Cecil Frank Powell developed nuclear emulsion methods for studying cosmic rays. A charged particle leaves a microscopic sequence of developed grains; track length, scattering and grain density can then constrain its energy and mass. His work with colleagues on connected stopping tracks helped identify the pion and its decay into a muon. The method combined long exposures with repeated microscope measurements, turning rare cosmic events into quantitative evidence.[ref]
The Cosmic Radiation by Cecil Frank Powell. 1950 authored lecture, photographic-emulsion methods and meson studies
A physicist.[ref]
https://www.bristol.ac.uk/university/distinctions/. Independent professional profile: Powell biographical paragraph: appointed Melville Wills Professor of Physics in 1948; full name and life years in heading
Portrait of Cecil Frank Powell
. Source. 
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