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A new theory of track formation in emulsion accounts for the tracks of charged particles on the basis of a theory developed earlier for the response of biological molecules and NaI(T1) to energetic heavy ions. The probability that an emulsion grain will remain undeveloped when exposed to δ rays depositing a mean energy E is assumed to be e-E/E0 where E0 is the dose at which l/e (37%) of the emulsion grains remain undeveloped, as in the one-or-more-hit cumulative Poisson distribution. The parameter E0 incorporates variations in emulsion properties and processing conditions. Calculation of the spatial distribution of the ionization energy deposited by δ rays is combined with the assumed emulsion response to yield the spatial distribution of developed grains about the path of the charged particle. Calculations are in agreement with experimental data for grain counts (up to the relativistic rise), blackness profiles, and track width.