Final answer:
The finger-like penetration profile in GMAW is promoted by argon-rich shielding gas. To shield against alpha particles, paper or skin suffices, whereas gamma rays require denser materials such as lead shielding.
Step-by-step explanation:
The finger-like penetration profile with Gas Metal Arc Welding (GMAW) is promoted by argon-rich shielding gas. Argon is used in GMAW for its inert properties and ability to contribute to stable arc conditions as well as deeper weld penetration. Given the options provided, the answer to the student's question would be b) argon-rich shielding gas.
As for the radiation shielding concept, gamma rays penetrate matter more deeply than alpha particles. Effective ways to protect against these emissions include using appropriate shielding materials. For alpha particles, a piece of paper or even human skin can provide effective protection. However, gamma rays require much denser materials such as lead shielding or concrete. The necessary thickness of lead or other materials will depend on the intensity of the gamma radiation.