Abstract: An arrangement for determining an energy spectrum of a beam of radiation or particles is disclosed. The arrangement comprises a plurality of polymeric bodies. Each of the plurality of polymeric bodies includes an optical waveguide. Each of the plurality of polymeric bodies has a scintillator disposed at a respective end of the optical waveguide. The scintillators are arranged relative to each other such that an energy resolution of a particle beam incident on the arrangement can be determined. Furthermore, a particle detector with the arrangement and an evaluation unit for reading out the particle detector are disclosed.