Determine the half-life of 90Y. A freshly isolated sample of 90Y was found to have an activity of 9.7 x 105 disintegrations per minute at the 1:00 P.M. on December 3, 2006. At 3:20 P.M. on December10, 2006, its activity was calculated again and found to be 1.5 x 105 disintegrations per minute.