William H. Patterson, Robert A. Heinlein: In Dialogue with His Century, volume 1, Tor Books, 2011 ch. 20-21; see also the description of Heinlein correspondence here
According to Heinlein biographer William H. Patterson, the Karst character is "an homage to Lise Meitner, who worked out the necessary mathematical support for the idea of fission in 1939 on a train fleeing Nazi Germany." Meitner, who was Hahn's co-worker, had to leave Germany in 1938 because of her Jewish origin. See Patterson's comments here
