Which type of radiation has a longer wavelength?

The type of radiation emitted from each of the isotopes listed corresponds to its distinctive properties, particularly its wavelength.

Cobalt-60 (Co-60) primarily emits gamma radiation, which is known for its short wavelength and high energy. Iridium-192 (Ir-192) also emits gamma radiation, but it has a higher energy than Co-60, resulting in even shorter wavelengths.

In contrast, cesium-137 (Cs-137) emits lower-energy gamma radiation compared to both Co-60 and Ir-192, leading to a longer wavelength. Iodine-131 (I-131) emits both beta particles and gamma radiation, but much of its gamma energy is again lower than that of Co-60 and Ir-192.

Understanding these characteristics, we find that the gamma radiation emitted from Cs-137 indeed has a longer wavelength compared to that from the other options provided. This makes Cs-137 a suitable choice when asked about radiation with a longer wavelength, as its energy level directly correlates with the wavelength of the radiation it emits.

The original answer mistakenly identified Ir-192, which emits radiation with shorter wavelengths due to its higher energy gamma emissions.