Neutronium

Neutronium was a grade 5 premium scavenged metal crafting material.

Source
Neutronium could be gathered from a Neutronium Speeder Parts node, which required 300 skill in scavenging. It was most commonly found on Belsavis and Voss.

The following materials could also be found in Neutronium Speeder Parts nodes:

It could also be obtained from grade 5 scavenging missions. Additionally, there was a chance to obtain Neutronium from reverse engineering the appropriate level and type of gear.