A sealed source containing cobalt-60 had an activity of 37 kBq when it left the manufacturer. On reaching a hospital some time later, its activity was found to be 29 kBq. The half-life of cobalt-60 is 5.27 years. (Take Avogadro constant NA=6.022×1023mol−1 and atomic mass of cobalt-60 = 60 g mol−1.)
(i) Calculate the time interval (in years) between the two activity measurements.
(ii) If the original mass of cobalt-60 in the source was 1.00 mg, determine the mass that remains when the activity is 29 kBq.