Stuck Radiography Source and the Survey Meter Reading
During industrial radiography with an Ir-192 source, the drive cable jams and the source does not return to the shielded camera. The radiographer's survey meter reads 2.5 mSv/h at 15 m from the guide tube. The source is estimated at 1.85 TBq; the supplier's gamma constant is 0.13 mSv/h per GBq at 1 m. The area is an open construction site with a temporary barrier at 40 m. Wind is light, and several workers are at 60 m. The radiographer wants to approach the guide tube with a long-handled tool to push the source back, because the meter reading is only 2.5. The radiation protection officer must decide whether the reading is consistent with the source activity, what cordon distance to set, and whether anyone should approach. The site has no remote manipulator, only a 2 m source handling tool and lead blankets.
1. Compute the expected dose rate at 1 m and 15 m and compare it with the measured value. Interpret the discrepancy.
Read the worked answer
At 1 m: 1.85 TBq = 1850 GBq; dose rate = 1850 × 0.13 = 240.5 mSv/h. At 15 m, inverse square gives 240.5/15^2 = 240.5/225 = 1.07 mSv/h. The measured 2.5 mSv/h is about 2.3 times higher than the simple point-source estimate. That is not a reason to relax: it may indicate the source is not fully in the guide, the distance is wrong, the meter is reading scatter or is out of calibration, or there is additional source material. The calculation is an order-of-magnitude check, not a substitute for a surveyed boundary.
2. Recommend the cordon and emergency response. Discuss whether the 2 m tool and lead blankets make an approach acceptable.
Read the worked answer
Use the measured survey, not the nominal activity, to set the controlled area. At 40 m the point-source estimate is 240.5/1600 = 0.15 mSv/h; at 60 m it is 0.067 mSv/h. A boundary should be set beyond where the measured dose rate falls to the facility action level, with guards and barriers, and workers at 60 m should be moved further back or removed. Notify the radiation protection officer and emergency response, secure the guide tube area, and do not allow the radiographer to approach. A 2 m handling tool would put the person in a field of about 240.5/4 = 60 mSv/h, so even 30 s is about 0.5 mSv to the body and much more to the hands; lead blankets may reduce the field but do not make an unplanned approach acceptable. Use a remote manipulator or source-recovery team with planned dose limits, and consider notifying the regulatory authority per the facility emergency plan.