Diamond Annual Review 2019/20
102 103 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 Detector Group Nicola Tartoni, Detector Group Leader Themost important achievement of the Detector Groupwithin the past year was to start the scientific exploitation of the time- resolved Tristan detector, developed entirely by the group, by carrying outmultiple user experiments fromAugust to December 2019. ThegoaloftheTristanproject istodeliveranareadetectorfortimeresolved crystallography experiments. Tristan is based on the Timepix3 read-out ASIC, which works in event driven mode (time stamp and location of an event sent out for any occurrence), rather than in frame based mode. Although initially designed for experiments on the Small-Molecule Single-Crystal Diffraction beamline (I19), this detector technology provides new capabilities for a variety of scattering techniques such as Small-Angle X-ray Scattering (SAXS). The area detector to be delivered for I19, known as Tristan10M, will tile ten detector modules. Each module uses a monolithic silicon sensor with 16 Timepix3 ASICs 1 bump-bonded for a total of more than one million pixels per module. All the hardware parts required to assemble a Tristan10M have been developed and procured, and are now ready to be assembled. The firmware and software for Tristan10M is ongoing. Whilst the delivery of Tristan10M is in the final stages, the Tristan project achieved full functionality for a single detector module system.TheTristan1M 2 has been used at Diamond in different user experiments on the Test beamline (B16) and I19. The most significant results with Tristan1M have been obtained at B16 where a set-up to carry out an experiment of Diffracted X-ray Tracking (DXT) was designed. Two experiments were performed by members of the Soft Condensed Matter Science Group, who specialise in bio-SAXS, in collaboration with Prof Yuji Sasaki (University of Tokyo) and Dr Hiroshi Sekiguchi (SPring-8). In these experiments a coiled coil protein, a protein-protein complex, and an RNA binding protein were investigated. A DXT experiment uses the white beam and measures the movement of Laue diffraction spots given by Au nanoparticles attached to the studied molecule under study. By tracking the movement of the diffraction spots the scientists were able to reconstruct the slow dynamic motion of the molecule. A fast detector is necessary to be able to track the fast movement of the spots and Tristan1M was ideal for this because it can time-stamp events with a time resolution of the order of a few nanoseconds. The data produced by Tristan1M were a stream of events with time and position stamps. These data were analysed after the experiment to reconstruct image sequences with the desired time resolution. Despite the high scattering background intrinsic to the white beam set-up, a pilot experiment in August 2019, and subsequent beam time in December 2019 have led to already impressive results. As shown in Fig. 1, the team could track the paths of the Laue diffraction spots within a 20 ms window, with a time resolution of 1 ms. The scattering signal was too weak in this experiment to allow for a finer time-binning of the data, which would have allowed for tracking with a better time resolution. However, Tristan will be able to bring the time resolution of DXT experiments comfortably into the microsecond regime due to its improved experimental conditions enabling higher signal to background ration. Tristan1M was employed during pump-probe experiments at I19 in September 2019. An Ag-Cu compound crystal was photo-excited with a 390 nm laser with a repetition rate of 10 kHz, and as the system relaxed into the ground state, the X-ray scattering determined the crystal structure. The experiments were carried out in stroboscopic mode to achieve sufficient statistics and were performed by both following a single reflection and obtaining full datasets. The trigger from the laser was fed into the Tristan continuous readout data stream. Tristan1M can acquire up to two external trigger signals and includes their time stamps in the data stream. By using the laser synchronism signal as the external trigger point, the delay for each event is determined with respect to the preceding trigger pulse. Histograms of the data for the 100 ms corresponding to the laser repetition period were producedwith 2ms bins.Tristan1Menabled data to be acquired 50 times faster than with the conventional technique of gating the detector and moving the delay of the gate with respect to the laser pulse. Data with healthy statistics have been achieved, thus opening the door for experiments covering a larger volume of reciprocal space for quantitative analysis. Moreover, the processing pipeline has been further optimised for quick feedback for future experiments. The group is also pleased to report significant progress with the development of the arc-detector for Pair Distribution Function (PDF) experiments at the X-ray PDF beamline (I15-1) 3 . The detector is an essential component of the beamline project that aims to radically improve the throughput of PDF experiments at Diamond and allow for a higher level of automation. The detector is a custom-made photon-counting detector based on a high Z sensor to benefit from negligible noise, large dynamic range and good efficiency at high photon energy. The conceptual architecture of the detector has been defined; the development of the parts is ongoing; and some parts including the sensor modules have already been delivered. Electron collection CdTe 1 mm thick sensors with Schottky contacts were selected due to their capability of quickly refreshing the polarisation effect by turning off and on the bias voltage 4,5 . The area of the CdTe sensor (14.2 mm × 42.6 mm) was chosen to match the area of three Medipix3RXv2 with readout ASICs of 55 μm× 55 μm pixel size 1,6 . The 48 low-voltage differential signalling (LVDS) data lines coming from two sensors (six ASICs) are sent to an Artix-7 FPGA which converts the data into a serial data stream. The data are then transmitted by a fibre optic to two data acquisition cards based on the Virtex7 FPGA located a few metres apart on a 19” rack. The fibre optic link between the detector head and the data acquisition cards enables the weight and size of the detector head to be substantially reduced. The data are then streamed at 25 fps (meeting I15-1 requirements) to a Linux server, via two QSFP+ transceivers on each FEM-II card, which runs the Odin DAQ framework 7 . The complete detector head consists of 24 modules, four modules are assembled in a single mechanical unit called super-module as shown in Fig. 2. Every module is arranged in an arc shape, covering a scattering angle of 100° for a total of 4.7 million pixels.The detector will be mounted on a goniometer in order to rotate and collect data from both above and below the beam, covering an even larger region of interest. The installation of the detector at I15-1 is currently planned for July 2021. References: 1. MEDIPIX Collaboration, CERN: https://medipix.web.cern.ch/home 2. Crevatin G. Oral communication at IEEE NSS-MIC 2019 3. Gimenez E.N. et al . NSS-MIC Conf. Record 2019 4. Gimenez E. N. et al . Development of a Schottky CdTe Medipix3RX hybrid photon counting detector with spatial and energy resolving capabilities. NIM A . 824 , 101-103 (2016) DOI: https://doi.org/10.1016/j.nima.2015.10.092 5. Astromskas V. et al . Evaluation of Polarization Effects of e - Collection Schottky CdTe Medipix3RX Hybrid Pixel Detector. IEEE TNS 63(1) , 252 – 258 (2016). DOI: 10.1109/ TNS.2016.2516827 6. Gimenez E. N. et al . Medipix3RX: Characterizing the Medipix3 Redesign With Synchrotron Radiation. IEEE TNS 62 (3) , 1413 – 1421 (2015). DOI: 10.1109/ TNS.2015.2425227 7. Yandell G. et al . Odin - a Control and Data Acquisition framework for Excalibur 1M and 3M Detectors. ICALEPS2017 Proceedings (2018). DOI: 10.18429/JACoW-ICALEPCS2017- TUPHA212 Figure 2: Reconstructed images (20 ms duration) of the path of the diffraction spots on the Tristan sensor. Figure 2: Arc-detector super-module drawing. The four grey rectangles represent the four sensors mounted on a printed circuit board in green. On the side, orthogonal to the sensor boards, the two cards which serialize the data by an Artix-7 FPGA are shown.
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