After reading caf's comment about Concorde's radiation detector, I came across this: "To protect passengers and crew from unnecessary radiation exposure, airworthiness authorities of Britain and France require that civilian aircraft, which fly above 50,000 ft, have a solar cosmic ray warning device. If the radiation level reachs 100 millirem/hr, the pilot is to descend to a lower altitude." - https://www.faa.gov/data_research/research/med_humanfacs/oam...

Guessing there aren't a whole lot of civilian planes affected by this rule :-D