Which radiation type poses the least danger externally?

Alpha particles pose the least danger externally because they are relatively heavy and positively charged. Due to their structure, alpha particles have very low penetration ability. They can be stopped by just a sheet of paper or even the outer layer of human skin. This limited penetration means that alpha radiation cannot penetrate into the body effectively from external sources, making them less harmful when exposure occurs outside the body.

In contrast, beta particles, gamma rays, and X-rays possess greater penetrating power. Beta particles can pass through the skin and potentially cause damage to internal organs if they enter the body. Gamma rays and X-rays, being forms of electromagnetic radiation, have high energy and can penetrate tissues and organs, leading to a higher likelihood of causing cellular damage. Therefore, while all forms of radiation should be treated with caution, alpha particles are the least hazardous in terms of external exposure.