In the illustrative embodiments of the invention, a neutron porosity well logging tool includes a D-T neutron accelerator (12), a fast-neutron monitor for monitoring accelerator output (14), near and far epithermal neutron detectors (16, 18) for deriving a primary porosity measurement, a neutron detector array (17) intermediate to the near and far epithermal detectors for providing enhanced spatial resolution, and a far-spaced thermal neutron detector (20). The epithermal neutron detectors are shielded and spaced relative to the accelerator to optimize tool response to formation porosity, whereby the tool affords accurate porosity sensitivity over the entire range of porosities of interest. The source monitor (14) may additionally be employed to measure capture gamma ray intensity vs. time between neutron bursts for deriving formation sigma. Also, the detector array may include both epithermal and thermal neutron detectors to afford an epithermal/thermal comparison measurement. The specially shielded near-epithermal is also independently useful as a monitor of the output of high energy neutron sources.
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