Radiation Safety and Applications

Ionising radiation can damage living cells by breaking chemical bonds in DNA and other molecules. Managing this risk while harnessing the benefits of radiation is one of the most important challenges in modern physics and medicine. This lesson covers the principles of radiation protection, dosimetry, and the key medical and industrial applications of radioactive materials.

Biological Effects of Radiation

When ionising radiation passes through living tissue, it can:

Ionise atoms and molecules in cells, creating highly reactive free radicals
Break chemical bonds in DNA, potentially causing mutations
Kill cells if the damage is severe enough
Cause cancer if DNA damage leads to uncontrolled cell division

The severity of the damage depends on:

The type of radiation (alpha is most damaging inside the body due to high ionising power)
The dose received
The duration of exposure
Which organs or tissues are exposed (rapidly dividing cells are most vulnerable)
Whether the source is external to or internal within the body

External vs Internal Hazards

Radiation	External Hazard	Internal Hazard	Reason
Alpha (α)	Low	Extreme	Cannot penetrate skin externally, but highly ionising if inhaled/ingested — deposits all energy in small volume of tissue
Beta (β)	Moderate	Moderate	Penetrates skin to a few mm; causes localised damage internally
Gamma (γ)	High	Lower per photon	Penetrates entire body from outside; but low ionising power means fewer ionisations per photon internally
Neutron	Very high	Very high	Highly penetrating, causes secondary ionisation and can activate atoms

The ALARA Principle

Radiation protection follows the ALARA principle: exposures should be As Low As Reasonably Achievable. This means reducing exposure to the minimum practical level, taking into account economic and social factors.

The three main protective strategies are:

1. Time

Reduce the time spent near radioactive sources. The dose received is directly proportional to exposure time — halving the time halves the dose.

2. Distance

Increase the distance from the source. For gamma radiation, the intensity follows the inverse square law (I ∝ 1/r²), so doubling the distance reduces the intensity to one quarter. Handling sources with long tongs rather than bare hands dramatically reduces dose to the hands.

3. Shielding

Place absorbing material between the source and the person:

Paper or thin plastic stops alpha
A few millimetres of aluminium stops beta
Several centimetres of lead or thick concrete significantly attenuates gamma

In practice, all three strategies are used together. A laboratory worker handling a gamma source might use tongs (distance), work quickly (time), and stand behind a lead screen (shielding).

Dosimetry

Several quantities measure radiation exposure:

Absorbed Dose (D)

The absorbed dose is the energy deposited per unit mass of tissue:

D = E/m

Unit: gray (Gy), where 1 Gy = 1 J kg⁻¹
This measures the physical energy absorbed but does not account for the biological effectiveness of different radiation types

Dose Equivalent (H)

Different types of radiation cause different amounts of biological damage for the same absorbed dose. The dose equivalent (or equivalent dose) accounts for this:

H = D × Q

where Q is the quality factor (or radiation weighting factor):

Radiation	Quality Factor Q	Meaning
Gamma / X-rays	1	Baseline biological effect
Beta	1	Same as gamma per gray
Thermal neutrons	5	5× more damaging than gamma
Fast neutrons	10–20	10–20× more damaging
Alpha	20	20× more damaging than gamma

Unit: sievert (Sv), where 1 Sv = 1 J kg⁻¹ × Q
The sievert allows comparison of biological risk from different radiation types
1 Sv is a large dose; in practice, millisieverts (mSv) and microsieverts (μSv) are commonly used

Dose Limits and Typical Exposures

Exposure	Approximate Dose
Eating a banana (K-40)	~0.1 μSv
Chest X-ray	~20 μSv (0.02 mSv)
Transatlantic flight (cosmic rays)	~40–80 μSv
Annual background (UK average)	~2.7 mSv
CT scan (abdomen)	~8 mSv
Annual dose limit for public	1 mSv (above background)
Annual dose limit for radiation workers	20 mSv
Dose causing acute radiation sickness	~1000 mSv (1 Sv)
LD50 (lethal dose for 50% of people)	~4000 mSv (4 Sv)

Worked Example 1: Dose Equivalent Calculation

A radiation worker receives an absorbed dose of 0.002 Gy from gamma radiation and 0.0001 Gy from alpha particles in one month. Calculate the total dose equivalent.

