Extremity dose of 512 mSv: a badge reading, a disputed explanation and a deadline
An industrial radiography contractor in Hong Kong uses a 1.85 TBq (50 Ci) Ir-192 source for weld inspection. A radiographer's quarterly dosimetry returns a whole-body Hp(10) of 18 mSv and an extremity dose of 512 mSv from a ring badge worn on the index finger. The extremity limit is 500 mSv per year, and the radiographer has accrued 512 mSv in the first three quarters. The ring badge is normally worn over the cotton glove but under the leather gauntlet; log sheets show 340 exposures and two source retrievals in the quarter. The RSO's preliminary view is that the badge reading is valid and the worker must be stood down and reported as an overexposure. The worker disputes this: he says the badge was left on a bench about 1.5 m from the source for 40 minutes on one afternoon while he completed paperwork, with the source in its shielded camera. The contractor faces a client deadline in 10 days.
1. Should the 512 mSv be entered as the worker's extremity dose and reported as an overexposure? Set out your reasoning and what you would do before any formal entry.
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Do not enter 512 mSv as the dose yet, and do not conclude overexposure from a single badge reading - but restrict the worker's radiography work in the interim.
Reasoning:
- The dose record should reflect the best estimate of dose received, not the raw badge output. A badge is an instrument with a measurement model and it can be exposed without the wearer.
- Test the worker's explanation quantitatively. For Ir-192 the gamma constant is roughly 0.13 mSv·m²/(GBq·h). With the source shielded inside the camera the dose rate at 1.5 m is very low; even if the camera leaked at, say, 0.1 mSv/h at 1 m, 40 minutes at 1.5 m gives of order 0.03 mSv. That is two to three orders of magnitude short of the excess. The story does not explain the reading, but it does establish that the badge was unattended, which undermines confidence in it.
- Reconstruct from the work records. 1.85 TBq unshielded at 1 m gives about 240 mSv/h (≈4 mSv/min); at 0.3 m, ≈45 mSv/min. A few minutes of unshielded handling at close range can plausibly deliver hundreds of mSv to a finger, so the reading is physically credible.
- Check badge integrity: issue/return log, whether it was worn over the glove that day, which hand handled the source, calibration and energy response.
If the reconstruction confirms >500 mSv, record it, notify the competent authority under the licence and the Radiation Ordinance, arrange a medical assessment and a formal dose investigation; the worker should not return to radiography until the assessment is agreed.
2. Explain the dosimetric factors that determine whether a finger ring badge represents peak skin dose, and how you would reconstruct the extremity dose.
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Factors:
- Energy response. Ring badges are filtered for a broad range; for Ir-192 (average ~0.38 MeV) the response is reasonably flat. For lower-energy scatter or a lower-energy nuclide, a lightly filtered badge can over-respond.
- Glove attenuation. A cotton glove is a few tenths of a millimetre and attenuates <5% at 0.38 MeV; a 1–2 mm leather gauntlet attenuates roughly 10–15%. Worn over the glove, the badge reads dose at the glove surface, conservative for skin but not representative of the basal layer.
- Position. A ring measures one point on the proximal phalanx. Peak skin dose often occurs at a fingertip or web space, which can be a factor of 2–3 higher when the hand is close to an unshielded source. A ring badge is an index, not a peak-skin monitor.
- Geometry. If the hand lies between source and badge the badge under-reads; if the badge faces the source it over-reads relative to the hand average.
- Practice. Tongs, collimators, distance and time dominate everything else.
Reconstruction: use the exposure log with source activity at each exposure, measured dose rates, and the actual hand geometry; supplement with an independent dosimeter (a TLD on a fingertip, or a ring on the other hand as a control) and, if needed, a hand phantom. Compare the reconstructed value with the badge result and record the agreed dose with its uncertainty. If the two disagree beyond the measurement uncertainty, the badge record should be corrected with a documented technical basis, not simply overwritten.
3. What corrective, ALARA and organisational actions would you put in place, and how would you handle the client deadline?
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Immediate: stand the worker down from radiography pending the investigation, on full pay, and explain why. Do not let the client deadline influence the dose decision.
Technical and procedural:
- Re-brief all radiographers on badge wearing and on never leaving badges unattended. A badge left near a source is a dosimetry failure, not a dose, and must be reported as such.
- Review the two source retrievals: they indicate either equipment problems or a training gap. Inspect the camera, drive cables and collimator; review the retrieval procedure and training records.
- Introduce an independent extremity dosimeter for high-dose tasks and a daily dose-rate check of the camera.
- Set a local investigation level (e.g. 5 mSv/month extremity) that triggers review well before the limit is approached.
ALARA: more remote handling, collimation and shielding; rotation of staff; planning of exposures so that the hand is never near the unshielded source.
Organisational: report through the RSO to the client and the authority as required; record the lessons in the safety committee; verify that the contractor's supervision is adequate - a supervisor who lets a badge sit on a bench near a source is a systemic warning sign.
Client deadline: offer a compliant alternative (a second qualified radiographer, or a revised sequence) rather than compressing the safety process. State in writing that the schedule will be recovered only after the dose assessment is closed.