The phrase "gold nuclei" refers to the tiny, central parts of gold atoms. They are like the cores or centers of the atoms and contain protons and neutrons. Full definition
They achieved this by smashing together gold nuclei at the Relativistic Heavy Ion Collider (RHIC). (newscenter.lbl.gov)
Some of them collided with gold nuclei in the foil and emitted piercing gamma rays. (sciencemag.org)
Five years ago, RHIC scientists announced that they had begun to understand the mash of melted protons and neutrons created by smashing gold nuclei in their machine, a collider ring nearly 4 kilometers around. (scientificamerican.com)