At 90 degrees from the patient and 1 meter away, the exposure measured by an ionization chamber is about what fraction of the incident radiation?

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

Answer: (A)

Scattered radiation from the patient is a small secondary portion of the primary beam. It results mainly from Compton interactions and is emitted in all directions, so only a tiny fraction travels to a detector located at 1 meter. The intensity also drops with distance according to the inverse square law. Together, at a distance of about 1 meter and at 90 degrees, the exposure from patient scatter is roughly 0.1% of the incident beam — about one thousandth. That’s why the reading is about 1/1000 of the incident radiation.