1. An imaging device comprising: a first-stage detector and a second-stage detector, wherein the first-stage detector is disposed between a gamma ray source and the second-stage detector; an event detection unit configured to detect an event in which a gamma ray that is Compton scattered by the first-stage detector is photoelectrically absorbed by the second-stage detector; and an arithmetic unit configured to generate a distribution image indicating a spatial distribution of the gamma ray source as an image in an image space enclosing the gamma ray source, on the basis of measurement data of interaction of each detector and gamma rays in a plurality of events, wherein the arithmetic unit individually sets a probability parameter indicating a probability that Compton-scattered gamma ray arrived from within the image space in each event, for each of a plurality of pixels in the distribution image, on the basis of the measurement data of each event, and the arithmetic unit generates the distribution image using the set probability parameter.