Here's why:

* Gamma rays have the highest energy and shortest wavelength among all types of electromagnetic radiation. This means they have the greatest penetrating power.

* X-rays also have high energy and good penetrating power, but they are less energetic than gamma rays.

* Beta particles are high-speed electrons, which are more penetrating than alpha particles but less penetrating than gamma rays or X-rays.

* Alpha particles are relatively large and slow, making them the least penetrating type of ionizing radiation. They can be stopped by a sheet of paper.

In summary:

Gamma rays > X-rays > Beta particles > Alpha particles

This is why gamma rays are used in medical imaging (PET scans) and industrial applications, while alpha particles are used in smoke detectors.