Answer
(b) because $^{123} I$ emits only gamma-ray photons, the radiation dose to the body is lower with that isotope.
Work Step by Step
To determine which isotope is suitable for imaging, we need to choose the least damaging for human cells.
Now let's see the types of decay of both isotopes.
$^{123} I$ undergoes gamma decay. RBE for gamma ray is 1
$^{131} I$ undergoes $\beta^-$ . RBE for electron is 1.0 - 1.50
The lower the RBE, the safer the isotope is. From here, we choose option (b) because $^{123} I$ emits only gamma-ray photons, the radiation dose to the body is lower with that isotope.
