Monthly Dosimetry Discrepancy and Hand Exposure
A radiographer’s February TLD report shows 12.5 mSv whole-body and 38 mSv to the hand. His electronic dosimeter recorded 1.8 mSv for the month. He performed 14 exposures on 12-inch pipe using a 1.85 TBq Ir-192 source. He admits he sometimes steadied the guide tube by hand at the collimator because the stand kept slipping, and he did not use the collimator on two shots. A survey at the barrier 15 m away reads 0.02 mSv/h; a check source confirms the meter responds. The electronic dosimeter was clipped to his belt, but he often left it in the van during setup because it alarmed. He is scheduled for a 10-exposure job tomorrow. The RSO must decide whether he can work, whether the dose is credible, and what to report.
1. Assess whether the TLD result is credible and explain the likely cause of the hand dose.
Read the worked answer
The TLD result is credible and should be treated as the legal dose record; the electronic dosimeter is a supplementary active device. A 1.8 mSv electronic reading against a 12.5 mSv TLD is a large discrepancy, but it is explained by the worker leaving the electronic dosimeter in the van during setup and part of the work. The TLD was worn and integrates all exposure. The hand dose is the stronger indicator: 38 mSv to the hand in one month is consistent with holding the guide tube near the collimator. At 1.85 TBq, the unshielded dose rate close to the source is very high; even short contact or near-contact with the guide tube can deliver millisieverts per minute to the fingers. The barrier reading of 0.02 mSv/h does not contradict this because the barrier is 15 m away and the hand was at the source. The missing collimator on two shots would also increase scatter and direct exposure. The RSO should not simply accept the electronic dose as the lower, more convenient number. The TLD, the worker’s statement, the job log and a reconstruction of time and distance should all be used to confirm the dose.
2. Determine whether dose limits are likely exceeded and decide whether the radiographer should work tomorrow.
Read the worked answer
Using a 20 mSv/y whole-body planning limit and a 500 mSv/y extremity limit, 12.5 mSv whole-body in one month is a high fraction of an annual budget if repeated, and 38 mSv to the hand is even more significant: twelve similar months would approach 456 mSv, close to the extremity limit. The monthly values do not prove an annual limit has been exceeded, but they exceed a reasonable investigation level and require formal dose assessment. The radiographer should not be automatically banned from all work, but he should not do the 10-exposure job tomorrow until the cause is corrected and the dose record is reviewed. If his remaining annual dose budget is adequate and he is retrained, he could return under direct supervision with an extremity dosimeter and a written restriction on hand-holding the guide tube. The RSO should notify the regulator if the investigation level or regulatory reporting threshold is crossed, record the assessment, and arrange a medical review if the extremity dose is confirmed. The decision should be documented as a fitness-to-work judgement, not an informal instruction.
3. Set out the ALARA and equipment controls that should be in place before he returns to work.
Read the worked answer
The primary control is to eliminate hand-holding of the guide tube. The crew should use a proper stand, clamps, a collimator and a remote crank, and should stop work if the stand slips rather than stabilising the tube by hand. The collimator should be mandatory for all shots unless a documented risk assessment approves otherwise. The radiographer should wear a ring or wrist dosimeter in addition to the whole-body TLD, and the electronic dosimeter should be worn on the trunk and checked at the start of each shift. Setup should be planned so the source is shielded and locked before anyone approaches the tube. The RSO should review the job with the radiographer and assistant, practise the crank operation, and set a pre-job dose estimate and stop-work trigger. The 15 m barrier at 0.02 mSv/h is acceptable for controlled-area posting, but the barrier does not protect the hands at the source. Records should include retraining, the revised procedure, dosimetry enrolment, the supervisor’s observation of the next job, and a follow-up TLD review after 30 days. The aim is to reduce the hand dose to well below the investigation level, not merely to stay under the limit.