SPEC 8463 • TOPIC 4.4.3.2 / 4.4.3.3 • PAPER 1 • PHYSICS ONLY
Different Half-Lives & Uses of Nuclear Radiation
Choose the wrong isotope and the application doesn't work. This lesson is the matching rulebook — half-life, emission type, and route of access.
Match the Isotope to the Job
Every radioactive isotope has three properties that decide what it's good for: half-life, type of radiation emitted, and how the radiation reaches the target. Match all three correctly and the application works. Get any one wrong and it fails — or becomes dangerous.
6h
Tc-99m tracer half-life
432y
Am-241 smoke detector
4.5×10⁹y
U-238 rock dating
What You'll Master
By the end of this lesson you'll be able to: define a tracer; explain why hazards vary with half-life (spec 4.4.3.2); justify the choice of isotope for medical uses (spec 4.4.3.3); and avoid the most-common exam trap of confusing "long half-life" with "more dangerous".
Key Definition: Tracer
Tracer
A small quantity of a radioactive isotope used to follow the path of a substance — for example through plumbing or a person's body.
The tracer is added to whatever is being studied. As it travels, it emits radiation. A detector tracks the radiation from outside the system, revealing the path the substance has taken — without opening the pipe or cutting open the patient.
Two Common Tracer Contexts
Medical tracers — injected/swallowed by a patient, used to image internal organs or find blockages.
Industrial tracers — added to fluids in pipes to find leaks or measure flow rates underground.
Spec 4.4.3.2: Hazards Vary With Half-Life
The spec requires students to "explain why the hazards associated with radioactive material differ according to the half-life involved". Two opposing principles, both true:
Short Half-Life
High activity per second — many decays in a small window. Intense radiation immediately, but decays away quickly. Dangerous on contact, but the danger ends within days/hours.
Long Half-Life
Low activity per second — few decays per unit time. Less intense, but emits for centuries or longer. Persistent contamination hazard if it gets inside the body or environment.
Exam Application
If asked "which isotope is most hazardous?" — never pick by half-life alone. The mark scheme wants you to consider half-life AND emission type AND location (inside vs outside body) AND route of entry (inhaled, ingested, external exposure).
The Three Properties to Match
Every "why is this isotope suitable?" answer needs to justify three things:
PropertyQuestion to ask
Half-lifeDoes it match the timescale the application needs?
Emission typeCan the radiation reach the target (or be detected) — and stop where it should?
Ionising powerDoes it cause enough damage for the job — without unwanted side effects?
Medical Tracer: Technetium-99m SPEC CORE
Spec 4.4.3.3 names "exploration of internal organs" as a required medical use. Tc-99m is the textbook example.
Tc-99m~6 hoursGAMMA
PropertyWhy it suits a tracer
Half-life ~6 hoursLong enough to inject the patient, allow it to reach the target organ, and image — but short enough that nearly all of it decays away within a day. Total dose to the patient is small.
Gamma emitterGamma penetrates body tissue, escapes the body, and is detected by an external camera. Alpha and beta would be absorbed in tissue and never reach the detector.
Least ionising of the threePer decay, gamma damages cells less than alpha or beta would inside the body.
Cancer Treatment: Cobalt-60 SPEC CORE
Spec 4.4.3.3 names "control or destruction of unwanted tissue" as a required medical use. Co-60 is a common external-beam therapy source.
Co-60~5.3 yearsGAMMA
PropertyWhy it suits cancer treatment
Half-life ~5.3 yearsLong enough that the source lasts years in the radiotherapy machine before needing replacement. Not so long that it can't be made and stored safely.
Gamma emitterGamma penetrates deep into tissue to reach internal tumours. Alpha or beta would be stopped before reaching the cancer.
Used externallyThe source stays outside the patient. Multiple beams are directed from different angles so the tumour gets a high dose while surrounding healthy tissue gets less.
Evaluation Skill (spec 4.4.3.3)
"Evaluate" questions weigh benefit against risk. For Co-60 therapy: benefit — destroys cancer cells, saves lives. Actual risk — also damages healthy tissue near the tumour; small statistical risk of inducing a second cancer years later. Worth it for life-threatening disease.
Perceived risks (spec language): patients often perceive radiation as more dangerous than statistics suggest because the word "radiation" carries strong public associations (Chernobyl, weapons). Good evaluation answers acknowledge that perceived risk can differ from actual risk shown in data.
