[en] The activation neutron detector for measuring integral fluxes of fast or thermal neutrons consists of a material which can be activated by neutrons and is distributed in the volume of a heat-hardenable neutron-inert polycondensation resin. The material is a mixture of at least two elements each of which is capable of forming radioactive isotopes with different decay energies under neutron irradiation. Suitable materials are mixtures of the salts of the metals cobalt, nickel, iron, indium, mercury or lutetium and a neutron-inert acid radical in a polycondensation resin or a copolymerisation product of phenol formaldehyde-resol resin with m-chlorophenol formaldehyde resin. These substances are mixed with aluminum oxide, magnesium oxide, the oxides of aluminium, nickel, iron and thallium, nickel oxide and aluminum oxide, mercury sulfide, iron sulfide or red phosphorus. An integral neutron flux of 10²⁰ n/cm² can be detected. (DG/RF)