Methods and systems for non-intrusively detecting existence of fissile materials by measuring energetic prompt neutrons. The unique nature of the prompt neutron energy spectrum from photo-fission arising from emission of neutrons from almost fully accelerated fragments is used to unambiguously identify fissile material. Angular distribution of prompt neutrons from photo-fission and energy distribution correlated to neutron angle relative to the photon beam are used to distinguish odd-even from even-even nuclei undergoing photo-fission. Independence of the neutron yield curve (yield as a function of electron beam energy or photon energy) on neutron energy is used to distinguish photo-fission from other processes such as (γ, n). Different beam geometries detect localized samples of fissile material and fissile materials dispersed as small fragments or thin sheets over broad regions. These signals from photo-fission are unique, allowing detection of any material in the actinide region of the nuclear periodic table.