Abstract: An apparatus for performing an operation in a borehole penetrating the earth includes a borehole tool and a neutron source within the borehole tool that emits neutron-alpha particles pairs as the result of a nuclear reaction. The apparatus also includes an alpha particle detector within the borehole tool arranged to detect when an alpha particle emitted by the neutron source strikes it and a gamma ray detector to detect gamma rays produced as a result of interactions between neutrons produced by the neutron source and at least one material in an area of interest outside of the borehole. The apparatus also includes a processing system that determines a characteristic of the area of interest based on a time difference between when the alpha particle strikes the alpha particle detector and a time when one or more gamma rays strikes the gamma ray detector.