Radium

From Turtledove

Radium is a radioactive chemical element which has the symbol Ra and atomic number 88. Its appearance is almost pure white, but it readily oxidizes on exposure to air, turning black. Radium is an alkaline earth metal that is found in trace amounts in uranium ores. It is extremely radioactive.

[edit] Radium in The Man With the Iron Heart

German scientists left a small batch of radium in a garbage dump in Hechingen when they were captured by the Allies at the end of World War II. In 1946, the German Freedom Front retook these scientists from the British, in the hopes of building an atomic bomb. While the scientists couldn't do this, Karl Wirtz shared his knowledge of the radium in Hechingen with Reinhard Heydrich.

While American troops had the sense that the GFF had something big planned for Hechingen, the town was in the French occupation zone, and American officials were loathe to share more specific information with their "allies". Thus, the GFF successfully raided the dump, found the radium, and subsequently used it against the American residency zone.