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How is RT exposure time calculated?

Exposure time depends on source activity (Curie), source-to-film distance (SFD), gamma constant of the isotope, and film speed. The inverse square law applies: doubling SFD requires 4× the exposure time for the same film density.

What is the safe exclusion zone for radiography?

The exclusion zone boundary is set where the dose rate falls to 2 mR/h (public dose limit). For a 50 Ci Ir-192 source, this is approximately 35 meters. The controlled area (50 mR/h) is set at a shorter distance where only radiation workers with dosimeters may enter.

What are the gamma constants for common RT sources?

Iridium-192: 0.48 R·m²/(h·Ci). Cobalt-60: 1.3 R·m²/(h·Ci). Cobalt-60 produces higher dose rates and requires larger exclusion zones for the same activity.

RT Exposure Time & Radiation Exclusion Zone Calculator

Source Type

Source Activity (Ci)

Source-to-Film Distance, SFD (mm)

Film Speed Factor (1 = D7)

Inverse Square Law

Radiation intensity decreases with the square of distance. Doubling the SFD requires 4× the exposure time for the same film density. This calculator uses the inverse square law for both exposure estimation and exclusion zone calculation.

Safety Requirements

Always barricade the exclusion zone with radiation warning signs and flashing lights. Use a calibrated survey meter to verify dose rates at the boundary. All RT personnel must wear personal dosimeters and maintain ALARA principles per IAEA Safety Standards SSG-46.