System for measuring earth formation thermal neutron lifetime utilizing a harmonically intensity modulated fast neutron source

A system is disclosed for measuring indications of the thermal neutron lifetime of earth materials in the vicinity of a well borehole. A harmonically intensity modulated source of fast neutrons is used to irradiate the earth formations in the vicinity of a borehole with fast neutrons at three different intensity modulation frequencies. Intensity modulated clouds of thermal neutrons at each of the three modulation frequencies are detected by a single spaced detector. The tangent of phase shift of the thermal neutron intensity with respect to the fast neutron population intensity is measured at each of the three frequencies. These measurements may be appropriately combined to derive the thermal neutron lifetime of earth materials in the vicinity of the borehole