I see others that are also assuming J and B_J to be the same for both isotopes, and some mention that it is indeed since we only change the number of neutrons, and this would not affect the electron cloud. But I would like more clarification on this, I’m not 100% convinced since I have no clue on how these J and B_J behave.
Moreover, J is the spin of the electron cloud, but how exactly is this defined? Can we be sure it remains constant? More info is always welcome!