Paper 3 Nuclear Physics Answers

Q1 Exponential Decay

1. $36\mathrm{h}$ is 3 half-lives, so $A=8.0\times 10^4/2^3=1.0\times 10^4\mathrm{Bq}$. [2]
2. $\lambda =\ln 2/T_{1/2}=0.693/(12\times 3600)=1.60\times 10^{-5}{\mathrm{s}}^{-1}$. [3]
3. Exponential decay halves repeatedly; it approaches zero but does not reach exactly zero in the mathematical model. [2]

Q2 Fission and Fusion Energy

1. In both cases, products are more tightly bound on average and have higher binding energy per nucleon; total binding energy increases and mass decreases. [4]
2. $E=0.190\times 931.5=177\mathrm{MeV}$. [2]
3. Neutrons emitted by one fission may induce further fissions in other fissile nuclei. [2]

Q3 Radiation Safety

1. Alpha: strongly ionising, weakly penetrating. Beta: moderate ionisation/penetration. Gamma: weakly ionising per path length but highly penetrating. [4]
2. Irradiation is exposure to radiation; contamination means radioactive material is present on/in the object. [2]
3. Increase distance, reduce time, use shielding such as lead/concrete, or use remote handling. [2]