Which scientific instrument, originally developed to study subatomic particles, led to breakthroughs in medical imaging and was later adapted for COVID-19 vaccine development?

The development of scientific instruments and technologies often involves fascinating breakthroughs and unexpected origins. Some of today's most revolutionary scientific tools began as simple ideas or were developed for entirely different purposes. This trivia question explores how a major scientific product came to revolutionize multiple fields of research.

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Electron Microscope
Cyclotron
Synchrotron
Mass Spectrometer