= Nuclear emulsion track experiment
In 1947, <Cesare Lattes>, <Hugh Muirhead>, <Giuseppe Occhialini> and <Cecil Frank Powell> reported <charged particle> tracks in which a stopping particle was followed by a secondary particle. Their paper treated the interpretation as provisional.<Processes Involving Charged Mesons by Cesare Mansueto Giulio Lattes and others>[Lattes, Muirhead, Occhialini and Powell, Processes Involving Charged Mesons, Nature 159 (24 May 1947), pp. 694–697, abstract and byline]{ref} The eventual distinction between <pions> and <muons> made a striking use of <photography>: a tiny permanent record could distinguish particles by how they slowed and transformed.
The method already had a history. In their jointly authored 1937 report, <Marietta Blau> and <Hertha Wambacher> described <photographic plates> exposed for five months at Hafelekar, $2300\,\mathrm m$ above sea level.<Disintegration Processes by Cosmic Rays with the Simultaneous Emission of Several Heavy Particles by Marietta Blau and Hertha Wambacher>[Blau and Wambacher, Disintegration Processes by Cosmic Rays with the Simultaneous Emission of Several Heavy Particles, Nature 140 (1937), p. 585, opening paragraph and joint byline]{ref} Neither shared the 1950 <Nobel Prize in Physics>, awarded to <Cecil Frank Powell>.<Research Profile: Cecil Powell by Luisa Bonolis>[Opening award statement: Powell's 1950 prize concerned development of the method and meson discoveries]{ref} This was a development of photographic <particle detection>, with earlier contributors, rather than its invention by one person.
In his own later account, <Cesare Lattes> recalls calibrating emulsion shrinkage and the <particle range>–<energy> relation with accelerator-produced particles. <Giuseppe Occhialini> took borax-treated and untreated plates to Pic du Midi for about a month. The untreated plates provided a control; the treated plates retained more events because <latent image> fading was reduced. Later, <Cesare Lattes> exposed plates at the higher Chacaltaya station in Bolivia and obtained more candidate events.<Meu Trabalho em Física de Mésons com Emulsões Nucleares by Cesare Mansueto Giulio Lattes>[Lattes, Meu trabalho em física de mésons com emulsões nucleares, 1984 recollection in Portuguese translation, paragraphs beginning “Foi-me dada a tarefa”, “Occhialini e eu decidimos”, “Quando Occhialini revelou” and “Assim, propus”; the recollection's altitude estimate is not adopted here]{ref} The Pic du Midi <cosmic ray> exposure was roughly $3\,\mathrm{km}$ above sea level.<Research Profile: Cecil Powell by Luisa Bonolis>[“The discovery of the pion”: Pic du Midi exposure altitude, given as 3000 metres]{ref}
\Image[/nuclear-emulsion-record-develop-scan.png]
{title=From <nuclear emulsion> exposure to a reconstructed <charged particle> track}
{description=Original functional diagram. Dots and paths are schematic, with no measured grain counts or spatial scale. The coating and joined-track drawings are separate schematic examples. The <microscope> reads developed material, rather than a whole <pion>.}
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