See explanation below.
Work Step by Step
Thorium-232 and Uranium-238 are the primary isotopes responsible for helium generation. As these isotopes continue in their decay chain (ultimately to stable lead isotopes), the three primary radioactive particles released are: alpha (a helium nucleus), beta (an electron), and gamma (X-rays). The alpha particle consists of two protons and two neutrons and when it attracts two electrons, a helium atom is born. This answer can be found in the chapter 24 reading.