Answer the following question given this paragraph: 

Early researchers in radioactivity found that the emissions from radioactivity could be classified into three distinct types according to their penetrating power. One type of radiation could barely penetrate a sheet of paper. The second type could pass through as much as 3 mm of aluminum. The third type was extremely penetrating and could pass through several centimeters of lead. They named these three types of radiation alpha , beta , and gamma respectively. Eventually, each type of radiation was further identified. Alpha particles are the nuclei of helium atoms, . Beta particles are electrons, and gamma rays are very high energy photons (even higher energy than x-rays).


Q: Alpha, beta and gamma are three types of what?


A:
radiation