@ JohnKSa
Aluminium jacketing (or as some say 'aluminum') IS invisible on standard X-rays used for medical diagnosis. Even on 'soft' X-rays when exposed directly you cannot see an aluminium jacket. You might see a very thick piece of aluminium on X-ray (such as an aluminium step wedge used for film/developer QA), but then again you'll see a thick piece of anything on X-ray. But unless you have a great big piece of aluminium, you won't see it on X-ray. That is why they make the ports on X-ray tube housings out of aluminium.
Edit: check out this thread where I X-rayed some samples:
http://www.thefiringline.com/forums/...11#post2022111