If the Hulk emits gamma radiation and that radiation makes vibranium vulnerable, then what about C. A. shield?

Captain America's shield is made up of vibranium and steel but in the Avengers animated movie, the alien ships are destroyed because they find out The Hulk's gamma radiation makes the material, vibranium, vulnerable and allows them to kill the space ships. So how come Captain America's shield is unaffected if he was able to continuously hit The Hulk and it was still indestructable?If the Hulk emits gamma radiation and that radiation makes vibranium vulnerable, then what about C. A. shield?
The shield *was* made of vibranium and steel. But when the to formulas bond it creates what is called a proto-adamantine. So it is no longer the two combined it one.If the Hulk emits gamma radiation and that radiation makes vibranium vulnerable, then what about C. A. shield?
Use "unobtainium" It worked for "Avatar"