In some embodiments, apparatus and systems, as well as methods, may operate to irradiate a portion of a geological formation with neutrons in a neutron burst generated by a switchable electronic source, to measure (with one or more detectors) a flux of gamma rays to provide a measured flux, at least a portion of the gamma rays being generated by the neutrons, and to determine one or more of a neutron porosity, a density, and/or a photoelectric factor of the geological formation based on the measured flux. Other apparatus, systems, and methods are disclosed.