= Track grain density
{title2=$g=N/L$}
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.<Meu Trabalho em Física de Mésons com Emulsões Nucleares by Cesare Mansueto Giulio Lattes>[1984 recollection, paragraph beginning “Devo adicionar”: scattering, grain density and an escaping secondary; no numerical grain count is reported here]{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.
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