Solution:

H_gamma = D × Q = 0.002 × 1 = 0.002 Sv = 2.0 mSv

H_alpha = D × Q = 0.0001 × 20 = 0.002 Sv = 2.0 mSv

Total H = 2.0 + 2.0 = 4.0 mSv

Despite the alpha absorbed dose being 20× smaller, it contributes equally to the biological risk due to its quality factor of 20.

Worked Example 2: Distance and Dose Rate

A researcher must perform a task near a gamma source. At 0.5 m, the dose rate is 120 μSv h⁻¹. The annual dose limit is 20 mSv.

(a) How many hours per year could they work at 0.5 m?

Time = 20,000 μSv / 120 μSv h⁻¹ = 167 hours

(b) If they stand at 2 m instead, what is the dose rate and how many hours could they work?

Dose rate at 2 m = 120 × (0.5/2)² = 120 × 0.0625 = 7.5 μSv h⁻¹

Time = 20,000/7.5 = 2667 hours

Quadrupling the distance gives 16× more working time — the inverse square law is a powerful protection tool.

Medical Applications

Diagnostic Use of Tracers

Radioactive tracers are used to image internal organs and detect disease:

A small amount of a radioactive isotope (e.g., technetium-99m) is injected, swallowed, or inhaled
The isotope accumulates in the target organ or tissue
A gamma camera detects the gamma rays emitted and builds an image

Ideal properties of a medical tracer:

Emits gamma radiation (penetrates the body to reach the detector)
Short half-life (hours, not days — reduces long-term dose)
Not chemically toxic
Quickly excreted from the body after the scan

Technetium-99m (t½ = 6 hours) is the most widely used medical tracer, accounting for over 80% of diagnostic nuclear medicine procedures.

Therapeutic Use

Radiation therapy destroys cancer cells by damaging their DNA:

External beam radiotherapy: A focused beam of gamma rays (from cobalt-60 or a linear accelerator) is directed at the tumour from multiple angles. This concentrates the dose at the tumour while spreading it thinly over surrounding healthy tissue.
Brachytherapy: Small radioactive sources are placed directly inside or next to the tumour (e.g., iodine-125 seeds for prostate cancer). The short range of beta or low-energy gamma radiation limits damage to nearby healthy tissue.
Radioiodine therapy: Iodine-131 (beta emitter, t½ = 8 days) is swallowed to treat thyroid cancer. The thyroid gland naturally concentrates iodine, so the radioactive isotope accumulates in the thyroid and delivers a targeted dose.

Industrial Applications

Thickness Gauging

In manufacturing processes (paper, metal sheet, plastic film), the thickness of the material can be monitored using a radioactive source and detector on opposite sides of the material:

Material	Appropriate Source	Radiation Type	Why
Paper / thin plastic	Strontium-90	Beta	Partially absorbed by thin material — sensitive to small thickness changes
Metal sheet (mm)	Cobalt-60	Gamma	Penetrates metal; absorption proportional to thickness
Very thin foil	Americium-241	Alpha	Very short range; sensitive to tiny changes

A feedback system adjusts the rollers to maintain constant thickness.

Sterilisation

Gamma radiation (from cobalt-60) is used to sterilise medical equipment, food packaging, and some foods:

The high-energy gamma rays kill bacteria and other microorganisms
The items do not become radioactive (irradiation ≠ contamination)
No heat is involved, so heat-sensitive items can be sterilised

Smoke Detectors

Americium-241 (alpha emitter, t½ = 432 years) is used in ionisation smoke detectors:

Alpha particles ionise air molecules between two electrodes, creating a small current
Smoke particles entering the chamber absorb alpha particles, reducing the ionisation current
The drop in current triggers the alarm

The long half-life ensures the detector works reliably for many years.

Contamination vs Irradiation

	Irradiation	Contamination
Definition	Exposure to radiation from an external source	Radioactive material deposited on or inside something
Duration	Stops when source is removed	Continues until material decays or is removed
Object becomes radioactive?	No	The contaminating material IS radioactive
Clean-up	Simply move away from source	Must physically remove material or wait for decay
Example	Standing near a gamma source	Radioactive dust on skin or clothing
Medical example	Having an X-ray	Swallowing radioactive iodine

Common exam mistake: Confusing irradiation with contamination. A patient having a chest X-ray is irradiated (the radiation stops when the machine is turned off). A patient given technetium-99m is contaminated (the radioactive material is inside their body until it decays or is excreted).

