A tool having a neutron source, a gamma ray detector, and a photomultiplier tube is provided. The gamma ray detector and the photomultiplier tube are at least partially surrounded by a layer of boron. The tool is used to make measurements, and the number of prompt gamma rays emitted by the boron is determined from the measurements. The number of neutrons detected may be inferred using the determined number of prompt gamma rays. The tool may also have a layer of neutron absorbing material different from boron or a layer of heavy metal at least partially surrounding the boron. The tool may be a logging tool used to delineate a porous formation and to determine its porosity. The tool may have a plurality of gamma ray detector/photomultiplier tube pairs and those pairs may be used to determine a formation hydrogen index and/or a borehole hydrogen index.