Cannot be gathered directly.
Unstable Isotopes occur naturally across all environments, at random times. Sometimes, when collecting Oxygen Gas , Ozone Gas or Methane Gas, the gathered gas will turn out to be added to your inventory automatically.
Unstable Isotopes cannot be gathered from Vespene, Amaranthine or Therazene.
Can be found at:
Unstable Isotopes gather from all gasses except vespene, amaranthine, and therazene
Highest rate of gather is probably from chests in depth >150 water, as a byproduct from fast oxygen and water gather... place a teleport pad to quickly return, as the oxygen and water fills up the cargo bay quickly.