully depleted monolithic silicon sensor with an 8 x 2 array of CMOS readout chips bump-bonded to it. Each sensor is a continuous array of 487 x 195 = 94'965 pixels without dead areas and covers an active area of 83.8 x 33.5 mm². This unit is wire-bonded to the mounting bracket with its readout electronics and forms the module (Fig. 3). These modules are mounted to a high-precision mechanical frame to realize multimodules setups with up to 60 modules (Fig. 4).