Equipment for the detection of undesired behind casing water flow in a producing well is provided. A well logging sonde having a modular fluid tight structure is disclosed. Basically, three modules which when joined together form a fluid tight assemblage comprise the sonde. A first module houses a 14 MEV neutron generator and a 125 kilovolt high voltage power supply for operating the neutron generator. A second module houses dual spaced gamma ray detectors for detecting characteristic gamma rays from the decay of radioactive nitrogen 16 at two different longitudinal distances from the neutron source. A third module houses control electronics and data transmission circuits for controlling the neutron generation and receiving detected signals from dual gamma ray detectors and for transmitting data to the surface from the sonde. The modules are constructed so that they may be assembled with the neutron generator either above or below the dual detectors while maintaining the same spacing distances from the generator to the detectors. Surface instrumentation for receiving and interpreting the gamma ray measurements are also provided.