Which of the following can be used on all materials (e.g., aluminum) - X-ray or Gamma ray?

X-ray is the correct choice because it is a versatile method of non-destructive testing that can be used effectively on a wide range of materials, including metals such as aluminum. X-rays are capable of penetrating various substances and can reveal internal defects, thickness variations, and other structural issues without damaging the material. This ability makes it suitable for diverse applications across industries, from aerospace to manufacturing.

In contrast, Gamma rays, while also a form of high-energy electromagnetic radiation, have limitations with certain materials. They may not be as effective for thinner or lower-density materials compared to X-rays. Additionally, Gamma rays are typically used in specific applications like radiography of thicker materials or in the presence of higher energy requirements.

The other options provided do not relate to the context of non-destructive testing. Emittance pertains to thermal radiation and does not have a direct application for testing material integrity like X-ray does. The statement about both being equally applicable is misleading because it overlooks the specific strengths and best use cases for each radiation type.