The exposure rate from an unshielded gamma source is the product of two things: how many photons the source emits, set by its activity A, and how energetic and penetrating those photons are, captured by the specific gamma-ray constant Gamma. Spread over a sphere of radius d, the result is D = Gamma x A / d^2. With Gamma in R.m^2/(h.Ci), A in curies and d in metres, D comes out in roentgen per hour.

The roentgen is a unit of exposure in air. For photon energies used in industrial radiography, 1 R corresponds to about 8.76 mGy of air kerma, and for practical dose accounting the field convention is 1 R = 8.76 mSv. That gives 8760 uSv per roentgen, the conversion used here. Older survey meters read directly in mR/h, which is why both scales are shown.

This is the number that sets everything on the safety side of the job: the barrier distance, the stay time, whether a colleague can work at the far end of the bay, and how long a source can sit exposed before someone accumulates a reportable dose. It assumes a bare source in free air. A collimator, the projector body itself and any intervening steel all reduce the real rate, and scatter from nearby walls and the ground adds a few percent back. Always confirm with a survey meter – the calculation tells you where to stand while you take the first reading, it does not replace it.

Worked example

Isotope	ir192
Present activity	50 Ci
Distance from source	5 m
Override gamma constant (0 = use table)	0
Gamma constant used	0.48
Dose rate	8409.6 uSv/h
Exposure rate	960 mR/h
Dose rate	8.4096 mSv/h

D = 0.48 x 50 / 5^2 = 24 / 25 = 0.96 R/h. In mR/h that is 960. Converting at 8760 uSv per R: 0.96 x 8760 = 8409.6 uSv/h, i.e. 8.4096 mSv/h.

Use at your own risk — verify before you act

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