Uranium oxide. the half-life of the uranium-238 isotope present in these dishes is about 4.5 billion years. this implies that most of the uranium-238 is still in the dishes and very little has decayed to emit ionizing radiation. uranium-238 initially decays into thorium-234 and an alpha particle as shown in the background, but thorium-234 can further decay to emit beta particles and gamma radiation. what is the decay reaction of thorium-234?