Two identical nuclei are accelerated in a collider to a kinetic energy of $621.38 \mathrm{GeV}$ and made to collide head on. If one of the two nuclei were instead kept at rest, the kinetic energy of the other nucleus would have to be 15,161.70 GeV for the collision to achieve the same center-of-mass energy. What is the rest mass of each of the nuclei?