SQA National 5 Physics Area 6 Radiation: a complete overview of nuclear radiation, dosimetry and half-life

Area 6 Radiation is about the radiation given out by unstable nuclei, how we measure the dose it delivers, and how radioactive sources decay over time. It has three key areas; each has its own answer page with worked examples, and this guide ties them together.

Nuclear radiation

The three types of nuclear radiation are alpha (a helium nucleus), beta (a fast electron) and gamma (a high-energy electromagnetic wave). Alpha is the most ionising but least penetrating (stopped by paper or skin); beta is in between (stopped by a few millimetres of aluminium); gamma is the least ionising but most penetrating (reduced only by thick lead or concrete). Ionisation (knocking electrons off atoms) is what makes radiation harmful to cells. The activity of a source is the number of decays per second, $A = \frac{N}{t}$, in becquerels.

Dosimetry

Absorbed dose is the energy absorbed per kilogram, $D = \frac{E}{m}$, in grays. Equivalent dose allows for how harmful the radiation is, $H = D w_R$, in sieverts, using the radiation weighting factor (about $20$ for alpha, $1$ for beta and gamma). The equivalent dose rate is $\dot{H} = \frac{H}{t}$. The dose is reduced by less time, more distance and shielding, and background radiation from natural and artificial sources is always present.

Half-life

The half-life is the time for the activity (or number of undecayed nuclei) to halve. Each half-life the activity halves again, so after $n$ half-lives it is $\left(\frac{1}{2}\right)^n$ of the start. Half-life is found from a decay graph or a table. Decay is random, so we predict the behaviour of many nuclei on average. Short half-lives suit medical tracers; long ones suit carbon dating and dating rocks.

How Radiation is examined

Radiation questions combine recall (the nature and properties of alpha, beta and gamma, the meaning of ionisation, ways to reduce a dose) with calculations ($A = \frac{N}{t}$, $D = \frac{E}{m}$, $H = D w_R$, $\dot{H} = \frac{H}{t}$, and half-life problems). For calculations, convert times to the right unit and, for half-life, count the number of half-lives first and halve the activity that many times. For dosimetry, keep the gray and the sievert distinct and remember the weighting factor.

For the official course specification

The SQA publishes the full National 5 Physics course specification, data sheet, relationships sheet and past papers at sqa.org.uk. Always revise from the current specification and SQA past papers.