​The DESY Stephenson Distinguished Visitor Programme

The Stephenson Distinguished Visitor Programme was set up at Deutsches Elektronen-Synchrotron (DESY), a national research centre for fundamental science located in Hamburg,  in 2014 after Dr. James D. Stephenson and his wife bequeathed their assets to the organisation. Born in Grimsby, Jim worked from the 1970s to the 1990s in Germany at the 'Fritz-Haber-Institut der Max-Planck-Gesellschaft' (MPG) and the 'Bundesanstalt für Materialforschung und -prüfung' (BAM) in Berlin. Jim published several scientific papers, mainly on synchrotron radiation topography and performed experiments at the former synchrotron source DORIS III at DESY as a scientific user.

The fund that Jim and his wife established provides support to allow renowned scientists to visit DESY, to work and to collaborate with scientists in the field of photon science research. The programme has a particular focus on supporting the work of young academics through innovative, advanced projects that initiate and expand connections with scientists at home and abroad. Over the last decade, the programme has facilitated visits to DESY from ~ 20 distinguished scientists from across Europe and the USA in support of the work of the Photon-Science Division. During the Summer of 2024, I was approached by Prof. Heinz Graafsma, the head of the Photon-Science Detector Group at DESY, to gauge my interest in taking part in the programme.

 

The Motivation for My Visit

The anticipated upgrade of the PETRA synchrotron at DESY​, much like the upgrade of our own Diamond-II here in the UK, will bring with it new beam lines that require instrumentation capable of operating under the intensities and X-ray energies that can be delivered by the latest generation of Diffraction Limited Storage Rings (DLSRs). Operation under these challenging conditions requires a new generation of detector systems produced from high-density compound semiconductors like cadmium zinc telluride (CdZnTe). These materials have the advantage of high densities, meaning they can stop high-energy X-rays (>10keV) and operate at room temperature without the need for cryogenic cooling, as well as being impervious to radiation damage compared to more traditional silicon-based detector systems. Having spent two decades working in the field, starting with a CASE-Plus PhD between the University of Surrey and RAL in 2005, I’ve developed considerable experience in the characterisation and use of these materials. My current research programme is focused on developing camera systems using these sensors that can operate under the most challenging of conditions found at modern light sources. 

Until recently, DESY didn’t have a member of staff dedicated to the development of detector systems based on these high-Z sensor materials. Debora Magalhaes joined the group in late 2023 with the responsibility of developing a capability for the high-Z compound semiconductor sensors for high-energy X-ray applications, as well as working towards a PhD with the University of Hamburg. Debora already had significant experience with detectors, with seven years of experience in the detectors group at the Brazilian Synchrotron Light Laboratory (LNLS), developing detectors for future beamlines. More recently, Debora worked at ESRF (France), developing high-rate photon-counting detectors for use at the new Extremely Bright Source (EBS). My aim, as part of the Stephenson Programme, is to mentor Debora in the characterisation of these high-Z materials and, alongside staff at the University of Hamburg, and help her to establish a Transient Current Technique (TCT) for studying the electrical field within these sensors. Understanding the inner workings of the sensors is particularly important when they are going to be in use under the extreme fluxes of X-rays.​ ​

My First Two Weeks at DESY

Arriving at DESY in July 2025, Debora and I set to work, adapting the existing equipment at the University of Hamburg to work with our CdZnTe sensors. This involved hunting around the labs for rare connectors, soldering wires onto PCBs and working out how best to mount our samples in the system; it was like being a PhD student again! With everything set up, we then attempted to see our first laser-induced signal in our sensors and failed miserably due to the high level of electronic noise in our setup. After some head scratching and reading of the literature, we attempted to AC-couple, rather than DC-couple, the amplifiers we were using to the sensors. DC-coupling transmits the entire signal, including the large, constant leakage current produced by the sensors, which can saturate the amplifier output. AC-coupling uses a capacitor to block the constant leakage current, allowing the amplifier to focus on the small, fast transient signals produced by each laser pulse. With this change, at the end of day one, we started to see some sensible signals in the sensors.

During subsequent days, Debora and I set about trying to optimise our setup, collecting every problem it is possible to have with a TCT setup along the way. From a poor electrical ground to broken amplifiers, damaged wire bonds and wandering sample stages, every day we encountered a new challenge but persevered, slowly making progress. One of the biggest phenomena to affect our results was interference from our mobile phones and laptops, which would light up the oscilloscope like a Christmas Tree whenever we forgot to put them in airplane mode. The need to keep this electrical noise to a minimum had the bonus of disconnecting us from any distractions like emails and Teams messages.

Personally, I found this enforced digital detox very enlightening! What it revealed was just how much of my post-COVID working day has become about fielding Zoom calls or answering Teams messages. A bit like the overused metaphor about a frog in a slowly heating pot, I hadn’t quite realised how much my ‘virtual’ workload had increased over the last five years. Sitting in that small lab, working together with Debora to make a success of the measurements, reminded me of why I had become a physicist in the first place. The intellectual challenge of chasing an elusive electrical noise source, the joy of getting that first clean measurement, the intrigue when what we observed didn’t quite align with the picture I had formed in my head, and then hypothesising and testing what in reality might be going on.

 

Reflections on the Experience

While I would still struggle to identify with the term ‘distinguished’, my time at DESY has helped me realise how 20 years of problem-solving has equipped me with the confidence to make a change to a setup, to try something that at first looks stupid just to see what happens, and a gut feeling for when things don’t quite look right. By the end of these first two weeks, Debora and I were already collecting results that were telling us new things about these CdZnTe sensors that had not been seen before. I’m sure that the results we took, and the ones that Debora will now take with the setup working, will lead to multiple publications and help inform our understanding of these important sensors and their use at light sources.

My first two weeks at DESY were a fantastic experience, and I would like to thank Heinz for the invitation to participate, Debora for being a fantastic host, and the rest of the DESY Detector Group for being so welcoming. I’m looking forward to welcoming Debora to STFC to show her our facilities within the Technology Department, as well as the second leg of my Stephenson Visit, which will likely coincide with testing of these sensors at the European XFEL. I believe that the trip has really helped build stronger relationships across our institutes. I really enjoyed our discussions of common challenges, and I’m excited about the ideas that have already been generated for more collaboration in the future. Finally, I would like to say thank you to the Stephenson Foundation for supporting my visits. I hope that both Debora and I have approached this in the spirit which Jim would have intended.  

​Written by Matt Veale