Source-to-Object Distance vs. Geometry Unsharpness on a Thick Nozzle Weld
You are the Level III reviewing a technique for a 75 mm thick carbon steel nozzle-to-shell weld. The current technique uses an Ir-192 source at 400 mm source-to-object distance (SOD), a 3.0 mm focal spot, D7 film with Pb screens, and a 2.0 mm geometric unsharpness limit per your written practice. A radiographer reports that the films 'look sharp enough' but a Level II reviewer flagged the Ug calculation. The source activity is 60 Ci, and the job is on a critical path with the vessel due to ship in 10 days. The client specification requires compliance with a maximum Ug of 2.0 mm and a minimum source-to-material distance consistent with that. You must decide whether to accept the technique, revise geometry, or change source/technique, and justify the decision for the client.
1. Calculate the geometric unsharpness for the current setup and determine whether the technique complies. Show the formula and the numbers.
Read the worked answer
Use Ug = F * t / (SOD - t), where F is the effective focal spot (3.0 mm), t is the object-to-film distance (assume worst case: source side of the weld to film, i.e., t = 75 mm plus any gap; conservatively t = 75 mm), and SOD = 400 mm.
Ug = 3.0 * 75 / (400 - 75) = 225 / 325 = 0.69 mm.
This is below the 2.0 mm limit, so the geometry as stated complies. Note: the exam-style trap is using SOD instead of (SOD - t) in the denominator, which gives 0.56 mm - still compliant but wrong. The real issue is whether t should include the full part thickness plus cassette gap. If t = 75 + 5 mm gap = 80 mm, Ug = 3.0*80/320 = 0.75 mm. Still compliant.
Conclusion: geometry complies; the reviewer's flag may be based on a different assumed t or a misread of the spec.
2. The client now insists on a 1.0 mm maximum Ug because of fatigue-critical geometry. What is the minimum SOD required with the same 3.0 mm focal spot and t = 80 mm? Is Ir-192 still practical, and what would you recommend?
Read the worked answer
Rearrange: SOD_min = Ft/Ug + t = 3.080/1.0 + 80 = 240 + 80 = 320 mm.
Wait - that gives 320 mm, which is less than the current 400 mm. That means the current 400 mm SOD already satisfies Ug <= 1.0 mm with F = 3.0 mm and t = 80 mm: Ug = 3.0*80/(400-80) = 240/320 = 0.75 mm. So the existing geometry meets 1.0 mm.
The judgement call: do not reflexively increase SOD. Verify F (effective focal spot, not nominal), verify t includes the weld reinforcement and cassette standoff, and confirm the film-side distance is the worst case. If F were actually 4.0 mm (a worn source or a larger actual focal spot), Ug = 4.080/320 = 1.0 mm - right at the limit, so margin is thin. Recommend measuring or confirming the effective focal spot and, if margin is needed, increasing SOD to 500 mm (Ug = 4.080/420 = 0.76 mm) or switching to a smaller-focal-spot source. Ir-192 remains practical; the constraint is exposure time, not geometry.
3. The radiographer proposes reducing SOD to 300 mm to cut exposure time and 'still be under 2.0 mm.' Evaluate this proposal against both the original 2.0 mm spec and the revised 1.0 mm spec, and state your decision.
Read the worked answer
At SOD = 300 mm, t = 80 mm, F = 3.0 mm: Ug = 3.0*80/(300-80) = 240/220 = 1.09 mm.
- Against the original 2.0 mm spec: compliant (1.09 < 2.0).
- Against the revised 1.0 mm spec: non-compliant (1.09 > 1.0).
Decision: reject the 300 mm proposal for the fatigue-critical case. The exposure-time saving is real (roughly (400/300)^2 ≈ 1.8x dose reduction), but it violates the client's revised acceptance criterion. Offer alternatives: keep 400 mm and accept longer exposure, use a faster film system or higher-activity source to recover time, or use a smaller focal spot. Document the Ug calculation in the technique sheet so the basis is auditable.