def METHOD_NAME(self, mat, units='atom/b-cm'):
    """Return atom densities for a given material
    Parameters
    ----------
    mat : str, int, openmc.Material or slice
        Material index.
    units : {"atom/b-cm", "atom/cm3"}, optional
        Units for the returned concentration. Default is ``"atom/b-cm"``
        .. versionadded:: 0.13.1
    Returns
    -------
    dict
        Dictionary mapping nuclides to atom densities
    """
    mat = self._get_mat_index(mat)
    normalization = (1.0e-24 if units == 'atom/b-cm' else 1.0) / self.volume[mat]
    return {
        name: normalization * self[mat, nuc]
        for name, nuc in self.index_nuc.items()
    }