Detector Group

Nicola Tartoni, Detector Group Leader

During the past year the Detector group at Diamond has consolidated the applications of the Medipix3 based detectors at the facility’s beamlines. A large Excalibur system (3 million pixels) is now in operation on I13 and five smaller Merlin systems1 (256 thousand pixels or 65 thousand pixels format) operating on five different beamlines (I07, I13, I16, B16, and I20). The availability of the debugged version of the Medipix3 chip (known as Medipix3RXv2) led to a major upgrade project of Excalibur. A new detector head is presently being assembled and it will be equipped with new hybrid modules built with the new version of the chip. Better image quality is expected together with the capability of detecting photons at lower energy than the present system. Furthermore the simultaneous read-out and counting mode, that will enable acquiring frames with no dead time, will be implemented.

The Merlin system was licensed to Quantum Detectors, which is now commercialising the complete detector system. Fig. 1 shows the commercial detector head as available from Quantum Detectors. Systems are now in use at synchrotron facilities and universities in Europe, Asia and North America having been delivered with custom software and integrated into existing infrastructure. The Detector Group at Diamond provides technical support to Quantum Detectors to enable this commercial activity.

Figure 1: The Merlin detector head commercialised by Quantum Detectors. The version shown is a detector head with a monolithic silicon sensor flip-chip bump-bonded to 4 Medipx3 chips. The number of pixels is 256 thousand.

The HiZPAD2 project – an EU funded joint research activity - was completed and good quality 110 micron pitch hybrid detectors were built with cadmium telluride (CdTe) sensors and Medipix3. The sensors used in the study were Schottky structures; they were driven with the Merlin system both in the standard mode of operation and in colour mode of operation. These sensors were tested at I15 and their quality proved to be good enough for future applications such as powder diffraction at high energy beam lines. An example of a powder diffraction pattern measured on I15 is shown in Fig. 2. The CdTe detectors were also tested in colour mode at the University of Surrey for an application in medical physics. The colour mode of operation may find its way into synchrotron experiments such as Laue diffraction or Talbot imaging. The results of the work done for the HiZPAD2 project have been presented orally at the 10th International Conference on Position Sensitive Detectors held in Surrey (UK) in September 2014 and at the 21st Symposium on Room- Temperature Semiconductor X-Ray and Gamma-ray Detectors within the IEEE Nuclear Science Symposium and Medical Imaging Conference, held in Seattle, USA, in November 2014. Publication of papers regarding this work in relevant scientific journals is planned for later in 2015.

Figure 2: Large reconstructed image of the CeO2 powder diffraction pattern from a total of 21 images acquired with the quad CdTe detector installed on a diffractometer arm. Data was collected using X-ray photons of 41 keV and the scattering angle varied from 0° to 42° in steps of 2°. The black crosses correspond to the simulated diffraction pattern CeO2, showing good consistency between the stitched image and the simulated data.

The Medipix3 chip was also used in the Lancelot system, a beam position monitor and profiler developed in collaboration with the University of Manchester. The first version of this system was successfully demonstrated with the X-ray beam2. A new version of the Lancelot system using the debugged version of the chip is under development and it will be installed on the beamlines I19 and I24.

Another important area where the detector group operated is in the area of spectroscopy detectors. In particular, the multielement monolithic germanium detectors were re-commissioned after the 64 element detector of I20 (Fig. 3) had developed a fault. Now the 64 element detector is back in operation on I20 and the 36 element detector is in operation on B18. The 36 element detector had been used by I20 while the 64 element detector was not available.

Figure 3: 64 element germanium detector installed in I20. The detector is read-out by the Xpress2 digital pulse processor developed by STFC. Xpress2 enables a counting rate in excess of 300 kcps per channel.

Although the multi-element germanium detectors installed on the beamlines achieve a counting rate of 300 kcps per element or higher with energy resolution sufficient for X-ray Absorption Fine Structure (XAFS) applications, a lot of work was done to understand how to further improve the performance of such detectors. A thorough investigation of the deterioration of the energy resolution due to cross-talk was carried out and then an algorithm to correct the effect of cross-talk was drawn up. This algorithm leads to considerably improved spectra and therefore it was decided that it should be implemented in hardware. In order to implement the cross-talk correction in hardware it is necessary to have a pulse processor where the channels can talk to each other. This cannot be achieved with the pulse processors presently in use at Diamond because their hardware architecture was not designed to support this feature. An activity is ongoing to define a development project for a new pulse processor capable of supporting the cross-talk correction. The results obtained with this algorithm were presented to the IEEE Nuclear Science Symposium and Medical Imaging Conference 2014, in addition a patent request was filed last year to protect possible commercial exploitation of the algorithm.

The group is also investigating a route to improve the detector head. A segmented monolithic germanium detector with a pitch of 1 mm was instrumented with CMOS preamplifiers to understand if this is a viable route to build multi-element spectroscopy grade detectors for XAFS application with a higher density of channels. The results appeared very promising and were published in an international journal3. Further investigations and developments are necessary before a full scale detector with a number of channels of the order of 100 can be designed and built.

References:
1. Plackett R., Horswell I., Gimenez-Navarro E., Marchal J., Omar D. and Tartoni N. Merlin a fast and versatile readout system for Medipix3. JINST 8 C01038 (2013).
2. Rico-Alvarez O., Kachatkou A., Marchal J., Willis B., Sawhney K., Tartoni N. and R.G. van Silfhout R.G. A compact and portable X-ray beam position monitor using Medipix3. JINST 9 C12036 (2014).
3. Tartoni N., Crook R., Krings T., Protić D., Ross C., Bombelli L., Alberti R., Frizzi T., and Astromskas V. Monolithic Multi-Element HPGe Detector Equipped With CMOS Preamplifiers: Construction and Characterization of a Demonstrator. IEEE Transactions on Nuclear Science, vol. 62, 387-394 (2015).