True or False? Gamma radiation emitted by a radioisotope of Co-60 has wavelengths that are longer than the radiation from Ir-192.

• A. True
• B. False
• C. Not enough information
• D. They are the same wavelength

Answer: (B)

The main idea here is that gamma-ray wavelength is determined by the photon energy through E = h c / λ, so higher energy means a shorter wavelength. Cobalt-60 emits two principal gamma rays at about 1.17 MeV and 1.33 MeV. Those energies correspond to very short wavelengths (on the order of a fraction of a picometer). Iridium-192 emits a spectrum of gamma rays with a range of energies, many of which are lower than Co-60’s, which gives those lines longer wavelengths than the high-energy Co-60 rays. Because Co-60’s gamma photons are higher in energy, their wavelengths are shorter, not longer. So the statement is false.