A system and method for detecting radiation from a source in a container is disclosed. A continuous optical fiber path is disposed in a medium which is part of or associated with a container and which totally encapsulates the inside volumetric space of the container. The optical fiber path provides a volumetric mass of optical fiber which is reactive to radiation from a radiation source in the container to cause an irreversible change in the light carrying capacity or other characteristic of the optical fiber. A light source is coupled to one end of the optical fiber path for introducing light having a predetermined characteristic. A light detector is coupled to the other end of the optical path for receiving light from the optical path. A circuit is coupled to the light detector and is operative to detect a change in the predetermined characteristic of the light and to provide an indication thereof in a fail-safe manner. The extraction of a radiation signal from background radiation noise is achieved using geometric methodologies. The detection system is operative to facilitate the detection of low level flux of thermalized neutrons.