Point & Extended Sources
6CEN Rule of Thumb
Estimates unshielded gamma exposure rate at 1 foot (30.48 cm) from a point source given activity in Curies, photon energy in MeV, and photon yield per disintegration.
Ẋ = 6 × C × E × N
Input Parameters
Calculation Goal
Radionuclide Presets
Co-60: E=1.25 MeV, N=2.00
Calculated Result & Visual Analysis
Nominal Solved Value Updated automatically
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Enter parameters on the left to calculate.
Geometry & Field Diagram
Interactive Model
Sensitivity Curve
Governing Equation & Parameters
Exposure Rate (R/hr at 1 ft) = 6 × Activity (Ci) × Energy (MeV) × Photon Yield (N)
| Symbol | Parameter | Default Unit | Definition |
|---|---|---|---|
| Ẋ | R/hr | Gamma exposure rate at 1 foot (30.48 cm) from the unshielded point source. | |
| C | Ci | Total activity of the point source (1 Ci = 3.7 × 10¹⁰ Bq). | |
| E | mev | Gamma photon energy per emission in Mega-electronvolts. | |
| N | photons/decay | Fractional number of gamma photons emitted per nuclear disintegration (e.g., 0.85 for Cs-137, 2.0 for Co-60). |
Health Physics Notes & Related Calculators
Operational Guidance
- Applies to gamma photon energies between 0.07 MeV and 4.0 MeV where the mass energy-absorption coefficient in air is nearly constant (~0.027 cm²/g).
- 1 foot (30.48 cm) corresponds directly to the 30 cm regulatory posting distance for Radiation Areas (5 mR/hr) and High Radiation Areas (100 mR/hr) in 10 CFR 20.1003.
- For radionuclides with multiple gamma lines such as Co-60 (1.173 MeV and 1.332 MeV), use the average photon energy (1.25 MeV) and summed yield (N = 2.0).
- SI equivalent at 1 meter: Dose Rate (mSv/hr at 1 m) ≈ 0.15 × Activity (GBq) × E (MeV) × N.
Related Calculators
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