Xenon, Tungsten, Carbon are basically nick names for our releases. We used to have HDR Light Studio version 2, 3, 4, 5. Then after 5 we moved to giving the releases nick names.
So in answer to your question, you can not buy an old Tungsten release... when buying a license for HDR Light Studio at our web site, you would be buying the latest version with all current features, that is currently Xenon Drop 4.