Source Decay, Exposure Time, and Technique Validity Across a Long Project
A project uses an Ir-192 source that was 80 Ci at calibration 120 days ago. The technique sheet was written at calibration and specifies a 4-minute exposure for a 30 mm steel weld at 500 mm SOD with D7 film and density 2.3. The project will run another 90 days. The radiographer reports that recent films are coming out at density 1.7 and wants to 'just double the time.' The client requires density 1.8-4.0 and the technique sheet must be re-validated if the source activity changes by more than 10%. You must decide whether the technique is still valid and specify how to correct it.
1. Calculate the current activity and the activity at the end of the project, and determine whether the 10% re-validation trigger has been exceeded.
Read the worked answer
Ir-192 half-life is approximately 74 days.
Current activity after 120 days: A = 80 * (0.5)^(120/74) = 80 * (0.5)^1.622 = 80 * 0.325 = 26.0 Ci.
Activity at end of project (120 + 90 = 210 days): A = 80 * (0.5)^(210/74) = 80 * (0.5)^2.838 = 80 * 0.140 = 11.2 Ci.
The activity has already dropped from 80 Ci to ~26 Ci - a 67% decrease, far exceeding the 10% re-validation trigger. The technique sheet is invalid and must be re-validated now and again before the end of the project (or on a defined schedule).
2. The radiographer proposes doubling the exposure time to fix density 1.7. Evaluate this and specify the correct correction.
Read the worked answer
Doubling time is the wrong correction and is not defensible. Exposure time scales inversely with activity for a given density, but the relationship is not linear in density - density is on the characteristic curve, and a 1.7 to 2.3 change requires a specific log E shift, not a doubling of time. Doubling time would overexpose and could push density above 4.0 or into the shoulder where contrast drops.
Correct correction: use the activity ratio. Required time = original time * (A_cal / A_now) = 4 min * (80/26) = 12.3 min for the current activity. At end of project: 4 * (80/11.2) = 28.6 min. Re-validate the technique sheet with the new times, confirm density with a step wedge, and verify the IQI sensitivity still meets 2-2T at the new exposure. Also confirm the source is still within its usable life and that the exposure time is practical (28 min may require a different approach, e.g., a higher-activity source or faster film).
3. What would you document and communicate to the client, and what is the risk if the technique is not re-validated?
Read the worked answer
Document: the decay calculation (current and end-of-project activity), the revised exposure times, the step-wedge verification of density, the IQI sensitivity result, and the Level III sign-off with the re-validation date. Communicate to the client that the technique sheet has been revised due to source decay exceeding the 10% trigger, and provide the revised technique for approval if required.
Risk if not re-validated: films at density 1.7 are below the 1.8 minimum, so they are non-compliant and could be rejected, causing rework and schedule delay. Worse, if the density is 'fixed' by arbitrary time increases without verification, sensitivity may be inadequate and defects could be missed - a safety risk on a pressure boundary. The 10% trigger exists precisely to prevent this drift, so treat it as a hard gate, not a guideline.