Figure 1.

(a) A gamma ray, produced by a proton beam incident on a tissue phantom, interacts in each of the three detector stages of a CCI system. The gamma origin and three interaction positions are indicated by stars. (b) The origin cone for a gamma ray detected by a three-stage CC. When a gamma produces three interactions in the detector, the scattering angles, θ₁ and θ₂ as well as the initial gamma energy, E₀, can be derived from the interaction positions and the two energies deposited, ΔE₁ and ΔE₂. The CC cannot determine the origin of the gamma ray, but because the scattering angle θ₁ is known, the origin of the gamma ray is restricted to the surface of the origin cone. This cone’s apex is the first scattering position, and its opening angle is 2θ₁.