Radiography Restricted-Area Boundary Distance

ScopeActivity times the gamma constant gives the dose rate at one foot, and the distance at which that falls to a permitted rate is the square root of the ratio -- which is a large number on a job site and the reason radiography shuts work down around it. THE SQUARE ROOT IS THE FACT WORTH CARRYING, because it governs what is worth doing about the boundary. Distance buys protection slowly: halving a boundary requires cutting the effective activity by a factor of four. No practical change to the source does that, and a smaller source means a longer exposure, which is not obviously safer. A COLLIMATOR does it, in every direction outside the beam, which is why collimated shots have a long boundary in one direction and a short one everywhere else and why collimation is the single largest control available. There are two limits and the stricter one governs the site. The boundary itself is the hourly one: 10 CFR 20.1301(a)(2) holds any unrestricted area to 2 mR in any one hour, which is why the 2 mR/h line is where the rope goes. But a member of the public is ALSO held to 100 mrem in a YEAR (20.1301(a)(1)), and someone who stays put beside a long job -- an occupied building, an adjacent tenant -- reaches that well under 2 mR/h. The occupied-location rate entered here is that annual figure spread over the hours the person is exposed, and its distance is correspondingly further; running only the 2 mR/h number beside an occupied building understates the boundary. Both limits are entered here because they are jurisdictional. AND THE CALCULATION IS A STARTING POINT RATHER THAN THE BOUNDARY ITSELF. The real field is not a point source in free air: scatter from surrounding steel, concrete and the ground makes it larger in some directions, and structures make it smaller in others. The boundary is established and maintained with a calibrated survey meter before and during every exposure, and a rope placed on a calculated number and never surveyed is not a controlled area. This is a point-source inverse-square estimate on entered figures. It does not perform a radiation safety analysis, determine any regulatory limit, size shielding or compute its attenuation, account for scatter, skyshine, or ground reflection, address personnel dosimetry, ALARA planning, exposure duration and total dose, source handling, or the emergency procedures a radiographer works under, or substitute for a survey. 10 CFR Part 34 or the agreement-state equivalent, the licensee's radiation safety officer, and the certified radiographer govern.

Formula or tabledose rate at 1 ft = source activity x gamma constant x any shielding transmission factor; the boundary distance is sqrt(dose rate at 1 ft / permitted rate), a point-source inverse-square estimate.

Source and editionA point-source boundary ESTIMATE. The gamma constant, both dose-rate limits and any collimator or shielding factor are entered because they are source-specific and jurisdictional. It is not a substitute for a survey.

Free accessOne inverse-square relation on entered constants.

What governsEstimate. AHJ and licensed professional govern.

API field namessource_activity_ci, gamma_constant_r_h_ci_ft, boundary_limit_mr_h, collimator_attenuation_factor, public_limit_mr_h, shielding_factor, dose_rate_1ft_r_h, boundary_distance_ft, collimated_distance_ft, public_distance_ft

• Point source in free air scatter and real geometry change the actual fielda calibrated survey meter
• Gamma constant at one FOOT published constants are usually per metre (Ir-192 about 0.48, Co-60 1.32 R/h per Ci at 1 m) and are 10.76 times larger at a foot (about 5.2 and 14.2); entering a per-metre figure here puts the boundary 3.3 times too closepublished nuclide data
• Limits are entered they are jurisdictional and differ for public areas10 CFR Part 34 or the state equivalent
• Not a radiation safety analysis no shielding design, dosimetry, ALARA planning or total dosethe licensee's radiation safety officer