Thorium 232 which alone makes up nearly all natural thorium is the most common isotope of thorium in the nature This isotope has the longest half life 1 4 x 10 10 years of all isotopes with more than 83 protons In fact its half life is considerably longer than the age of earth Therefore 232 Th belongs to primordial nuclides 232 Th decays via alpha decay into 228 Ra