Therapy Radiopharmaceuticals Practice Test

I-131 mode of decay?

Alpha decay Beta positive Beta negative and gamma Electron capture

Correct Answer:

Beta negative and gamma

Explanation:

I-131 decays by beta minus emission to xenon-131, and the daughter nucleus emits gamma radiation as it de-excites. In this process, a neutron in I-131 converts to a proton, increasing the atomic number by 1 (Iodine, Z = 53, becomes Xenon, Z = 54) while the mass number stays at 131. The resulting xenon-131 is typically left in an excited state and releases a gamma photon to drop to a lower energy state, which is why gamma emission accompanies the beta particle. This combination—beta minus decay followed by gamma emission—is characteristic of I-131 and is what the answer reflects. Other decay modes, such as alpha decay, beta plus emission, or electron capture, do not describe this isotope’s typical decay path.

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