A downhole segmented radiation detector tool measuring formations surrounding a borehole is provided, the tool including at least a plurality of segments capable of detecting radiation, wherein the segments return to the tool after interacting with material surrounding the tool; and radiation shielding configured to allow radiation directly from a radiation source to pass internally through the downhole tool to a reference segment. A method of measuring formations surrounding a borehole is also provided, the method including at least: lowering a downhole tool into a borehole surrounded by a subterranean formation; detecting a first plurality of X-rays or gamma-rays that return to the downhole tool after interacting with materials surrounding the downhole tool using a first segment of an array segmented radiation detector; and configuring associated radiation shielding to allow radiation directly from a radiation source to pass internally through the downhole tool to the reference segment.