ROOT documentation and source code is somewhat unclear on the subject of the relative abundances / fractions of elements and mixtures:
void TGeoMixture::AddElement(TGeoElement *elem, Double_t weight);
The second parameter is stat.weight (molar fraction) or fraction by weight?
void TGeoElement::AddIsotope(TGeoIsotope *isotope, Double_t relativeAbundance);
the relativeAbundance should be provided in molar of weight fractions?
Thank you,
Andrey