The present invention relates to a logging unit which can be used for measuring radioactive gamma energy spectrum naturally-existed in the borehole. Its technical scheme includes probe portion, mechanical structure portion and circuit portion, in which the mechanical structure portion mainly includes upper connector, external shell, power supply skeleton, circuit skeleton, scale mark and lower connector. The circuit portion mainly includes power supply plate, main control plate and PHA plate, the power supply plate is mounted on the aluminum alloy skeleton, and the main component of the probe portion includes a cesium iodide crystal, an optically-coupled piece and a photomultiplier, on the pins of photomultiplier are welded a voltage-dividing resistor and a filter capacitor, the described component is packaged in an aluminum metal cylinder, and the upper end of the probe is equipped with three lead-out wires which respectively are high-voltage power supply wire, signal output wire and grounding wire. The circuit composition of main control plate and PHA plate includes energy spectrum preamplifier circuit, energy spectrum communication interface, energy spectrum measurement and control circuit, pulse height analysis circuit and M5 data remote transmission control circuit. Besides, said invention also provides the concrete application field and range.