Activity: decays per second, in becquerels
Activity is the number of nuclear decays per second, measured in becquerels (Bq) — 1 Bq = 1 decay per second. Each decay leaves one fewer undecayed nucleus, so over time fewer nuclei remain to decay and the activity falls. Decay is random (you cannot predict which nucleus decays or when) and spontaneous (unaffected by temperature, pressure or chemical state), so real readings scatter about a smooth decay curve.
Half-life: what halves, and it is fixed
Half-life is the time for half the undecayed nuclei to decay — equivalently, the time for the activity to fall to half its value. Say what is halving; being vague loses the mark. The half-life is constant for an isotope: it does not change with amount, temperature, or how much has decayed. Different isotopes have very different half-lives — carbon-14 is about 5700 years. After each half-life the activity halves: to one half, then a quarter, then an eighth.
Irradiation vs contamination
Irradiation is being exposed to radiation from a source outside the body; no material is transferred, and it stops when the source is removed. Contamination is getting radioactive material onto or inside the body; it keeps emitting radiation until removed or decayed. Inside the body, alpha contamination is especially dangerous — strongly ionising, its energy dumped in nearby tissue — whereas for external irradiation alpha is least dangerous (stopped by skin).
Drawn from real examiner reports.
Half-life: name what halves
No credit: the time for it to halve, or for the mass/element to halve. Full credit: the time for half the undecayed nuclei to decay, or for the activity to fall to half. Do not say the half-life changes with more source or higher temperature — it is constant for the isotope. And a source never fully runs out: it only halves each half-life, so never quite reaches zero.
Nov 2024 1P Q4(c)(i): imprecise descriptions of what is being halved lost the mark; keep it simple and name the quantity.
Reading half-life from a graph
For "use the graph" questions: read the time-axis scale carefully — misreading it throws the whole half-life out. Find the half-life as the time for a chosen activity to fall to half, then confirm with the quarter value. Always quote actual numbers read from the graph; describing only the shape scores nothing. Subtract any given background before halving.
Nov 2024 1P Q4(c)(ii), Q4(d) and Jun 2023 1P Q1(c)(iii): misreading the time-axis scale, and answers with no quantitative data, lost marks.
Carbon-14 dating traps
In a carbon-dating context, a sample with more carbon-14 than expected appears younger, not older; and the half-life of carbon-14 does not change because the atmospheric concentration differs. Writing that the trees would die, or that the half-life gets longer or shorter, earns nothing. The half-life of an isotope is fixed regardless of the amount present or the conditions.
Scatter is because decay is random
Asked why data points scatter about a smooth decay curve, do not answer only that "background radiation is present". That is insufficient — you must say the count in any interval varies because radioactive decay is random (and the background itself varies too). It is the randomness, not merely the existence of background, that explains the scatter.
Jun 2023 1P Q1(c)(iv): stating background was present was insufficient; the mark needed that the background or the decay varies (is random).
Activity vs half-life
Do not confuse these. Activity is how many decays happen per second right now, measured in becquerels, and it changes (falls) over time. Half-life is a fixed time — constant for the isotope — for the activity to halve. One is a rate that decreases; the other is a period that stays the same. A source with a long half-life still has an activity that slowly falls.
Alpha danger flips inside vs outside
The danger ranking of alpha reverses with location. For external irradiation, alpha is the least dangerous — it is stopped by skin or a few cm of air, so it barely reaches living tissue. But as internal contamination, alpha is the most dangerous — it is strongly ionising and dumps all its energy into nearby cells. So "alpha is the least harmful" is only true outside the body.
Half-life calculation method
Each half-life multiplies the activity by one half. To find a time, count how many times the activity halves; time = that count × the half-life. To find a final activity, halve the start that many times. Subtract any given background first; quote activity in becquerels.
Read decay graphs carefully
On a decay graph, read the time-axis scale first. Find the half-life as the time for the activity to fall to half a value, then confirm with the quarter value. When justifying, quote numbers from the graph — the shape alone scores nothing.
Definitions and dangers: be precise
Name what halves: the undecayed nuclei, or the activity. Separate irradiation (from outside, stops when the source goes) from contamination (material on/in you, keeps irradiating). Dangers: radiation damages cells and causes mutations; reduce risk by shielding, distance and time.
This topic mixes a calculation strand (activity and half-life, 7.11-7.13) with a describe strand (uses, contamination vs irradiation, dangers, 7.14-7.16). Marks are won by counting half-lives correctly, reading decay graphs carefully, and using precise definitions.
There is no formula sheet equation to memorise here — half-life works by repeated halving:
where = starting activity (Bq), = activity after half-lives (Bq), and = number of half-lives = ( = time elapsed, = half-life, same units).
Key definitions:
Define the ACTIVITY of a radioactive source and give its unit.
A radioactive source has an activity of Bq. Its half-life is hours.
(a) State what is meant by an activity of Bq.
(b) Calculate the activity of the source after hours.
