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We present the results of modelling archival observations of type Ib SN1999dn. In the spectra, two He I absorption features are seen: a slowercomponent with larger opacity, and a more rapid He I component with smalleropacity. Complementary results are obtained from modelling the bolometric lightcurve of SN 1999dn, where a two-zone model (dense inner region, and less denseouter region) provides a much better fit than a one-zone model. A key result wefind is that roughly equal amounts of radioactive material are found in bothregions. The two-zone analytical model provides a more realistic representationof the structure of the ejecta, including mixing and asymmetries, which offersa physical explanation for how the radioactive material is propelled to, andmixed within, the outer regions. Our result supports the theoreticalexpectation that the radioactive content in the outflow of a type Ib supernova(SN) is thoroughly mixed. We fit our model to six additional SNe Ibc, of whichthe majority of the SNe Ib are best described by the two-zone model, and themajority of the SNe Ic by the one-zone model. Of the SNe Ic, only SN 2007gr wasbest fit by the two-zone model, indicating that the lack of helium spectralfeatures for this event cannot be attributed to poor mixing.

Type Ib SN 1999dn as an example of the thoroughly mixed ejecta of Ib supernovae