Abstract: The porosity of an earth formation is derived in a plused neutron logging tool by use of a directionalized and borehole-shielded eptithermal neutron detector array and by processing detector count rate measurements of the epithermal neutron die-away curve in accordance with a multi-parameter fit to obtain as one parameter the epithermal neutron decay time of the formation and as a second parameter an indication of detector standoff (environmental) effects. The multi-parameter fitting procedure separates the formation and standoff effects on the measured count vs time curve, and yields a decay time measurement that is primarily representative of formation porosity and substantially free of environmental effects. To enhance count rate statistics, the neutron source and one or more of the epithermal neutron detector may be located at the same depth in the borehole. The neutron burst centroid is monitored an used to define a fiducial window for processing the die-way curve data.