Radiation Physics and Instrumentation
Core concepts
Candidates must be fluent in atomic structure, the nature of alpha, beta, gamma, X-ray and neutron emissions, and the inverse-square law. Understand photon interactions (photoelectric effect, Compton scattering, pair production) and how each dominates at different energies and atomic numbers. Know that beta particles are a continuous spectrum, that gamma emission follows nuclear de-excitation, and that characteristic X-rays arise from electron-shell transitions. Instrumentation concepts include gas-filled detectors (ionization chamber, proportional counter, Geiger-Mueller), scintillators, and semiconductor detectors, plus the electronics chain (preamp, amplifier, discriminator, scaler).
Traps
A common trap is confusing exposure rate with activity, or forgetting that the inverse-square law applies to a point source in a vacuum and fails at close range or with extended sources. Another is treating GM dead time as negligible at high count rates. Watch for questions that swap the roles of the photoelectric effect (low energy, high Z) and Compton scattering (mid energy, low Z).
Worked example
A point source gives 400 µSv/h at 2 m. At 4 m: 400 × (2/4)² = 400 × 0.25 = 100 µSv/h. Rule of thumb: doubling distance quarters the dose rate.
Instrumentation detail
Know that ionization chambers measure dose or exposure with flat energy response, proportional counters can discriminate particle type, and GM counters are sensitive but saturate and cannot measure dose rate accurately. Semiconductor detectors offer superior energy resolution for spectroscopy.