Mass stopping power for alpha particles in mylar (polyethylene terephthalate) and air at atmospheric pressure.  For mylar the density is 1.4 g/cm3 and mean ionization energy \(I=78.7 eV\), while for air the density is  1.20479 10-3 g/cm3 and \(I=85.7 eV\).\cite{NIST}  The table of data is associated with this plot on the Authorea version of the document.