4.4.3.2Different half-lives of radioactive isotopes
AQA GCSE Physics Foundation (8463), Foundation tier · Atomic structure › Hazards and uses of radiation
Practise Different half-lives of radioactive isotopes. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Radioactive isotopes have half-lives from fractions of a second to billions of years. You need to explain why the hazard from a radioactive material depends on its half-life, and use half-life to choose a suitable isotope for a job. GCSE Physics only, not in Combined Science.
Key facts
Half-lives range from fractions of a second to billions of years
Short half-life → activity falls quickly → hazardous for a short time
Long half-life → activity falls slowly → hazardous for a long time
Medical tracer: half-life of a few hours
Smoke alarm or thickness gauge: half-life of years
Long half-life waste must be stored safely for thousands of years
Notes
A very wide range of half-lives
Different radioactive isotopes have very different half-lives, from fractions of a second to billions of years.
Examples: technetium-99m about 6 hours; iodine-131 about 8 days; cobalt-60 about 5 years; carbon-14 about 5700 years; uranium-238 about 4.5 billion years.
How half-life affects the hazard
diagram
Short half-life: the activity falls quickly, so the material is only hazardous for a short time (hours or days).
Long half-life: the activity falls very slowly, so the material stays hazardous for a very long time. Nuclear waste containing such isotopes must be stored safely for thousands of years.
Same starting activity: the short half-life source is soon safe; the long half-life source stays hazardous.
The hazard also depends on the type of radiation emitted, and whether the material is inside or outside the body.
Choosing an isotope for a job
Medical tracers need a short half-life (a few hours): long enough to take the images, short enough that the activity in the patient soon falls.
Smoke alarms and thickness gauges need a long half-life (years), so the count rate stays nearly constant and the source does not need replacing often.
When choosing from a table, check the half-life AND the type of radiation.
Don’t lose marks
Saying a long half-life isotope is more dangerous in every way. For the same number of nuclei its activity is lower; the problem is that it lasts.
Choosing a medical tracer with a long half-life (the patient would stay radioactive) or an alpha emitter (it would not leave the body).
Using only the half-life, or only the type of radiation, when choosing an isotope from a table.
More tips
Memory tricks
Short half-life: 'bright but brief'. Long half-life: 'weak but lasting'.
Tracer half-life: think of a hospital visit, so hours, not years.
Gauge or alarm half-life: think of the life of the machine, so years.
Exam technique
In 'choose an isotope' questions, give reasons for your choice and say why the others are unsuitable.
Quote the half-life from the table in your answer, e.g. '6 hours is long enough to...'.
For 'explain the hazard', build a chain: half-life → how fast the activity falls → how long it stays dangerous.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
State that half-lives vary widelyHalf-lives range from fractions of a second to billions of years.
Grade 5
Link half-life to how fast activity fallsA short half-life means the activity falls quickly; a long half-life means it falls slowly.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Which isotope’s activity falls to a very low level more quickly?
P
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy2 marks
Isotope P has a half-life of 2 hours. Isotope Q has a half-life of 20 years.
(a) Which isotope’s activity falls to a very low level more quickly?[1]
(b) Which isotope would need to be stored safely for longer?[1]
Show the answer and mark scheme
(a)Answer: P
P
(b)Answer: Q
Q (it stays radioactive for much longer)
Question 2Medium6 marks
Two samples, A and B, contain the same number of radioactive nuclei. The isotope in sample A has a half-life of 2 hours. The isotope in sample B has a half-life of 5000 years.
(a) Which sample has the higher activity at the start? Give a reason for your answer.[2]
(b) Explain why sample A is less hazardous than sample B one week later.[2]
(c) Explain why the hazard from a radioactive material depends on the half-life of the isotope it contains.[2]
Show the answer and mark scheme
(a)Answer: A: its nuclei decay much more quickly, so there are more decays per second.
A
with a shorter half-life, a larger proportion of its nuclei decay each second
(b)Answer: After 84 half-lives A has almost completely decayed, but B’s activity is practically unchanged.
one week is 84 half-lives of A, so almost all of A has decayed / its activity is negligible
the activity of B has hardly changed (because its half-life is so long), so it is still hazardous
(c)Answer: Short half-life: intense but short-lived hazard. Long half-life: weaker but very long-lasting hazard.
a short half-life gives a high activity, but only for a short time
a long half-life means the material stays radioactive (hazardous) for a very long time, although its activity is lower