Improved borehold logging methods and apparatus for detecting and measuring photon and other radiation from earth formations. Such radiation may be, for example: natural; generated by a source and modified by interaction with formation constituents; produced during or immediately following interactions between neutron irradiation and formation constituents; or produced by decay of constituents rendered artificially radioactive by prior neutron activation. The radiation is detected by a scintillator and photomultiplier tube, the scintillator comprising one or more crystals containing gadolinium, for example gadolinium orthosilicate doped with cerium. Such a detector provides advantageous operating characteristics, including relatively high detection efficiency and energy resolution and the ability to operate in the borehold environment without special protection against contamination or temperature effects. Depending upon the origin of the radiation to be measured, the crystal may be surrounded with a layer of material for attenuating low-energy gamma radiation, such as lead, and/or a layer of neutron absorbing material such as boron-10. The crystal may be shaped by truncating its corners parallel to the axis of the photomultiplier tube, thereby improving the usage of space and obtaining an improvement in energy resolution.