7.在三棱锥P−ABCP-ABCP−ABC中,PA⊥PA\botPA⊥平面ABCABCABC,PA=6PA=6PA=6,BC=3BC=3BC=3,∠CAB=π6\angle CAB=\frac{\pi }{6}∠CAB=6π,则三棱锥P−ABCP-ABCP−ABC的外接球半径为((( )))