Speaker
Description
AGIPD (Adaptive Gain Integrating Pixel Detector) is a 2D hybrid pixel detector system designed and developed for the European XFEL (Eu.XFEL) [1]. At the Eu.XFEL photons will arrive in pulse trains every 100 ms (or at the fundamental repetition frequency of 10 Hz). Each pulse train consists of up to 2700 pulses that arrive within 600 µs (i.e. a bunch spacing of 222 ns, meaning 4.5 MHz frame rate) followed by 99.4 ms between trains. Each single pulse consists of 10$^{12}$ X-ray photons in less than 100 fs and with an energy tunable between 250 eV up to 25 keV. The challenges for the detector at the Eu.XFEL include a dynamic range up to 10$^{4}$ photons per pixel, single photon sensitivity, a frame rate up to 4.5 MHz, as well as high radiation tolerance. In order to cope with the large dynamic range, the first stage of each pixel of the AGIPD ASIC [2] is a charge sensitive preamplifier with three different gains that are dynamically switched during the charge integration. Dynamic gain switching enables single photon resolution (with a measured S/N ratio of 11 at 12.4 keV) in high gain and a dynamic range of 10$^{4}$ x 12.4 keV photons in low gain. The time structure of the beam of the Eu.XFEL does not allow a continuous readout of the single frames during the bunch train, thus each pixel of the AGIPD ASIC (area 200 x 200 µm$^{2}$) is equipped with an in-pixel memory which consists of 2 storage cell matrices of 352 storage cells each onto which the frames are stored before being read out in the gap between trains. One of the most critical aspects of this detector concerns its calibration [3] and many aspects have to be taken into account such as linearity, calibration time, radiation damage and high dynamic range to probe. A high dynamic range test suite is needed which must be linear to the required degree over the entire dynamic range. Due to the high brilliance the radiation damage expected at the Eu.XFEL is significant and may cause a variation of many parameters of the detector such as noise, baseline and gain. The detector will therefore need a periodic re-calibration, taking into account that the calibration suite itself may be affected by radiation damage. This is especially true for the on-chip sources. This contribution will be focused on the calibration concept of the AGIPD detector. Different calibration techniques will be shown and compared such as internal current source, backside pulsing, IR pulsed laser, LED light and mono-energetic protons (see figure). Moreover an update of the status of the AGIPD detector will be given and some relevant results of the first experiments will be shown.
[1] M. Altarelli et al., European XFEL technical design report, (2006), ISBN 978-3-935702-17-1.
[2] X. Shi et al., Challenges in chip design for the AGIPD detector, Nucl. Instrum. Meth. A 624 (2010) 387.
[3] D. Mezza et al., 2016 New calibration circuitry and concept for AGIPD JINST 11 C11019.
