The radiation exposure (intensity) measured by an ionization chamber at 90 degrees from the patient at one meter is about what fraction of the incident radiation?

• A. 1/1000
• B. 1/100
• C. 1/10
• D. The same as

Answer: (A)

When the patient is irradiated, only a tiny portion of photons are scattered out of the beam toward the operator. Most interactions in tissue are Compton scatter, and photons scattered at 90 degrees toward a point 1 meter away are spread over a large area and have reduced energy, making their flux much smaller than the primary beam. The combination of the low probability of scattering into that exact angle and the inverse-square spreading with distance means the exposure at 90 degrees and 1 meter from the patient is about one thousandth of the incident primary radiation. That’s why the measured exposure is roughly 1/1000 of the incident radiation.