A coordinated approach

Detection of light is one of the basic principles of measurements and diagnostics in science and many applications. With about 600000 PMTs/year medical diagnostic instrumentation is the largest consumer of PMTs; PMTs are used in Positron Emission Tomography (PET) scanners, Gamma cameras, and many applications in Life Sciences. Besides specific applications of PMTs, e.g., in oil drilling industry, large scale experiments in basic research are consumers of up to several 100000 low light-level sensors.

All innovation with respect to low light-level sensors is driven by the challenging demand by research projects and infrastructures. The demand of reaching lower levels in light detection efficiency with the highest precision in astroparticle, particle, and nuclear physics experiments is one of the main R&D drivers in the domain of low light-level detection.

Various European research groups have played and will play an important role in further advancing this technology. However, a coordination and structuring of these groups – especially in Europe – is currently missing; while competition is often speeding up developments especially in the prototyping together with industry, duplication in testing and evaluating available devices for research projects is a standard.

With SENSE R&D activities of European research groups, industry, and strategic partners worldwide shall be coordinated and supported with the clear goal to develop the ultimate low light-level sensors. Beyond current scientific collaborations, the involved groups from academia and industry will be guided in developing together a roadmap with the necessary technological steps and coordinating their synergetic work to maximize technological innovation. Of importance for SENSE is the attraction of young academics for this close-to-application R&D work, to ensure a long-term European perspective in this very innovation field with many new applications to be realized in the future.

The concept followed by SENSE is based on preparatory work and experiences made during the ERA-NETs ASPERA and ASPERA-2 funded by the European Commission during the Framework Programs 6 and 7. The technology fora dedicated to innovative technologies required in world class astroparticle projects have been continued within the frame of APPEC (Astroparticle Physics European Consortium). Furthermore, it integrates the experience made with the series of LIGHT conferences and a workshop series dedicated to advanced SiPMs.

Any improvement in the PMT technology evolving from science projects has allowed medical diagnostics industry to immediately come up with advanced products. Together with this immediate market there is a clear positive societal impact: a doubling of the light sensor’s efficiency in a medical diagnostic instrument will allow to half the radiation doses for patients and thus to reduce the possible negative consequences of irradiation. PET scanners allow studies of the functional activity in vivo, which is important for cancer research, Alzheimer studies as well as drug tests. When replacing PMTs with SiPM devices it might be possible, for instance, to install PET inside the very strong magnetic field of MRI coils. Also, one will profit from ultra-fast time resolution of SiPM, for the so-called time-of-flight reduction of the background.

Silicon Photo Multipliers (SiPMs) are the future of PMT technology. SiPMs are robust and feature numerous advantages compared to PMTs, such as compactness, lightweight, mass producibility, insensitivity to magnetic fields, low bias, high photo-detection efficiency. Nonetheless, they are still so compact that instrumenting large area requires complicated readout electronics.

A first attempt in the ultimate sensor with immediate application has been made by Philips with a digital SiPM approach. This comes with an accompanying test set up which has still high costs. Moreover, the digital-SiPM cannot currently be employed in ns-timing applications because of its large dead time. Nevertheless, a breakthrough in the technology also addressing the SiPM noise issue is going to take place in the next years.

In addition to these two technologies, a complete proof-of-the-concept exists for a novel design of low light-level detectors, called Abalone, which has to be evaluated for its commercialization. Other novel ideas like the superconducting transition edge sensors (TES), the Neganov-Luke light devices, or new generation organic image sensors require further R&D before becoming a commercial product.

The leading industrial partners confirmed that the scientific and technological challenge of physics experiments is the main driver for developments in the field of low light-level detector devices and electronics. For the industry alone – especially for SMEs – it is very difficult to improve the quality of low light-level sensors. The demands are very specific and if there is not enough market for the novel products, it is hard to justify the related development and investment costs.

For significant improvements it is necessary to produce test series of 10 to 100 sensors. However, it has been noted that the costs of such small series are difficult to transfer to the customers and from the physics experi-ments alone companies cannot make their living. Newly developed products must pay off, allowing the companies selling these in a wide market. For companies the time scale for a return of an investment is shorter than it is usual in the academic world.

Concerning the research projects, the challenge for industry is to develop and produce the right technology at the right time. To cope with the demand of several hundred thousands of PMTs within several years is not easy and new production capacities must be built up. To realize such a large production capacity a concrete commercial prospect is required. Furthermore, for such a high number of required units automation of the production processes and investments in extended production capacity are obviously needed.