Final answer:
The addition of 1%-2% of thorium to the filament enhances thermionic emission and extends tube lifespan. Thorium's electron emission properties are key in such applications. Molybdenum-99, though a fission product of uranium-235, is unrelated to this context.
Step-by-step explanation:
The addition of 1%-2% of thorium to the filament increases efficiency of thermionic emission and prolongs tube life. This is due to thorium's ability to emit electrons when heated, which is crucial for devices such as X-ray tubes where a filament generates a stream of electrons that produce X-rays upon striking a target material like copper. In nuclear physics, concerning the alpha decay process, a uranium nucleus emitting an alpha particle becomes a thorium nucleus, since two protons and two neutrons are lost in the process, conserving charge Molybdenum-99 is a notable fission product of uranium-235, which decays into technetium-99m used in medical imaging. However, molybdenum is not related to the improvement of thermionic emission in tube filaments.