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. It was identified in 1898 by French chemists Marie and Pierre Curie. Marie often worked without protection, as radiation's full danger was not understood at the time, and subsequently died of resulting cancer.
Radium was used in self-luminous paints for watches, aircraft switches, clocks and instrument dials in the early 20th century. A typical self-luminous watch that uses radium paint contains around 1 microgram of radium. Hank Jeter's great grandfather's pocket watch had a glow-in-the-dark radium dial which stopped glowing during an experiment he and Irv Farmer were carrying out.
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". Likewise, the French were more suspicious of the intentions of the Americans at this point, and so failed to take precautions in Hechingen. Thus, the GFF successfully raided the dump, found the radium, and subsequently used it against an American military base in Frankfurt.
The sample of radium was indeed left in a garbage dump in Hechingen in OTL. However, its ultimate fate is unknown as of 2015.
Sgt. Ludwig Rothe owned a watch with radium-glowing hands. On more than one occasion, while waiting in the dark for an attack to go in, Rothe would glance at his glowing watch, note how much time remained, then an hour later look again and see only 10 minutes had passed.
↑See, e.g., There Will Be War Volume VIII, loc. 1747, ebook.