Development of the DEPFET Sensor With Signal Compression: A Large Format X-Ray Imager With Mega-Frame Readout Capability for the European XFEL

Abstract

We present the development of the DSSC instrument: an ultra-high speed detector system for the new European XFEL in Hamburg. The DSSC will be able to record X-ray images with a maximum frame rate of 4.5 MHz. The system is based on a silicon pixel sensor with a DEPFET as a central amplifier structure and has detection efficiency close to 100% for X-rays from 0.5 keV up to 10 keV. The sensor will have a size of approximately 210 210 mm composed of 1024 1024 pixels with hexagonal shape. Two hundred fifty six mixed signal readout ASICs are bump-bonded to the detector. They are designed in 130 nm CMOS technology and provide full parallel readout. The signals coming from the sensor are processed by an analog filter, immediately digitized by 8-bit ADCs and locally stored in an SRAM, which is able to record at least 640 frames. In order to fit the dynamic range of about photons of 1 keV per pixel into a reasonable output signal range, achieving at the same time single 1 keV photon resolution, a non-linear characteristic is required. The proposed DEPFET provides the needed dynamic range compression at the sensor level. The most exciting and challenging property is that the single 1 keV photon resolution and the high dynamic range are accomplished within the 220 ns frame rate of the system. The key properties and the main design concepts of the different building blocks of the system are discussed. Measurements with the analog front-end of the readout ASIC and a standard DEPFET have already shown a very low noise which makes it possible to achieve the targeted single photon resolution for 1 keV photons at 4.5 MHz and also for 0.5 keV photons at half of the speed. In the paper the new experimental results obtained coupling a single pixel to an 8 8 ASIC prototype are shown. This 8 8 ASIC comprises the complete readout chain from the analog front-end to the ADC and the memory. The characterization of a newly fabricated non-linear DEPFET is presented for the first time.Index Terms-DEPFET, DSSC, silicon radiation detectors, X-ray detectors, X-ray free electron lasers.

Strategy for calibrating the non-linear gain of the DSSC detector for the European XFEL

“…Here. the offset has been calibrated such that a zero signal (no incident photon) is centered on bin 4. consequently the signals from I. been set to a value of 50 electrons ENC (equivalent noise charge), as expected for operation of the DSSC at 4.5 MHz read-out speed [11]. For simplicity, an ideal 8 bit ADC (i.e.…”

“…Conversely, it is possible to maximize dynamic range at the expense of reduced photon counting resolution. The flexibility in DSSC operation is explained in detail in [11].…”

“…The DSSC design goal is to achieve at the same time single photon detection and high dynamic range, both for photon energies down to 0.5 ke V and read-out speeds up to 4.5 MHz [10], [11]. To realize this design goal, an accurate calibration of multiple DSSC properties is required, including quantum efficiency, spectral response, and charge sharing among neighbouring pixels.…”

Calibration of the non-linear system response of a prototype set-up of the DSSC detector for the European XFEL

“…The DSSC develoment status is summarized in [8], [9]. As described in detail in [7], the DSSC is based on the integrated detector/amplifier structure DEPFET (Depleted P channel Field Effect Transis tor).…”

“…The DSSC design goal is to achieve at the same time single photon detection and high dynamic range, both for photon energies down to 0.5 keY and read-out speeds up to 4.5 MHz [8], [9]. To realize this design goal, an accurate calibration of multiple DSSC properties is required.…”