I think it is rather unusual task to do Co-60 exposures on 260 mm (steel) wall thickness. Below I am pointing out some problems with such examination.
1. The European standards EN 444 and EN 1435 covering RT testing of welds limit the use of Co-60 to 200 mm of steel for Class A and to 150 mm of steel for Class B. So you can have a formal problem with the standard compliance.
2. For 260 mm thickness of steel the scattered radiation Build-up Factor for Co-60 emission lines (1,17 and 1,32 MeV) is about 11. It means that the image forming primary radiation stands for less than 10% of the total radiation reaching the film. You surely obtain very poor IQI sensitivity.
3. To meet the EN 1435 requirements for geometric unsharpness for 260 mm wall thickness and typical 5 mm source size you should use SFD not less than 3315 mm.
4. For such SFD, even using 100 Ci source and D7 film you get exposure times of the order of 1000 hours !!!. Even if you break the rules and use SFD=1000 mm you get unacceptable exposure times of the order of 100 hours.
So my advice is: forget about Cobalt radiography for 260 mm thick steel and think about TOFD or other UT technique.
I'm completely agree with you, but the final user doesn't accept TOFD examination.
I'm making some test in the bunker, but i'm not sure to obtain a satisfactory result.
Canyou explan better tour point 2?
How are you able to say that the scattered radiation Bulid-up Factor is about 11 and the image forming primary radiation stands for less than 10% of the total radiation reaching the film?
Doubt you will find many welding defects this way unless they are huge :)
You can suggest to do x-ray at different weld stages such as after first run, 1/3 and 1/2 welded butt. This way you have some record of the welded joint and complement this with Ultrasonic, MPI/DPI and Visual (even at the same stages?).
The radiation Build-up Factor, B is not a kind of thing easy to explain in a few words. In short, it is a number by which you have to multiply the intensity of primary radiation (i.e. radiation not scattered within the material) passing the certain material thickness to obtain the TOTAL intensity of radiation passing through this thickness. This TOTAL is the sum of primary and scattered radiation.
So if, the build up factor for your 260 mm steel slab is B=11 it means that the total radiation intensity registered behind your slab is 11 times greater than the intensity of its primary component. In other words good, image building, primary radiation stands only for 1/11 (9,1%) of the total radiation registered by your film.
The comprehensive tables of build-up factors for different materials and radiation energies you can find in ANSI/ANS-6.4.3.
Theory is a theory but you mentioned that you have made some practical trials with Cobalt exposures of 260 mm thick steel. I am really curies to your results. What exposure parameters did you use and what film densities and IQI sensitivities did you really obtain.
I'm sorry Mondonico but i have to ask this - have you asked your end-user to state why they insist on Gamma ray under such unusual and dangerous conditions - especially relevant to the excellent considerations put forward by Slawomir? And have they given a clear and unequivocal answer? Or conceded the need, as per Anjafo's post, for intermediate and supplementary inspections? It should be relatively easy for you to provide evidence after a literature search that the TOFD method has a higher probability of detection (POD) than Cobalt gamma ray for heavy wall weld examination. And if you are going ahead with a formal reported inspection make certain you record your doubts in a formal and traceable manner. A signed accepting report with no inadequacy statement is of no benefit to you in case of any manufacturing history investigation - after all 260mm thick steel has a special application.
Perhaps there are conditions such as the acceptance criteria or expected defect types which mitigate for Cobalt gamma, but I would be wondering about the hidden agenda of your end-user. Clue: are they the real end-user or are they the EPMC?
I concurr with Mr. Armstrong. TOFD will find all significant defects. Cobalt 60 at that thickness will only find HUGE defects, and that lack of sensitivity will be shown by your penetrameter / Image Quality Indicator. The people you are talking to may not want you to find moderate sized defects.
Any reputable engineer would not demand that you perform NDT outside of the ASME and EN limits. Those limits were established based on experience, and proven [and disproven] capabilities. You are trying to go beyond a physical limitation, as evidenced by Mr. Mackiewicz's calculations.
I strongly recommend that you stay within ASME and/or EN limits. Knowingly going outside established Codes and Standards opens your company up to masssive liability.