Directional epithermal neutron detector

Claim 1

A borehole tool for epithermal neutron die-away logging of subterranean formations surrounding a borehole, comprising: (a) a pulsed source of fast neutrons for irradiating the formations surrounding a borehole, (b) at least one neutron counter for counting epithermal neutrons returning to the borehole from said irradiated formations, (c) a neutron moderating material, (d) an outer thermal neutron shield providing a housing for said counter and said moderating material, (e) an inner thermal neutron shield dividing said housing so as to provide a first compartment bounded by said inner thermal neutron shield and a first portion of said outer thermal neutron shield and a second compartment bounded by said inner thermal neutron shield and a second portion of said outer thermal neutron shield, said counter being positioned within said first compartment and said moderating material being positioned within said second compartment, and (f) means for positioning said borehole tool against one side of said borehole wall and azimuthally orienting said borehole tool such that said first chamber is in juxtaposition with said borehole wall, whereby formation epithermal neutrons penetrating into said first chamber through said first portion of said outer thermal neutron shield are detected by said neutron counter for die-away measurement, thereby maximizing the directional sensitivity of said neutron counter to formation epithermal neutrons, while borehole fluid epithermal neutrons penetrating into said second chamber through said second portion of said outer thermal neutron shield are largely slowed down and lowered in energy by said moderating material and absorbed by said inner thermal neutron shield before penetrating into said first chamber, thereby minimizing the directional sensitivity of said neutron counter to borehole fluid epithermal neutrons.