Final Report Summary - SIPM IN-DEPTH (Development of novel analytical and experimental approaches for an in-depth characterization and optimization of Silicon Photomultipliers) The detection of extremely weak light signals starting from single photons is of critical importance for a very wide range of scientific and technical applications. This includes high energy physics and astrophysics, medical imaging, biotechnology, scientific instrumentation, communication, environmental monitoring, homeland security, and many others.The development of Silicon Photomultipliers (SiPMs) – a new emerging generation of photosensors with superior photon number and timing resolution at room temperature, high reliability and robustness, insensitivity to magnetic fields and low operating voltages – came as a real breakthrough in photon detection in the XXI century. Furthermore, SiPMs are compatible with commonly used CMOS technology and inexpensive in mass-production, especially in comparison with conventional vacuum photomultiplier tubes and solid state single photon counters.The first applications of SiPM technology in high energy particles calorimetry and positron emission tomography (PET) started about a decade ago and revealed its enormous potential. However, it was at the same time that also specific drawbacks and bottlenecks were discovered that had to be addressed to allow a secure and efficient large-scale implementation of SiPMs for various applications.In order to evaluate their full potential for a specific application, it is necessary to quantify their fundamental parameters also as a particle detector in combination with scintillators or optical fibres used for signal generation and transport in detail. The development of comprehensive analytical and experimental approaches for an in-depth characterization and optimization of SiPMs was the goal of this project and aimed at providing valuable support for the implementation of SiPM technology in a variety of applications. This development furthermore provides an excellent basis for future advanced designs of SiPMs.Since a number of ongoing and planned large and medium-scale European projects would benefit from an improved theoretical basis of the intrinsic limitations of this novel technology, the “SiPM in-depth” project was started. It aimed at a comprehensive transfer of knowledge on SiPM technology to the European host institution and the wider research community, as well as the development of advanced theoretical, experimental and methodical approaches to fully exploit this technology. Furthermore, the project meant to actively build bridges between developers, researchers and application specialists in the UK, EU, USA, Russia and other parts of the world.The “SiPM in-depth” project has been carried out by the senior research fellow Dr. Sergey Vinogradov, an expert in R&D of solid state photodetectors and SiPMs and a pioneer of analytical probabilistic modelling of SiPM response and performance. The project comprises the collaboration of Dr. Vinogradov and host scientist Prof. Dr. Carsten P. Welsch, an expert in charged particle beam diagnostics and accelerator instrumentation, leader of the QUASAR Group at the University of Liverpool (UoL) and the Cockcroft Institute (CI) of Accelerator Science and Technology, and coordinator of several very large Marie Curie training networks – DITANET, LA3NET, and OPAC.An efficient knowledge transfer was delivered through an extensive series of lectures and seminars at the host institution during the first year of the project. This covered all essential aspects of SiPM technology and its applications, as well as hands-on training, supervision, support, and consulting on a wide range of experimental studies with SiPMs, the methodology of measurement and characterization of SiPMs, probabilistic modeling, simulation, and the analysis of SiPM response and performance. As an important part of the knowledge transfer, several dedicated experimental setups were developed and commissioned in the optical lab of the Cockcroft Institute and in the beam instrumentation department at CERN where part of the QUASAR Group is hosted.The development of novel analytical and experimental approaches for an in-depth characterization and optimization of SiPMs was the main scientific goal and research challenge of the project. This concerned in particular an analytical modelling of stochastic processes as part of SiPM response to a given signal. Internationally recognized research results that emerged from this project include an advanced analytical model of SiPM time resolution. This model is in high demand for time-of-flight applications such as 4D imaging particle calorimetry or time-of-flight PET and promises a significant improvement in image resolution and quality. It is the first implementation of the correlated filtered marked point process technique to SiPM modeling and takes into account correlated noise. It was found to be in good agreement with experiments and appears was established as a new and powerful analytical tool for the analysis and improvement of SiPM time resolution. In addition, first theoretical and experimental studies into transient SiPM response to intense light signals were carried out. The results included the development of a novel reward-renewal Markov process model of nonlinear transient response, pave the way for high dynamic range detection, for example, in Cherenkov fibre beam loss monitoring systems at particle accelerators. This can help reduce the cost and optimize the operational control of such systems and was a direct result of the collaboration within this project. Furthermore, a new method for the calibration of low gain photomultipliers was developed and now provides a robust measurement of the gain and other key parameters in high-noise environments. It utilizes the Erlang probability distribution between single electron responses instead of the commonly used measurement of their charge or amplitude distributions. New results were also achieved in studies and probabilistic analysis of the timing properties of a Geiger discharge in a single SiPM pixel, in the methodology of dark current generation measurement and the analysis of the underlying mechanisms based on dark IV characteristics referenced to corresponding photo-IVs.All projects results were widely disseminated during the main phase of the project. This included 10 publications in journals and conference proceedings, as well as more than 40 presentations at several dozens of recognized international conferences, workshops and meetings. The Fellow was also directly involved in the organization of expert meetings, such as for example, a workshop on the factors influencing the timing resolution of SiPMs, held in Calvi, Corsica, France on 7th/8th May 2015. This allowed detailed discussions about how to realize 10 ps time resolution.Outreach activities were carried out as part of the well-established activities of the hosting QUASAR group. A dedicated project web page was established right at the beginning of the project and has been updated on a regular basis. All essential project news such as participation in conferences, invited talks and advances in research were frequently posted in the news section of the groups, as well as of the Cockcroft Institute. The Fellow was also involved in major public engagement events organized by Prof. Welsch including a large Symposium on Lasers and Accelerators for Science & Society that was held in Liverpool, UK on June 26, 2015. The “SiPM in-depth” project was also highlighted in an interview given by Dr. Vinogradov for the Astroparticle Physics European Consortium (APPEC) on March 11, 2015. This was coupled with a description of SiPM concepts, technology, and applications targeting the general public. Both materials were published in APPEC news.The outstanding achievements of Dr. Vinogradov in research, development, characterization, modelling, analysis, and applications of Silicon Photomultipliers have recently been recognized by the IEEE Nuclear and Plasma Sciences Society by electing him to the level of Senior Member of IEEE in June 2015. All major project goals and objectives were successfully achieved within the given time frame. The joint research between the Fellow and host scientist have led to some excellent results and opened up new research avenues at the interface between advanced photon detectors and accelerator instrumentation R&D. After the end of his Fellowship, Dr. Vinogradov was given an honorary affiliate status at the University of Liverpool and collaboration with Prof. Welsch will be continued beyond the initial two years. The results to date will be an ideal ground for further studies that are now planned between both researchers into advanced particle detection techniques, complex signal analysis and R&D into the wider applications of SiPMs. final1-sipm-logo.png
