11.6. Edges $S A$, $S B$, and $S C$ of the tetrahedron $S A B C$ are mutually perpendicular, and their lengths are 2, 3, and 6, respectively. a) Find the radius $R$ of the sphere circumscribed around the tetrahedron. b) Does there exist a sphere with a radius smaller than $R$ that contains the given tetrahedron?