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A developed grain is a visible deposit of silver produced during photographic processing. The chain of developed grains in a nuclear emulsion records interactions along a charged particle's path. Its width is a property of the recording and viewing process, not a measurement of the particle's diameter.

A nuclear emulsion is a photographic emulsion prepared for recording charged particles. The sensitive material serves as both target and record: a particle may slow, stop, decay or cause a nuclear reaction inside it. Reading that record requires track reconstruction, not recognizing a miniature picture of a particle. Unlike a cloud chamber photograph, the exposed nuclear emulsion itself retains the record until it is developed and examined.

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.^[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.^[ref] Neither shared the 1950 Nobel Prize in Physics, awarded to Cecil Frank Powell.^[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.^[ref] The Pic du Midi cosmic ray exposure was roughly $3\mathrm{km}$ above sea level.^[ref]

A photographic emulsion is a coating that responds to light to record an image in photography, for example on a photographic plate. Silver halide crystals, commonly silver bromide, are dispersed in gelatin. Exposure leaves a latent image. A developer produces dark silver grains and fixing removes remaining sensitive material. A microscope can reveal a charged particle's path through the coating: the basis of nuclear emulsion particle detection.^[ref]

The track grain density is the number $N$ of resolved developed grains along reconstructed track length $L$, divided by $L$. It is a response variable, not a universal conversion from darkness to energy. Overlapping developed grains, exposure fading, processing and optical spatial resolution can all change what is counted. Calibration must use comparable material and viewing conditions.

Cesare Lattes specifically recalls distinguishing meson tracks from proton tracks using their greater deflection by scattering and changing track grain density toward the stopping point. He also recounts a secondary track that left the coating, requiring extrapolation rather than a directly measured stopping distance.^[ref]

For a candidate joined track, the useful questions are whether both branches actually meet in depth, whether the incoming branch is consistent with stopping there, and whether the outgoing branch has the expected particle range and track grain density. A visible change of direction alone cannot distinguish a decay from scattering or an accidental crossing. Examining successive focal depths tests a projected crossing; comparison with ordinary stopping tracks tests the identification. These are measurement checks, not a claim that the original plates provided time-resolved movies.

A nuclear emulsion integrates exposure over time. It does not by itself supply the time between two apparently joined branches. Finite spatial resolution also limits how tightly one can establish their common origin: developed marks and the microscope response are much larger than a subatomic particle. There is no single resolution number applicable to every emulsion, developer and optical arrangement.