Panoramic Shot, a Public Footpath and an Inadequate Barrier
A crew is radiographing a 16-inch gas pipeline tie-in with a 1.5 TBq Ir-192 source. The company procedure sets the controlled-area boundary at 100 µSv/h and the public boundary at 7.5 µSv/h. The source is uncollimated for a panoramic shot. A survey at 1 m from the source reads 190 mSv/h. The planned barrier is a 2 m high fence 32 m from the source. A public footpath crosses the site 60 m from the source. Two workers will be at the crank 25 m from the source. The client wants the shot during daylight and refuses to close the footpath. The radiographer has one assistant and no extra barriers.
1. Calculate the controlled-area and public boundaries and assess the planned barrier and footpath.
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Use the inverse-square relationship, since the survey gives 190 mSv/h at 1 m. At 32 m the unshielded dose rate would be 190 mSv/h × (1/32)² = 190/1024 = 0.185 mSv/h, or 185 µSv/h. That is above the 100 µSv/h controlled-area boundary, so the 32 m fence is too close. The required distance for 100 µSv/h is √(190,000 µSv/h ÷ 100 µSv/h) = √1900 ≈ 43.6 m. For the public boundary of 7.5 µSv/h, the distance is √(190,000 ÷ 7.5) = √25,333 ≈ 159 m. At 60 m the dose rate would be 190,000/3,600 ≈ 52.8 µSv/h, which is well above 7.5 µSv/h. Therefore the footpath is inside the public boundary and cannot be left open on the assumption that the fence is adequate. The 25 m crank position also needs checking: 190,000/625 = 304 µSv/h, so the crank is inside the controlled area and must be controlled, shielded or relocated.
2. Set out the controls needed and decide whether the shot can proceed as planned.
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The shot cannot proceed as planned. The controlled barrier must be moved to at least 44 m, and the public footpath must be closed, diverted or guarded out to about 160 m unless shielding or collimation reduces the measured dose rates. A 2 m fence at 32 m is both too close and not a complete barrier to a footpath. The crew should use the collimator, point the beam into the pipe and away from the footpath, and place additional shielding if practical. The crank should be relocated behind shielding or to a position where the dose rate is as low as reasonably achievable and within the controlled-area rules. The radiographer should post warning signs and barriers, use a continuous survey meter, and have the assistant guard the footpath side if diversion is not possible. The client should be told that the footpath must be controlled during the exposure; daylight does not change the dose rate. If the client refuses, the radiographer should stop and escalate to the RSO. A job that relies on the public staying out of an unguarded 60 m area is not acceptable.
3. What changes if the source is collimated and directed away from the footpath?
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Collimation changes the dose distribution, not the need for control. The 190 mSv/h at 1 m is an uncollimated or panoramic value; a collimated beam will produce much lower dose rates to the side and behind the camera, but the forward beam may be higher. The radiographer must repeat the survey at the actual barrier, crank and footpath positions with the collimator fitted and the beam in its final direction. The controlled-area boundary should then be set where the measured dose rate is 100 µSv/h, and the public boundary where it is 7.5 µSv/h. If the survey at 60 m is below 7.5 µSv/h, the footpath may remain open only with periodic surveys, barriers and a guard if occupancy is high. If the beam is directed away from the footpath but scatter or skyshine still gives more than 7.5 µSv/h, the path must still be controlled. The decision should be recorded with the survey results, collimator setting, beam direction and barrier distances.