AbstractThe fast-neutron spectrum has been measured at several positions in the IRR-1 pool-type reactor using seven activation detectors. The spectrum was calculated by the simultaneous analysis of the data from the seven detectors. Adjacent to the core the spectrum was found to be close to the fission spectrum, with a slight dip between 3 and 4.5 MeV. At increasing distances from the core, the spectrum becomes harder, with a pronounced dip between 3 and 4.5 MeV.