Which of the following is not a form of radiation?

Proton radiation is not considered a form of radiation in the same context as alpha, beta, and gamma radiation. Alpha, beta, and gamma radiation are types of ionizing radiation that result from the decay of atomic nuclei.

Alpha radiation consists of helium nuclei and is characterized by its low penetrative ability; it can be stopped by paper or skin. Beta radiation involves electrons or positrons and has greater penetration power, requiring materials like plastic or aluminum to block it. Gamma radiation is high-energy electromagnetic radiation, highly penetrating and requiring dense materials, such as lead or several centimeters of concrete, for shielding.

Proton radiation, while it can occur under certain circumstances, is not a typical example of radiation in the context of radioactive decay. In radiation therapy, protons are used because they can be directed at tumors with high precision but they are not a naturally occurring form of radiation released during nuclear decay processes. Thus, distinguishing between these categories helps clarify why proton radiation does not fit the same classification as the others listed.