Why might ¹²³I be preferred for imaging over ¹³¹I?

(a) The atomic mass of ¹²³I is smaller, so the ¹²³I particles travel farther through tissue.

(b) Because ¹²³I emits only gamma-ray photons, the radiation dose to the body is lower with that isotope.

(c) The beta particles emitted by ¹³¹I can leave the body, whereas the gamma ray photons emitted by ¹²³I cannot.

(d) ¹²³I is radioactive, whereas ¹³¹I is not.

Iodine in the body is preferentially taken up by the thyroid gland. Therefore, radioactive iodine in small doses is used to image the thyroid and in large doses is used to kill thyroid cells to treat some types of cancer or thyroid disease. The iodine isotopes used have relatively short half-lives, so they must be produced in a nuclear reactor or accelerator. One isotope frequently used for imaging is ¹²³I; it has a half-life of 13.2 h and emits a 0.16-MeV gamma-ray photon. One method of producing ¹²³I is in the nuclear reaction ¹²³Te + p → ¹²³I + n. The atomic masses relevant to this reaction are ¹²³Te, 122.904270 u; ¹²³I, 122.905589 u; n, 1.008665 u; and ¹H, 1.007825 u.

The iodine isotope commonly used for treatment of disease is ¹³¹I, which is produced by irradiating ¹³⁰Te in a nuclear reactor to form ¹³¹Te. The ¹³¹Te then decays to ¹³¹I. ¹³¹I undergoes b- decay with a half-life of 8.04 d, emitting electrons with energies up to 0.61 MeV and gamma ray photons of energy 0.36 MeV. A typical thyroid cancer treatment might involve administration of 3.7 GBq of ¹³¹I.