Which type of radiation can be stopped by a sheet of paper?

The correct answer identifies alpha particles as the type of radiation that can be stopped by a sheet of paper. Alpha particles are relatively heavy and consist of two protons and two neutrons, making them positively charged and relatively large compared to other forms of radiation. Due to their size and charge, alpha particles have a very low penetrating ability. They can be easily blocked by thin materials like paper, clothing, or even the outer layers of human skin. This characteristic makes them less hazardous when outside the body but potentially harmful if ingested or inhaled.

On the other hand, beta particles, gamma rays, and X-rays possess significantly greater penetrating power. Beta particles can penetrate through paper but are stopped by materials like plastic or aluminum. Gamma rays and X-rays are electromagnetic radiation that can penetrate much deeper and require more substantial shielding, such as lead or several centimeters of concrete, to be effectively stopped. Thus, when considering the ability of different radiations to be impeded by a simple barrier like paper, alpha particles clearly stand out as the only type sufficiently hindered by such a thin layer.