State what is meant by the binding energy of a nucleus.
Some masses are shown in the table below.
Calculate the binding energy of a nucleus of lanthanum-141.
The nuclide lanthanum-141 (
) has a half-life of 3.9 hours. Initially, a radioactive source contains only lanthanum-141. The initial activity of the source is 
.
Calculate the time for the activity of the lanthanum-141 to be reduced to 
.
Suggest why the total activity of the radioactive source measured at the time calculated in (i) may be greater than .
.
