(a)[2]
State what the binding energy of a nucleus means.
(b)[4]
Fig. 12.1 gives the masses below. Proton ($^{1}_{1}\text{p}$) has mass $= 1.007\,\text{u}$, neutron ($^{1}_{0}\text{n}$) has mass $= 1.009\,\text{u}$, and the nucleus of lanthanum-141 ($^{141}_{57}\text{La}$) has mass $= 140.911\,\text{u}$. Calculate the binding energy of a nucleus of lanthanum-141.
(c(i))[3]
The nuclide lanthanum-141 ($^{141}_{57}\text{La}$) has a half-life of $3.9\,\text{hours}$. At the start, the radioactive source contains only lanthanum-141. Its initial activity is $A_0$. Calculate the time taken for the activity of the lanthanum-141 to fall to $0.40A_0$.
(c(ii))[1]
Suggest why the total activity of the radioactive source measured at the time calculated in (i) might exceed $0.40A_0$.