Which isotope requires a thicker minimum material thickness?

• A. IR 192
• B. CO 60
• C. IR 192 and CO 60 have the same minimum thickness
• D. Neither requires a minimum thickness

Answer: (B)

Higher energy gamma rays penetrate thicker materials, so the source that emits more energetic photons requires a thicker minimum material thickness to achieve adequate attenuation and imaging. Cobalt-60 produces gamma rays at about 1.17 and 1.33 MeV, which are significantly more penetrating than iridium-192’s photons, which are in the hundreds of keV range. Because of this greater penetrating power, cobalt-60 needs a thicker minimum material thickness (or a thicker section to be radiographed) than iridium-192. Ir-192, with its lower-energy photons, works with thinner sections, so the minimum thickness is smaller. This is why cobalt-60 is used for thicker components, while iridium-192 is preferred for thinner ones.