Instrument Fails the Check Source Mid-Shift
Third exposure of the night at a remote compressor station: 55 Ci iridium-192, crank-out camera, 20 ft source guide tube. You crank the source back in; the lock ring seats and the crank flag lines up. Your procedure then requires a survey of the camera before it is moved. You check the meter on its check source and get no response above background, and a survey of the camera surface and the guide tube path reads 0.05 mR/h everywhere. The battery check passes and the calibration sticker is current. Your backup instrument is at the shop, 90 minutes away. The client wants the six remaining shots finished before 05:00, and the crew is two people.
1. State your immediate actions and explain why the crank and lock-ring evidence does not authorize you to move the camera.
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Immediate actions: stop. Do not crank further, do not touch the camera or guide tube, do not run another exposure. Move both crew members away from the camera, post the area, and call the RSO. Have the backup instrument brought out (or bring the camera's condition to the RSO and let the RSO direct recovery). Log the time, the failure, and the actions.
Why the mechanical evidence is not enough: the crank turns and lock-ring position tell you what the drive cable did, not where the source is. A source can be left in the guide tube while the crank still turns and the lock ring seats - that is precisely the failure mode the post-exposure survey exists to catch. The procedure requires a survey after each exposure for that reason; if you cannot perform it, you cannot certify the source is shielded.
Why the reading of 0.05 mR/h proves nothing: an instrument that fails its check source is not measuring. A dead detector reads background everywhere, including the surface of a camera holding an exposed source. The only safe interpretation is that you have no information about the source's location.
Worst case drives the boundary. If the 55 Ci source were fully out of the shield, the 2 mR/h distance would be d = √(0.48 × 55 / 0.002) = √13,200 ≈ 115 m (about 375 ft). Until a functioning meter says otherwise, treat the area as if that were true: pull everyone back, keep the area under observation from a distance, and do not send anyone in to 'take a quick look.'
2. A colleague argues that the alarm dosimeters are still working, so the crew can use those to confirm the source is shielded and finish the job. Evaluate that argument.
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It fails on three counts.
- Wrong instrument for the job. An alarm dosimeter is a personal dose/dose-rate device with a threshold set for personal protection, not a survey instrument for locating and quantifying radiation fields. It will not tell you whether the guide tube or the camera port is the hot spot, and at the distances involved it may simply not respond.
- Wrong sensitivity and no spatial resolution. You need a reading, at a known distance, with a known instrument response, to make an inverse-square argument. 'It did not alarm' at an unknown distance is not a measurement.
- It does not satisfy the requirement. Radiography operations require an operable, calibrated survey instrument at the location, and a survey after each exposure. A personal dosimeter cannot substitute for that survey; finishing the six shots would mean running exposures with no way to verify the source returned each time.
Use the dosimeters for what they are for: confirm the crew's own accumulated dose and establish that nobody has picked up an unexpected increment. Do not use them to certify the camera.
3. The backup meter arrives. Describe the verification sequence and the records that must come out of this event.
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Verification sequence:
- Respond to the check source on the backup instrument first (source present, response within the expected range, battery check). A meter that has not been checked is no better than the failed one.
- Establish background at a point well away from the camera, then approach from the far side and survey the guide tube, the camera port end, the lock ring area, and the surface - at contact and at 1 m and 3 m.
- Compare to the camera's expected shielded readings. Only when the readings are at the expected shielded level and fall off with distance as a shielded source should should be considered back in service.
- If a hot spot is found, the camera stays put and the RSO takes over recovery.
Records:
- Survey log entries with time, instrument serial number and calibration due date, distances, and readings.
- A description of the instrument malfunction, including that it passed battery check and failed the check source.
- The failed instrument tagged out of service and removed from the kit until it is serviced and its calibration re-verified; it must not go back into a survey kit on the strength of a fresh battery.
- A note on the exposure record for the interrupted job and the reason for the delay, and a written communication to the client that the remaining shots were not run.
The root cause to write up is not 'the meter broke' - instruments break. It is that a crew was sent to a remote site at night with a single survey instrument and no realistic backup.