There were concerns about German use of radiological warfare with regards to D-Day, and plans were made to deploy teams with Geiger counters during the invasion (Operation PEPPERMINT). In the end, they weren't put into effect, though, because there was no strong evidence of a German radiological warfare program. I have written a bit about this here.

Whether such a thing was used prior to FDR's visit to Casablanca, I don't know specifically, but it's not totally anachronistic.

In the late 1920s women used to paint a glow-in-the-dark substance on the faces of watches called radium for the United States Radium Corporation and the Luminite Corporation. In the late 20's many of them began to suffer debilitating periodontal disease that one doctor seemed to think resembled the symptoms that Swedish workers endured after painting phosphorus on matches. The workers would often lick the paint brushes to obtain a fine point. This turned out to be radium poisoning and several large lawsuits were filed by the women who did this work against the two corporations.

The Geiger counters were to “to guard against the presence of radium which may be used in an attempt to injure the President.” It was a standard Presidential protection measure that had been added along with many others after Pearl Harbor. Obviously the effects of radium poisoning were well-known at the time.

Steve Gillon wrote about the changes to White House protection after Pearl Harbor in his book, some of which you can read here.

Edit: sorry, Swedish workers, not Swiss.