A thermal shield for an XRF measurement tool is formed from a heat shield and a heat shield cowling. These components protect the X-ray head assembly that includes the x-ray generation and detection columns and the head control electronics, communications and cooling systems. The heat shield is directly below the X-ray head, parallel to the x-ray head plane and plane of the PV substrate, and perpendicular to the primary beam output from the x-ray head. The heat shield is fabricated of machined copper with several ports machined through the shield. These ports provide a path for primary beam x-rays through the heat shield and for the return of fluoresced X-rays from the PV substrate back to the detector in the X-ray head, while preventing damage to the X-ray head due to the heat emitted from the PV substrate.