Film Density Falls After 74 Days with an Ir-192 Source
An Ir-192 projector was loaded with 80 Ci (2.96 TBq), and the exposure chart was made on the day of loading. The chart specifies 90 s at 600 mm to produce density 2.0 on D7 film for a 20 mm steel weld. After 74 days, the same technique produces density 1.1. The radiographer wants to double the exposure time to 180 s and continue. The film is from a new box; the developer is at 20 °C, but the replenishment log has not been signed for a week. A step-wedge exposure made that morning looks normal, and the source cable is free. The Level II must decide whether the density change is explained by source decay, whether doubling the time is sufficient, and whether the film or processing system is suspect.
1. Calculate the activity after 74 days and the exposure time predicted by decay alone. Is 180 s guaranteed to restore density 2.0?
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For Ir-192, the half-life is about 74 days, so activity after 74 days is approximately 80 × 0.5 = 40 Ci (1.48 TBq). Exposure is proportional to activity for the same geometry, so the time should double from 90 s to about 180 s. That explains the direction and approximate size of the change. However, 180 s is not guaranteed to restore density 2.0: film density is not linear with exposure, and the 90 s point may have been on a different part of the characteristic curve. Doubling exposure in the straight-line region changes density by roughly the film gamma times 0.3, which may be less than the 0.9 density difference observed. The correct approach is to expose a step wedge at several times, plot density against log exposure, and select the time that gives the required density with the actual film batch and processor. Also confirm the source activity from the decay curve and the loading record.
2. How would you separate source decay from a processing or film-batch problem, and what would you record before issuing a revised technique?
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Use the step wedge as a transfer standard. If the whole step wedge is uniformly lighter than the previous chart by about the factor predicted by decay, source decay is the main cause. If the low-density steps are normal but the high-density steps are compressed, or if base fog has increased, suspect developer exhaustion, temperature, or replenishment. Check developer temperature and time, replenishment records, fixer, and film batch; run a reference film strip with a known density. Expose a fresh step wedge with the same source and geometry and compare densities step by step. If the required density cannot be achieved within the procedure’s exposure-time limit, do not simply keep increasing time; correct the processing first. Record the source serial number, calibration date, activity, decay calculation, new exposure chart, film batch, developer temperature and replenishment action, and the step-wedge results. Revise the technique sheet under document control and re-radiograph the weld if the original density is outside the acceptance range.