Scatter and Backscatter on a Pipe Weld with a Lead Screen Failure
A crew is radiographing a 150 mm diameter, 12 mm wall carbon steel pipe weld with Ir-192 using a single-wall single-image technique. The films show a foggy, low-contrast image with a dark band along one edge. Density in the weld is 2.1, but the background is 1.4 and the IQI hole is barely visible. The radiographer says the film is 'just a little fogged' and wants to accept it. The lead backing is a 0.5 mm sheet placed behind the cassette, but the cassette is bowed away from the pipe on one side. The client requires density 1.8-4.0 and 2-2T sensitivity. You must decide acceptance and corrective action.
1. Identify the likely causes of the fog and the dark edge band, and explain why the IQI sensitivity is poor despite an in-range weld density.
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The fog and dark edge band are classic backscatter and side-scatter signatures. The 0.5 mm Pb backing is adequate in thickness for Ir-192 only if it is in intimate contact and covers the entire cassette; the bowed cassette creates a gap where backscatter from the floor/table reaches the film, fogging it and creating the dark band where the backing is displaced. The background density of 1.4 is below the 1.8 minimum, confirming fog is raising the base density unevenly.
IQI sensitivity is poor because contrast is reduced by the fog: the film is operating on a compressed portion of the characteristic curve, and scatter reduces the subject contrast of the 2T hole. In-range weld density (2.1) does not guarantee sensitivity when the background is fogged - the density difference between weld and background is small, so the effective contrast is low.
2. Specify the corrective actions and the acceptance decision for the current films.
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Acceptance decision: reject the current films. Background density 1.4 is below the 1.8 minimum and the 2T hole is not clearly visible, so the films do not meet the client's density and sensitivity requirements. 'A little fogged' is not an acceptance category.
Corrective actions:
- Re-establish intimate contact between cassette and pipe - use a flexible cassette or a curved cassette, and tape/secure it so it follows the pipe curvature.
- Increase and verify lead backing: use at least 0.5-1.0 mm Pb, fully covering the cassette and extending beyond its edges, with no gaps.
- Add lead masking around the weld and collimate the beam to the area of interest to reduce side scatter.
- Check for scatter sources (floor, walls, nearby steel) and use a lead sheet on the floor or move the source/part.
- Re-shoot and verify background density is at or above 1.8 and the 2T hole is visible before submitting.
3. How would you verify that backscatter is eliminated on the re-shoot without adding a separate exposure?
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Use the standard backscatter check: place a small lead letter 'B' (or a lead marker) on the back of the cassette, outside the area of interest but within the beam. On the processed film, if the 'B' image appears as a light (less dense) image, backscatter is negligible; if it appears dark or is not visible, backscatter is reaching the film and the backing is inadequate. This is a qualitative check that requires no extra exposure and should be part of the technique for every cassette. Document the result on the technique sheet. If the 'B' is dark, increase backing thickness and re-shoot.