^{1.) 238}Pu is a manufactured nuclide that is used as a power source on some space probes - Cassini, for instance, which entered orbit around Saturn in 2005 and gathered data until it was intentionally destroyed in September of 2017.

Find the radius of a²³⁸Pu nucleus in femtometers, assuming it is well-described by a tightly packed sphere of its nucleons.

r= ?