The invention relates to a system for treating gases and/or liquids with radiation or for detecting radiation in gases and/or liquids, including at least one optical system and a reactor. The reactor has a hollow body shape which includes lateral surfaces, connecting parts, and an inner chamber which may be open to the front and rear sides, through which a medium flows or in which the medium is present. The reactor is designed, at least partially, in the form of a radiation reflector and is divided into first and second functional areas. This enables a particularly homogeneous radiation distribution in the inner chamber of the reactor, which increases the efficiency of the treatment or detection. Also, due to this arrangement, the system is more compact.