Choosing the Right Source for an Application

flowchart TD
    A["What is the\napplication?"] --> B["Medical tracer"]
    A --> C["Cancer treatment"]
    A --> D["Thickness gauge"]
    A --> E["Smoke detector"]
    A --> F["Sterilisation"]
    B --> B1["Gamma emitter\nShort half-life (hours)\ne.g., Tc-99m (6 h)"]
    C --> C1["Beta emitter for targeted\ntherapy (I-131, 8 days)\nor gamma for external beam"]
    D --> D1["Match source type to\nmaterial thickness:\nα for foil, β for paper,\nγ for metal"]
    E --> E1["Alpha emitter\nLong half-life\ne.g., Am-241 (432 yr)"]
    F --> F1["Gamma emitter\nLong half-life\ne.g., Co-60 (5.3 yr)"]

Real-World Application: Radon Gas in Homes

Radon-222 is a naturally occurring radioactive gas produced by the decay of uranium-238 in rocks and soil. It seeps into buildings through cracks in foundations and accumulates in poorly ventilated spaces, particularly basements and ground floors.

Radon is the single largest source of background radiation in the UK (~42% of average dose). It decays into polonium-218 and then polonium-214, both alpha emitters that can be inhaled and deposited in the lungs. The alpha radiation from these decay products directly damages lung tissue, making radon the second largest cause of lung cancer after smoking.

The UK has an action level of 200 Bq m⁻³ for radon concentration in homes. Areas of granite geology (Cornwall, Devon, parts of Scotland) have the highest radon levels. Mitigation involves improving ventilation beneath floors and sealing cracks in foundations.

A-Level Deep Dive: Radiation Safety and Applications

Spec mapping

Edexcel 9PH0 specification Topic 11 — Nuclear and Particle Physics (paraphrased) addresses the nature, properties and biological effects of ionising radiation, the meaning of activity and absorbed dose, and the use of radioactive sources in medicine, industry and dating (refer to the official specification document for exact wording). The "safety and applications" strand draws on Topic 11 background — radioactive decay, half-life, the three principal emissions (α, β, γ) — and links forward to Topic 12 (Gravitational and Electric Fields, where the inverse-square law governs gamma intensity) and Topic 9 (Thermodynamics, relevant to deposited energy and heating). It is examined principally in Paper 2 — Advanced Physics II, with shorter recall items possible in Paper 1, and synoptic application in Paper 3. The Edexcel formula and data booklet provides the activity equation, the exponential decay law and the inverse-square relation, but candidates must memorise the qualitative differences between α, β and γ in penetration, ionisation and biological hazard.

Worked example with full mark scheme

Question (8 marks):

A medical physicist is using a sealed cobalt-60 source for external-beam radiotherapy. Cobalt-60 emits beta-minus particles followed by two gamma photons of energies 1.17 MeV and 1.33 MeV. The mean energy delivered per decay to a patient's tumour, after collimation and absorption modelling, is taken as 1.25 MeV.

(a) State, with reasons, why the beta emission is not the principal hazard to the patient during external-beam treatment, while the gamma emission is. (3)

(b) The activity of the source incident on the tumour after collimation is $4.0 \times 10^{8}$ Bq, and 12% of decay energy is deposited in a tumour of mass 0.20 kg over a 60 s exposure. Calculate the absorbed dose, in grays, delivered to the tumour. (5)

Solution with mark scheme:

(a) B1 — beta-minus particles have a range of only a few millimetres in tissue and are absorbed by the source housing or the patient's skin layer, so they do not reach an internal tumour from an external source.

B1 — gamma photons are highly penetrating, with attenuation lengths in soft tissue of order tens of centimetres at MeV energies, so they can deliver energy to a deep tumour.

B1 — gamma is therefore the useful radiation for external beam therapy; beta would be relevant for an internal (sealed-implant or radio-pharmaceutical) source. Common error: writing "gamma is most ionising" — this is wrong; alpha is the most ionising per unit length, but its penetration is too low for external use.

(b) Step 1 — energy per decay deposited in tumour.

Energy released per decay reaching the tumour: $E = 0.12 \times 1.25\,\text{MeV} = 0.150\,\text{MeV}$ .

