Theme E — Nuclear and Quantum Physics

E.3.8 Applications of Radioactivity — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 25 of 45 1 quiz

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Lesson summary

E.3.8 Applications of Radioactivity — IB PHYSICS SL notes: Explain how half-life and the properties of radiation are used in dating, medicine, tracing and industry.Key factors — the penetrating power of the radiation and the half-life of the isotope.The chemical behaviour of the …

Living organisms keep a constant carbon-14 to carbon-12 ratio
After death no more carbon-14 is absorbed
Reliable range
Uncertainty
It applies only to once-living material
Rocks start with uranium-238 and no lead

Part 1 of 7Choosing a radioactive isotope

Key factors

Key factors the penetrating power of the radiation and the half-life of the isotope.
The chemical behaviour of the element also matters for tracers.

The half-life must be long enough for the job and short enough to leave no lasting hazard.

Uses include dating, medicine, leak detection, thickness control, smoke detectors and nuclear power.

Part 2 of 7Carbon-14 dating

Carbon-14 dating

Formation

Formation cosmic rays release neutrons that hit nitrogen-14 to form carbon-14: n + ¹⁴N gives ¹⁴C + p.

Living organisms keep a constant carbon-14 to carbon-12 ratio

Living organisms keep a constant carbon-14 to carbon-12 ratio because carbon is continually replaced while they live.

After death no more carbon-14 is absorbed

After death no more carbon-14 is absorbed the activity falls with a half-life of about 5730 years.

Age is found by comparing present activity (or the C-14 to C-12 ratio) with that of living tissue.

Reliable range

Reliable range samples between about 500 and 60 000 years old.
Younger samples have too high a ratio to show small changes accurately.
Older samples have an activity too low to separate from background radiation.

Uncertainty

Uncertainty carbon-14 production in the atmosphere varies with location and may not have been constant over time.

It applies only to once-living material

It applies only to once-living material not to rocks.

Part 3 of 7Uranium-lead dating

1Rocks start with uranium-238 and no lead

Rocks start with uranium-238 and no lead uranium decays through a chain to stable lead-206.

2Uranium-238 has a half-life of 4.5 billion years

Uranium-238 has a half-life of 4.5 billion years so it suits very old rocks.

3The ratio of lead-206 to uranium-238 atoms rises with time

The ratio of lead-206 to uranium-238 atoms rises with time this ratio gives the age of the rock.

It showed that the Earth is billions of years old.

Part 4 of 7Medical uses

Radiotherapy

Radiotherapy gamma rays are focused on a tumour to destroy cancer cells while surrounding tissue is protected.
Lead sheets and beams from several directions spare healthy tissue.
Beta radiation can treat skin cancer by direct application.

Tracers

Tracers a short-lived isotope is injected so its path in the body can be followed.
A short half-life gives a high initial activity from a small sample and a low long-term risk.
Technetium-99m is a gamma emitter (about 140 keV) with a half-life of 6 hours and is made at the hospital.
Gamma escapes the body and is the weakest ioniser; iodine-131 (half-life 8 days) is taken up by the thyroid.

PET scanning

PET scanning a positron emitter such as fluorine-18 is attached to glucose.
Positrons annihilate with electrons into two gamma photons in opposite directions, which locate high metabolic activity.

Sterilisation

Sterilisation gamma radiation kills bacteria on equipment, even through packaging.
The equipment does not become radioactive, because radiation affects outer electrons and not the nucleus.

Part 5 of 7Industrial uses

Leak detection

Leak detection a gamma emitter is added to the fluid in an underground pipe.
A detector above ground shows a raised count rate at the leak.
Gamma is used because it is the most penetrating;
- sodium-24 (beta and gamma, half-life about 15 hours) is typical.
The half-life must be long enough to stay detectable and short enough not to remain in the supply.

Thickness control

Thickness control a beta source and detector sit on opposite sides of a moving sheet of foil, paper, plastic or steel.
The count rate falls as the sheet gets thicker, and feedback adjusts the rollers.
Alpha would be fully absorbed and gamma would pass almost unchanged, so beta is used.
A long half-life avoids frequent source replacement.

Smoke detectors

Smoke detectors americium-241 ionises air between two electrodes, giving a small current.
- (alpha, half-life 460 years)
Smoke absorbs the alpha particles, the current drops and the alarm sounds.
Alpha is safe because it cannot leave the casing; the long half-life means the source need not be replaced.

Weld inspection

Weld inspection gamma radiography reveals cracks in metal.

Part 6 of 7Risks and safe use

Risks and safe use

Every application must balance benefit against the risk of ionising radiation.

Good practice

Good practice smallest activity, shortest exposure, shielding and monitoring of those exposed.

Choose isotopes that decay to stable products

Choose isotopes that decay to stable products and match the radiation type to the penetration needed.

Disposal depends on half-life

Disposal depends on half-life short-lived sources are stored until decayed, long-lived ones need secure storage.

Dating assumes the original isotope proportion is known and the sample is uncontaminated

uncertainty rises as the remaining activity gets small.

Part 7 of 7More from the specification

1Nuclear Stability and Strong Nuclear Force

In stable atomic nuclei, the attractive strong nuclear force overcomes the electrostatic repulsive force acting between positively charged protons.

2Alpha and Beta Emission Modes

Nuclei with an excess of protons and neutrons undergo alpha decay to reduce size,
while neutron-rich nuclei emit beta particles;
when an unstable nucleus de-excites, excess energy is emitted as gamma photon emission.

3Binding Energy per Nucleon and Nuclear Fission

Nuclear binding energy per nucleon peaks around iron-56;
when massive nuclei undergo fission, the total binding energy increases,
and an enormous amount of kinetic and radiative energy is released.

4Proton

Positive particle in the nucleus, relative charge +1 and relative mass 1.

5Binding energy

The minimum energy required to separate the nucleons of a nucleus to infinity.

6Nuclear notation

Symbol showing mass number and atomic number.

7Beta-plus decay

Proton changes to a neutron with emission of a positron and a neutrino.

8Decay equation

An equation in nuclide notation with balanced nucleon and proton numbers.

Key terms

Term	Meaning
Tracer	Radioactive isotope used to follow a process or locate a structure.
Carbon dating	Estimating age from the remaining activity of carbon-14.
Radiotherapy	Using radiation to destroy cancer cells.
PET	Positron emission tomography, imaging based on positron annihilation.
Smoke detector	Device using an alpha source and an ionisation current.
Thickness gauge	Device using beta absorption to monitor sheet thickness.
Uranium-lead dating	Dating rocks from the ratio of lead-206 to uranium-238.

Avoid these mistakes

Choosing a tracer with a very long half-life — the patient would stay radioactive for a long time.

Using an alpha source for thickness gauging — it is absorbed completely.

Saying carbon dating can date rocks — it applies only to once-living material.

Forgetting dating assumes the original ratio was known and constant.

Choosing gamma tracers because they ionise strongly — they are chosen because they escape the body and ionise weakly.

Last updated: 18 October 2026