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Before the simulators, the vocabulary. What the different kinds of radiation actually are, what stops them, what the units mean, and how an instrument turns something invisible into a number you can act on.
A heavy particle — two protons and two neutrons — thrown out by elements like uranium, radium and americium.
Stopped by: A sheet of paper, or the dead outer layer of your skin.
Why it matters: Harmless outside the body, but dangerous if the material is inhaled or swallowed, because then there is no barrier.
A fast electron emitted as a neutron turns into a proton. Common from strontium-90 and tritium.
Stopped by: A few millimetres of aluminium or a thick sheet of plastic.
Why it matters: Can burn skin at close range and is a concern if the source is ingested; easy to shield against.
Pure energy — light far beyond the visible range. Gamma comes from the nucleus, X-rays from electrons or machines.
Stopped by: Centimetres of lead or tens of centimetres of concrete — and even then only partly.
Why it matters: This is what most detectors look for: it travels far, passes through cargo and walls, and carries the isotope's fingerprint.
Uncharged particles released by fission and by a few special sources such as californium-252.
Stopped by: Water, polyethylene, concrete — anything rich in hydrogen.
Why it matters: Rare in everyday life. Neutrons alongside gamma is a strong indicator of fissile material, so detectors treat it seriously.
Three different units get called "radiation", and they measure three different things.
How active a material is: one decay per second. It says nothing about the effect on a person.
How much energy the material absorbed — one joule per kilogram. Physics, not biology.
Absorbed energy adjusted for how damaging that type of radiation is to tissue. This is the number that matters for health.
A sievert is enormous, so real doses are quoted in millisieverts (1/1,000) and microsieverts (1/1,000,000).
Figures are typical values used across this site for comparison; real doses vary with equipment, altitude, geology and the individual.
Radiation passing through a detector crystal releases a flash of light or a burst of charge. Electronics count those pulses — that is where 'counts per second' comes from.
A simple Geiger counter tells you how much is arriving, not what it is. Granite, bananas, fertiliser and a genuine threat can all raise the same needle.
Better detectors measure the energy of each pulse. Plotted together, those energies form a spectrum with peaks unique to each isotope — potassium-40 at 1461 keV, caesium-137 at 662 keV, and so on.
The detector learns the normal background of the place it is standing in, then flags what rises above it. That is why the same instrument behaves differently at a port and inside a nuclear plant.
An alarm stops a truck. An identification tells the operator it is a shipment of ceramic tiles, or that it genuinely is not. That difference is the whole point of edge-AI analysis.
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