Sterilising Medical Equipment
Surgical instruments, syringes, and dressings are sterilised by exposing them to gamma radiation. Bacteria and viruses are killed because gamma photons ionise their DNA.
GAMMA
Industrial source (often Co-60)
PropertyWhy it suits sterilisation
Gamma penetratesPasses through plastic packaging and the equipment itself to kill organisms throughout. Beta or alpha wouldn't reach the centre.
Irradiation onlyThe equipment is exposed but never touches the radioactive source. Equipment doesn't become radioactive — this is irradiation, not contamination.
Lethal to bacteriaIonising radiation damages microbial DNA enough to prevent replication. Effective for items that can't be heat-sterilised (some plastics) or wet-sterilised.
Smoke Detectors: Americium-241
A small alpha source inside the detector ionises the air in a chamber. The ions allow a small current to flow. Smoke disrupts the ions → current drops → alarm sounds.
PropertyWhy it suits a smoke detector
Half-life ~432 yearsActivity stays essentially constant over the detector's lifetime (10-20 years). The owner never has to replace the source.
Alpha emitterAlpha is strongly ionising — perfect for ionising the air in the chamber and creating the detection current.
Safe in this contextAlpha is stopped by a few cm of air and by the detector casing. Outside the device, virtually no radiation reaches the user. Safe as long as the device isn't opened .
Dating: Carbon-14 & Uranium-238
The matching rule shines here: the half-life must match the timescale being measured. Date a 5,000-year-old skeleton with U-238 and you'd see no measurable decay. Date a billion-year-old rock with C-14 and there'd be no C-14 left at all.
C-14
~5,730 y
Carbon dating — organic remains
Living things constantly absorb C-14 from the atmosphere. When they die, intake stops; existing C-14 decays. Measuring the remaining C-14 dates the time of death. Range: thousands to tens of thousands of years.
U-238
~4.5×10⁹ y
Geological dating — rocks
U-238 decays through a chain to stable lead-206. The ratio of U-238 to Pb-206 in a rock dates when the rock solidified. Range: millions to billions of years. Used to estimate the age of the Earth.
Thickness Gauges: Beta Emitters
Used in factories making paper, foil, or sheet metal. A beta source sits on one side of the moving sheet; a detector sits on the other.
BETA
Source on one side, detector on the other
PropertyWhy beta is the right choice
PenetrationBeta is partly absorbed by the material. Alpha would be completely blocked (no signal). Gamma would barely be affected (no measurable change).
Sensitivity to thicknessSheet too thick → less beta gets through → detector reading drops → rollers automatically adjust. Sheet too thin → more beta gets through → reading rises → rollers compensate.
Long half-life chosenSo the source's activity stays steady over years of factory operation, removing one variable from the thickness measurement.
How to Justify Any Isotope Choice
When asked "Explain why isotope X is suitable for use Y", work the three properties in order. Every property needs a mechanism, not just a statement.
StepWhat to sayExample phrasing
1. Half-lifeState the half-life and link it to the timescale needed."6 hours is long enough to image but short enough to limit dose"
2. Emission typeState alpha/beta/gamma and link it to what the radiation needs to do."Gamma penetrates tissue so it can escape the body to be detected"
3. Ionising power / safetyLink the ionising property to the desired outcome or safety."Alpha is highly ionising, so it strongly ionises the chamber air"
Mark-Scheme Reward
Each property scores its own mark when paired with a mechanism. Three mechanisms = up to 3-4 marks. A bare statement like "short half-life is safer" won't score — the mark scheme wants the link explained.
PEE Worked Example
"Explain why technetium-99m is suitable for use as a medical tracer."4 marks
P
Technetium-99m emits gamma radiation with a short half-life of about 6 hours.
E
Gamma can escape the body and be detected externally, so the tracer's path through the patient can be imaged.
E
A short half-life means the tracer's activity falls rapidly within hours, so the patient's total radiation exposure is low.
E
Gamma is the least ionising of the three radiation types, so the per-decay damage to body tissue is minimised compared with alpha or beta.
Examiner Alert: Half-Life ≠ Hazard
Jun 2023 Paper 1 Q05.4 — <1% scored full marks
Given a table of four beta-emitting isotopes with different half-lives, students were asked which posed the biggest risk to health based only on half-life. Most wrongly picked the one with the longest half-life. The correct answer was the one with the shortest half-life — the mark scheme awards "greatest activity → greatest dose of radiation absorbed per second".
