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(1928-2000) Greek-American physicist who specialized in the construction of sensitive detectors used in particle physics. Ypsilantis came from a distinguished family. In 1821, Prince Alexander Ypsilantis, a general in the Russian army, led a charge across the Danube against the Ottoman Turks at the beginning of the fight for Greek independence.

Born in 1928 into a Greek family living in Salt Lake City, Ypsilantis studied physics at Berkeley where he obtained his master's degree and then became a graduate student under the direction of Emilio Segré. Together with Owen Chamberlain and Clyde Wiegand, Ypsilantis joined the 1955 experiment at the new Berkeley Bevatron that observed the first antiprotons, which became the subject of his Ph.D. thesis. After postdoctoral positions in the U.S. and after playing a pioneer role in teaching modern physics in Greece, he came to CERN in 1968, where he met Jacques Séguinot. Ypsilantis and Séguinot, working in Max Ferro-Luzzi's group, proposed the technique later named Ring Imaging Cherenkov (RICH) counter.

Together with Tord Ekelöf, they introduced this technique for high-energy physics: the first large-scale application was for the DELPHI experiment at LEP. More recently they worked in the framework of the LAA Project on noble-liquid calorimetry and on a very large water neutrino detector based of the fast-RICH technique. Ypsilantis also made a major contribution to the LHCb experiment at CERN. Over the years, Ypsilantis was associated with CERN, Ecole Polytechnique, Collège de France and INFN.

At times, he worked without a position and a salary. His office at CERN was always open for those who wanted to consult and discuss with him, even at weekends.