Neutronium is a key element in Asgard technology and is used in the construction of Asgard ships as well as in technology such as the time dilation device used on Hala. It is also essential for the creation of human-form Replicators. The new Asgard homeworld, Orilla, was rich in neutronium, and Fifth came to Orilla seeking neutronium to replicate. Fifth was defeated by SG-1 and the Asgard at Orilla, however, he was able to escape, and had collected enough neutronium from the planet to create a new human-form Replicator in the image of Carter.
Cross Reference: Fifth, Orilla, Replicators, Time Dilation Device
Episode Reference: Unnatural Selection, New Order