A well logging instrument includes a source of high energy neutrons
arranged to bombard a formation surrounding the instrument. A
scintillator sensitive to gamma radiation resulting from interaction of
the high energy neutrons with the formation is disposed in the
instrument. A neutron shielding material surrounds the scintillator. A
neutron moderator surrounds the neutron shielding material. An amplifier
is optically coupled to the scintillator.