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Jacques Séguinot (1932 – 2020)

Jacques Séguinot (1932 – 2020)
Jacques Séguinot (Image: CERN)

Jacques Séguinot, a great detector physicist and a founding father of the Ring Imaging Cherenkov technique, passed away on 12 October 2020.

Jacques was born in 1932 in a small village in Vendée. After a baccalauréat in mathematics and technology, he studied electromechanical engineering at the University of Caen and received his PhD in physical sciences in 1954. This solid base of engineering became visible in every single drawing and experiment set-up that Jacques designed and built throughout his long career. Jacques followed a classic career path in the French academic system, from a stagiaire de recherche in 1954 to a directeur de recherche in 1981, a rank that he held until his official retirement in 1990.

His first studies led him to airy heights. He spent several months at the French cosmic ray laboratory on the Col du Midi (3500 m), not far from Mont Blanc. The ride up there in a ropeway for material transport must have been impressive! From 1960 he worked on accelerator-based experiments, first at Saturne (CEA Saclay) and from 1964 onwards at the CERN PS, studying strong interactions with pion and kaon beams. At the end of the 1960s, while working on one of these experiments, Jacques met Tom Ypsilantis and the two began to collaborate closely in a very complementary and fruitful way.

In a 1977 seminal paper, co-authored by Tom Ypsilantis, Jacques established the basis of an innovative particle identification technology that became known as RICH (Ring Imaging Cherenkov Counter, CRID on the other side of the ocean). The central idea was to use the recently introduced Multiwire Proportional Chambers, filled with a photosensitive gas, to detect and localise UV photons emitted by fast charged particles in a radiating medium, and use a suitable optical arrangement to create a ring pattern whose radius depended on the particle momentum. Combined with magnetic analysis, the method made it possible to identify the particle’s mass in a wide range of energies. In further work, Séguinot and Ypsilantis developed algorithms to evaluate and optimise the momentum resolution of the detectors, as well as studying the characteristics of radiators adapted to cover different momentum ranges.

The work raised enormous interest, since it promised to cover a region of momenta where other technologies were ineffective. In its original design, the detector would extend over most of the solid angle around the target or colliding beams intersections; this was particularly relevant for the experiments around newly commissioned accelerators: LEP at CERN, SLD at SLAC and others. The early RICH devices were in fact successfully deployed for fixed target experiments: the OMEGA magnetic spectrometer at CERN and the E605 experiment at Fermilab. The RICH detector for DELPHI at CERN’s electron–positron collider (LEP) came close to the original design, with nearly 4π angular coverage; Jacques’ contribution to the design and realisation of the detector was of utmost relevance.

In view of the growing interest in meson factories, Jacques and Tom worked on faster RICH devices, with shorter photo-conversion length, and soon also on CsI solid photo-converters. This fundamental R&D work led to applications in the CLEO RICH (at the CESR storage ring) and the CsI-based RICHes in ALICE, COMPASS and other experiments. Another very ambitious R&D programme started in the mid-1990s, aimed at the development of highly segmented photodetectors sensitive to visible light, so-called Hybrid Photo Detectors (HPD). Their design was optimised for use in the two RICH detectors of LHCb. Even though this development wasn’t crowned by selection for application in LHCb, Jacques’ HPD design and the achieved performance impressed and inspired many detector groups around the world. Jacques also saw the potential of the segmented photodetectors for applications in medical imaging. He proposed an innovative design of a positron emission tomography (PET) device in which matrices of long scintillation crystals are read from both sides by HPDs. In the meantime, SiPM photodetectors had become available, with a number of practical advantages over HPDs. In the AX-PET collaboration, Jacques and several other physicists and engineers built a fully operational axial PET with SiPM readout.

With the passing of Jacques Séguinot, the HEP community loses an excellent detector physicist with an extraordinary sense of engineering. His groundbreaking ideas live on, also in the most recent detectors, such as BELLE-II in Japan. But we will also remember his fine personality, patience, politeness and decency.

Christian Joram and Fabio Sauli