Certain radioactive isotopes decay by positron emissionfor

Chapter 32, Problem 32.18

(choose chapter or problem)

Certain radioactive isotopes decay by positron emissionfor example, oxygen 15 8O. In the medical diagnostic technique known as PET scanning (positron emission tomography), such isotopes are injected into the body, where they collect at specific sites. A positron ( 0 1e) emitted by the decaying isotope immediately encounters an electron ( 0 1e) in the body tissue, and the resulting mutual annihilation produces two -ray photons + ), which are detected by devices mounted on a ring around the patient. As Figure 32.11a shows, the two photons strike oppositely positioned detectors and, in so doing, reveal the line on which the annihilation occurred. Such information leads to a computer-generated image that can be useful in diagnosing abnormalities at the site where the radioactive isotope collects (see Figure 32.12). Which conservation principle accounts for the fact that the photons strike oppositely positioned detectors, the principle of conservation of (a) linear momentum or (b) energy? (0