Radiation hazards, protection and shielding

(a) Health hazards

Ionising radiation (alpha, beta, gamma, X-rays, neutrons) breaks chemical bonds and damages cells.

• Acute (deterministic) effects from a high dose in a short time: radiation sickness (nausea, hair loss, burns), and death above about 4–5 Sv whole-body.
• Stochastic (delayed) effects: cancer (leukaemia, thyroid, lung) and shortening of life, which appear after years; their probability rises with dose.
• Genetic effects: mutations in reproductive cells that are passed on to children.
• Internal exposure by inhaling or swallowing radioactive material (e.g. iodine-131 in the thyroid, radon, strontium-90 in bones) is especially dangerous for alpha emitters.

Units: activity in becquerel (Bq); absorbed dose in gray (Gy); equivalent dose in sievert (Sv). The public limit is about 1 mSv per year and the limit for radiation workers is about 20 mSv per year (averaged, ICRP).

Principles of protection

1. Time: reduce the time of exposure; dose is proportional to time.
2. Distance: dose falls with the square of the distance from a point source, $I \propto 1/r^2$.
3. Shielding: place absorbers between the source and the person: lead and concrete for gamma and X-rays, plastic or perspex for beta, water or polythene (hydrogen-rich) and boron/cadmium for neutrons, and paper or skin for alpha.

Other measures are containment, ventilation, personal monitoring badges (TLD) and the ALARA principle (as low as reasonably achievable).

(b) Shielding calculation

Attenuation law: $I = I_0 e^{-\mu x}$.

Half-value layer:

$$x_{1/2} = \frac{\ln 2}{\mu} = \frac{0.693}{0.806} = 0.860\ \text{cm}$$

For 1 % transmission, $I/I_0 = 0.01$:

$$x = \frac{\ln 100}{\mu} = \frac{4.605}{0.806} = 5.71\ \text{cm}$$

Distance effect:

$$D_4 = D_1\left(\frac{1}{4}\right)^2 = \frac{4}{16} = 0.25\ \text{mSv/h}$$

Answer: HVL = 0.86 cm; thickness for 1 % = 5.7 cm of lead; dose rate at 4 m = 0.25 mSv/h.