Fission neutrons are neutrons which have retained their recorded kinetic energy during nuclear fission and have not slowed down. The FRM II has a secondary source (the so-called converter facility), which consists of two uranium plates. They are arranged approximately 1 m from the reactor core and irradiated with thermal neutrons in this position. Through uranium fission, fast neutrons are released and fall with a certain probability influenced by the geometric conditions into a horizontal beam tube that guides them to their destination.
The beam tube ends at the instrument MEDAPP. MEDAPP facilitates the irradiation of tumour patients and a number of other experiments; for example, for physical and biological dosimetry, studies of material resistance to neutron irradiation (functional life of semiconductors, devices in aerospace) and non-destructive materials testing. The flux density of fast neutrons at the irradiation position is approximately 3,2 x 108 cm-2s-1.