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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.

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RT Exposure Time & Exclusion Zone Calculator

Radiographic testing safety per IAEA / ASNT

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.

The RT exposure time and exclusion zone calculator is an essential safety tool for NDT radiographers and radiation safety officers. It computes the estimated exposure time for radiographic film based on source activity, source-to-film distance, and film speed, then calculates the radiation exclusion zone and controlled area boundaries. This tool helps ensure compliance with IAEA Safety Standards and ASNT guidelines for site radiography.

How Exposure Time Is Calculated

The exposure time formula is t = (Target Dose × SFD²) / (Activity × Gamma Constant). The target dose depends on the film type — a standard D7 film requires approximately 2.0 R for proper density. The inverse square law means that doubling the source-to-film distance quadruples the required exposure time. This is why longer SFDs produce sharper images but take significantly more time.

Radiation Safety Zones

Exclusion Zone (2 mR/h boundary): The public must be kept out. Barricade with radiation warning signs and flashing lights.
Controlled Area (50 mR/h boundary): Only trained RT personnel with personal dosimeters may enter. This is the inner zone.
The calculator computes both distances from the source activity and gamma constant using the inverse square law.

Ir-192 vs Co-60 Sources

Iridium-192 has a gamma constant of 0.48 R·m²/(h·Ci) and a half-life of 74 days. It is the most common source for industrial radiography of steel welds (typical thickness 5–50 mm). Cobalt-60 has a gamma constant of 1.3 R·m²/(h·Ci) — nearly 3× higher — and a half-life of 5.3 years. Co-60 is used for thicker sections (50–150 mm) but requires much larger exclusion zones and heavier shielding.

Site Safety Requirements

Before any radiographic exposure: barricade the exclusion zone, post radiation warning signs, use a calibrated survey meter to verify dose rates at the boundary, ensure all personnel wear personal dosimeters, and maintain ALARA (As Low As Reasonably Achievable) principles per IAEA SSG-46. A radiation safety officer (RSO) must authorize the work, and local regulatory permits may be required.

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