A downhole tool includes a window cap located in a cover that is positioned between an electromagnetic radiation detector in the downhole tool and a geological formation into which a borehole is formed and where the downhole tool is to be positioned. The electromagnetic radiation detector is to detect an electromagnetic radiation from the geological formation. The downhole tool includes a window core positioned behind the window cap relative to an external environment of the downhole tool, wherein the window core is positioned between the window cap and the electromagnetic radiation detector.