The mark scheme also rejected the common phrase "decays the fastest" on its own — without linking it to amount of radiation emitted in a given time period.
Hazard depends on four things, not just half-life:
FactorWhy it matters
Type of radiationAlpha is most ionising (worst inside body); gamma most penetrating (worst outside body)
Half-lifeShort = high dose rate now, gone soon. Long = low dose rate, lingers indefinitely.
Location (inside / outside body)Alpha inside = catastrophic; alpha outside = stopped by skin
Route of entryInhaled / ingested = direct internal contamination; external exposure = irradiation only
Mark-Scheme Phrasing
To score, link half-life to activity (decays per second), then activity to dose absorbed per second. The chain — half-life → activity → dose rate — is what the examiner is looking for.
Common Errors That Cost Marks
✗ Weak
"Short half-life = safe / long half-life = dangerous."
✓ Strong
Hazard depends on type, half-life, location and route. Short half-life = high initial activity; long half-life = persistent low-level emission.
✗ Weak
"Tracers should last a long time so we can keep imaging."
✓ Strong
Tracers need a short half-life so the patient's total dose is low — imaging is done within hours, not days.
✗ Weak
"Alpha is dangerous so we can't use it anywhere."
✓ Strong
Alpha is dangerous inside the body. In a sealed device (smoke detector), alpha is the ideal choice — its strong ionisation is exactly what's needed, and it can't escape the case.
✗ Weak
"Decays the fastest" on its own (Jun 2023 Q05.4 mark-scheme reject).
✓ Strong
"Greatest activity, so greatest dose of radiation absorbed per second" — the time period is what scores the mark.
✗ Weak
"Short half-life is safe so it's used for tracers." (1 mark out of 4)
✓ Strong
State the half-life value, link it to dose, state the emission type, and link each property to a mechanism. Three linked claims = 3-4 marks.
Your Turn
A hospital uses iodine-131 to investigate the function of a patient's thyroid gland. Iodine-131 has a half-life of 8 days and emits both beta and gamma radiation. It is given to the patient by mouth.
Evaluate whether iodine-131 is a suitable choice for this medical use.4 marks
This is a standard 4-mark "evaluate" pattern. Use the 3-property framework — half-life, emission type, route of entry — and reach a supported conclusion.
1
Half-life of 8 days is reasonably short — the iodine will decay within a few weeks, limiting the patient's total dose, but is long enough to reach the thyroid and emit detectable radiation while present.
2
Gamma emission escapes the body to be detected externally, allowing the thyroid to be imaged. This is suitable for diagnosis.
3
Beta emission deposits its energy in nearby tissue. Inside the thyroid, this damages thyroid cells — useful if the aim is also to destroy overactive tissue, but harmful for purely diagnostic use.
4
Conclusion: suitable for combined imaging/treatment of the thyroid because the iodine concentrates in the thyroid gland naturally. Less suitable if pure imaging is needed — a gamma-only emitter like Tc-99m would give a lower dose.
Checkpoint
You should now be able to:
✓
Define a tracer using mark-scheme wording.
✓
Explain why hazards differ by half-life (spec 4.4.3.2) — short = high activity, brief; long = low activity, persistent.
✓
Justify isotope choice for medical uses (spec 4.4.3.3) — exploration of organs (Tc-99m), destruction of unwanted tissue (Co-60), sterilisation (gamma).
✓
Match other uses to isotope properties — smoke detector (Am-241 alpha), carbon dating (C-14), rock dating (U-238), thickness gauge (beta).
✓
Apply the 3-property framework — half-life vs timescale, emission vs access, ionising power vs safety — to any "is X suitable for Y?" question.
✓
Avoid the "long half-life = more dangerous" trap — hazard depends on four factors, not just half-life.
✓
Evaluate perceived vs actual risks (spec WS 1.5) — public perception of radiation risk often differs from statistical risk shown in data.
✓
Use half-life data in standard form (spec MS 1b) — half-lives range from microseconds to billions of years, so standard form (e.g. 4.5×10⁹ years) is the practical notation.
Next Lesson
Lesson 4.07 — Nuclear fission and fusion (physics only). How splitting heavy nuclei powers reactors, and how fusing light nuclei powers stars.
Next: Match the Isotope to the Job