Radiation dose and safety – irradiation, contamination, grays and sieverts
PhysicsQuantum & Nuclear PhysicsAges 17–18
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Sign in to playChoose a source (α, β, γ or neutron), its activity, the distance, a shield (paper, aluminium, lead or concrete) and the exposure time to work out the absorbed dose D (Gy) and the equivalent dose H = w_R·D (Sv). The Contamination screen shows that radioactive material on the skin or inside the body keeps irradiating you after you walk away, unlike external irradiation. A dose ladder compares your dose with natural background, X-rays, CT scans and dose limits, with a graph of dose against time and a results table.
Lesson: Absorbed dose, equivalent dose and radiation weighting factors; irradiation and contamination; radiation safety
What it shows
Radiation harms cells by depositing energy in them. The absorbed dose D is the energy absorbed per kilogram of tissue, measured in grays (1 Gy = 1 J/kg). Because alpha particles do far more damage per joule, the equivalent dose H = w_R·D, in sieverts, multiplies by a radiation weighting factor: 20 for alpha, 1 for beta and gamma. Irradiation from an outside source stops when you move away or shield it. Contamination is different: radioactive material on the skin or inside the body keeps irradiating you until it is washed off, decays or leaves the body.
How to use
On the Irradiation tab, pick a Source, then set Activity A, Distance d (or drag the person), Shield and Thickness x, and Time near the source t. Read the dose rate, D and H, and see where H sits on the dose ladder. On the Contamination tab, choose On the skin or Inside the body and compare the graphs. Press Record result to build a table.
Parameters you can change
- Screen External irradiation, Contamination
- Source type α – americium-241, β – strontium-90, γ – cobalt-60, Neutron – californium-252
- Source activity A (external irradiation) 0.01–100000 MBq
- Distance d from the source to the person 0.02–10 m
- Shield None, Paper, Aluminium, Lead, Concrete
- Shield thickness x 0.1–500 mm
- Time t (near the source, until washed off, or since intake) 0.01–8760 h
- Type of contamination On the skin, Inside the body
- Activity of the contamination 0.01–100000 kBq
Questions to explore
- Why is an alpha source almost harmless outside the body but very dangerous inside it?
- By what factor does the gamma dose fall when you move from 1 m to 2 m away?
- Why does leaving the room end irradiation but not contamination?
