  Microwave spectroscopy was used to probe the superfluid-Mott Insulator
transition of a Bose-Einstein condensate in a 3D optical lattice. Using density
dependent transition frequency shifts we were able to spectroscopically
distinguish sites with different occupation numbers, and to directly image
sites with occupation number n=1 to n=5 revealing the shell structure of the
Mott Insulator phase. We use this spectroscopy to determine the onsite
interaction and lifetime for individual shells.