Convert to joules: $E = 0.150 \times 10^{6} \times 1.6 \times 10^{-19} = 2.4 \times 10^{-14}\,\text{J}$ per decay.

M1 — using $1\,\text{eV} = 1.6 \times 10^{-19}\,\text{J}$ and applying the 12% absorption fraction.

Step 2 — total decays in 60 s.

$N = A\,t = 4.0 \times 10^{8} \times 60 = 2.4 \times 10^{10}$ decays.

M1 — using $N = A\,t$ for a source whose activity is approximately constant over the exposure (justified because 60 s is negligible compared with the 5.27-year half-life of Co-60).

Step 3 — total energy absorbed.

$E_{\text{total}} = 2.4 \times 10^{10} \times 2.4 \times 10^{-14} = 5.76 \times 10^{-4}\,\text{J}$ .

M1 — multiplying decays by energy per decay.

Step 4 — absorbed dose.

$D = \dfrac{E_{\text{total}}}{m} = \dfrac{5.76 \times 10^{-4}}{0.20} = 2.88 \times 10^{-3}\,\text{Gy} \approx 2.9\,\text{mGy}$ .

A1 — correct numerical value.

A1 — correct unit (Gy or mGy) and answer to 2 s.f. consistent with input data.

Total: 8 marks (3 + 5).

Specimen question modelled on the Edexcel 9PH0 Paper 2 format

Question (6 marks): A worker stands 1.5 m from an unshielded gamma source and receives a dose rate of 80 µGy h⁻¹. The dose rate is dominated by the inverse-square attenuation of the gamma flux in air (negligible absorption in 1.5 m).

(a) Calculate the dose rate the worker would receive at a distance of 4.5 m, assuming the same source. (2)

(b) The maximum permitted whole-body annual dose for a classified radiation worker is 20 mGy. Estimate, to 1 s.f., the maximum number of hours per year the worker can spend at 4.5 m from this unshielded source. (2)

(c) Discuss one operational measure, other than increasing distance, that the worker could use to reduce dose, referring to the ALARP principle. (2)

Mark scheme decomposition by AO:

(a)

M1 (AO2.2) — applying $I \propto 1/r^{2}$ , so dose rate $\propto 1/r^{2}$ . Ratio $(1.5/4.5)^{2} = 1/9$ .
A1 (AO1.1b) — $80/9 \approx 8.9$ µGy h⁻¹.

(b)

M1 (AO2.1) — annual budget in matching units: $20\,\text{mGy} = 2.0 \times 10^{4}$ µGy.
A1 (AO1.1b) — hours $= 2.0 \times 10^{4} / 8.9 \approx 2.2 \times 10^{3}$ h, so $\sim 2000$ h to 1 s.f.

(c)

B1 (AO1.2) — naming a valid measure: shielding (lead, concrete) reducing transmitted intensity, or reducing exposure time per task.
B1 (AO3.5) — linking to ALARP: dose should be As Low As Reasonably Practicable, so the worker still applies time and shielding controls even when the legal limit is not reached.

Total: 6 marks split AO1 = 3, AO2 = 2, AO3 = 1. Edexcel uses inverse-square / dose questions to test fluent unit conversion (µGy ↔ mGy) and qualitative engagement with safety culture (ALARP) — the AO3 mark is reserved for showing the worker's behaviour is shaped by principle, not only by the legal ceiling.

Synoptic links

Connects to:

Radioactive decay (Topic 11): dose calculations begin with activity $A = \lambda N$ and the exponential decay law $N = N_0 e^{-\lambda t}$ . For long-lived sources (Co-60, Cs-137) over short exposures, activity is treated as constant; for short-lived medical isotopes (Tc-99m, $t_{1/2} = 6$ h) decay during a procedure must be accounted for.
Half-life (Topic 11): application choice depends critically on $t_{1/2}$ . Diagnostic tracers need short half-lives to minimise patient dose after imaging; radiotherapy sources need long half-lives so calibration remains valid over months; smoke detectors need decade-scale half-lives so they remain functional over the device's life.
Radiation detection (Topic 11): Geiger–Müller tubes, scintillation counters and ionisation chambers all rely on the ionising property of radiation. Detector efficiency varies with radiation type (GM tubes are poor for gamma compared with scintillators), which constrains experimental design in the required practical.