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IEC 60325: Radiation Protection Instrumentation — Alpha/Beta Surface Contamination Monitors
Why Alpha and Beta Contamination Cannot Be Measured With the Same Detector
IEC 60325:2002 specifies alpha/beta surface contamination monitors. Alpha particles (helium nuclei) travel only a few centimetres in air (stopped by a sheet of paper); beta particles (electrons) travel several metres (stopped by a few mm of aluminium). This means α detection requires an extremely thin-window detector (typically ZnS(Ag) scintillator), while β detection can use thicker windows.
| Parameter | α Detection | β Detection |
|---|---|---|
| Detector | ZnS(Ag) scintillator, window ≤0.1 mm | Plastic scintillator or GM tube |
| Detection Distance | ≤5 mm (air absorption) | ≤50 mm |
| Typical Efficiency | ²³⁹Pu: 20–40% | ⁹⁰Sr/⁹⁰Y: 30–60% |
In nuclear power plant radiologically controlled areas, α/β monitors must be separately calibrated using different reference sources (²⁴¹Am for α, ⁹⁰Sr/⁹⁰Y for β). A β detector will read zero on α contamination — not because there is none, but because α particles cannot penetrate the detector window.
TNLab — Alpha and beta are both “particle radiation,” but they interact with matter completely differently. You cannot measure both with the same ruler.
