The Paul Scherrer Institute (PSI) is a multi-disciplinary research institute for natural and engineering sciences in Switzerland. It is located in the Canton of Aargau in the municipalities Villigen and Würenlingen on either side of the River Aare, and covers an area over 35 hectares in size. Like ETH Zurich and EPFL, PSI belongs to the Swiss Federal Institutes of Technology Domain of the Swiss Confederation. The PSI employs around 2,100 people. It conducts basic and applied research in the fields of matter and materials, human health, and energy and the environment. About 37% of PSI's research activities focus on material sciences, 24% on life sciences, 19% on general energy, 11% on nuclear energy and safety, and 9% on particle physics. PSI develops, builds and operates large and complex research facilities and makes them available to the national and international scientific communities. In 2017, for example, more than 2,500 researchers from 60 different countries came to PSI to take advantage of the concentration of large-scale research facilities in the same location, which is unique worldwide. About 1,900 experiments are conducted each year at the approximately 40 measuring stations in these facilities. In recent years, the institute has been one of the largest recipients of money from the Swiss lottery fund. History The institute, named after the Swiss physicist Paul Scherrer, was created in 1988 when EIR (Eidgenössisches Institut für Reaktorforschung, Swiss Federal Institute for Reactor Research, founded in 1960) was merged with SIN (Schweizerisches Institut für Nuklearphysik, Swiss Institute for Nuclear Research, founded in 1968). The two institutes on opposite sides of the River Aare served as national centres for research: one focusing on nuclear energy and the other on nuclear and particle physics. Over the years, research at the centres expanded into other areas, and nuclear and reactor physics accounts for just 11 percent of the research work at PSI today. Since Switzerland decided in 2011 to phase out nuclear energy, this research has primarily been concerned with questions of safety, such as how to store radioactive waste safely in a deep geological repository. Since 1984, PSI has operated (initially as SIN) the centre for Proton Therapy for treating patients with eye melanomas and other tumours located deep inside the body. More than 9,000 patients have been treated there until now (status 2020). The institute is also active in space research. For example, in 1990 PSI engineers built the