Problems of registration of Cherenkov radiation using photomultiplier tubes (PMTs) with flat and hemispherical photocathodes are considered. Configurations of several PMTs with flat photocathodes allow to design a module that has the properties of spherical PMT. Besides the independence of its response on the direction of incident Cherenkov radiation, such quasi-spherical module (QSM) allows to estimate this direction.
The module with six PMTs oriented along the axes of an orthogonal coordinate system – the simplest configuration of quasi-spherical module – was proposed for the first time in 1979 by the MEPhI group and was used in practice in the Cherenkov water detector (CWD) NEVOD.
Common principles of designing the quasi-spherical modules are considered. The properties of isotropy of the response of QSM depending on the distance to a track of charged particle and on the number of PMTs in QSM are discussed. The relevance of the problem of studying the properties of quasi-spherical modules is confirmed by the decisions to use multi-PMT QSMs in modern large-scale projects such as KM3NeT and next generation of IceCube.