Which best blocks x-ray particles and can therefore be observed more closely through the use of this technology

Answer;

Bones

Explanation;

The x-ray particles pass through the body.  Structures that are dense such as bone will block most of the x-ray particles.

Different parts of the body absorb the x-rays in varying degrees. Dense bone absorbs much of the radiation while soft tissue, such as muscle, fat and organs, allow more of the x-rays to pass through them. As a result, bones appear white on the x-ray, soft tissue shows up in shades of gray and air appears black.

not sure if this what   you'r looking for would be percent composition