Re:Rad. therapy (Score 1)

It is almost certainly gamma emissions which are detected in this case, not beta. You are not taking into account the full mechanism of beta decay. The beta minus decay of iodine-131 has an associated gamma emission, which occurs immediately after the beta emission when the daughter nuclide goes to the ground state. In terms of other isotopes, technetium-99m is a gamma emitter, albeit a low energy one at 140keV. Nuclides used in PET all emit 511 keV gammas.