Final answer:
The characteristic X-ray in thallium decay is characteristic of thallium. This is due to the filling of inner-shell vacancies upon decay. When uranium emits an alpha particle, it decays to thorium.
Step-by-step explanation:
The characteristic X-ray in thallium decay is characteristic of thallium itself. This is because when a nucleus undergoes decay, it emits radiation that can leave an inner shell electron vacancy. The resulting characteristic X-ray is a consequence of another electron falling into this vacancy and emitting a photon in the process. The energy of this photon is associated with the specific levels of the electron shells of the atom in question, in this case, thallium. Since the inner-shell electrons are tightly bound, especially in heavy elements like thallium, the characteristic X-ray energies are significantly high.
When a uranium nucleus emits an alpha particle, the resulting nucleus is thorium. This is because the atomic number drops by 2 (the atomic number of helium, which is the alpha particle), and the chemical element with atomic number 90 (92 - 2 for uranium) is thorium.