A radiation image forming apparatus, comprising: a plurality of Compton cameras arranged annularly to surround a region in which a specimen is placed, wherein each of the plurality of Compton cameras includes a plurality of radiation detection devices and detection modules , wherein the plurality of radiation detection devices is configured to detect an electron generated by a track of a recoil electron generated by Compton scattering and configured to output a detection signal configured to specify a position of a pixel that has detected the electron and a time when the pixel has detected the electron, and wherein the detection modules are configured to detect an incident position of scattered Y rays generated by the Compton scattering, wherein the plurality of radiation detection devices each comprises a first radiation detection device and a second radiation detection device , wherein the first radiation detection device includes a first pixel electrode portion including a plurality of first pixel electrodes, and the second radiation detection device includes a second pixel electrode portion including a plurality of second pixel electrodes, wherein the first radiation detection device and the second radiation detection device share a chamber into which a gas is introduced, and a first insulating member provided in the chamber, wherein the first radiation detection device is provided on a first surface of the first insulating member , and the second radiation detection device is provided on a second surface located on an opposite side to the first surface of the first insulating member , and the second radiation detection device annularly surrounding the outer circumference of the first radiation detection device and the first radiation detection device being arranged to annularly surround a region in which a specimen is placed. a plurality of Compton cameras arranged annularly to surround a region in which a specimen is placed, wherein each of the plurality of Compton cameras includes a plurality of radiation detection devices and detection modules , wherein the plurality of radiation detection devices is configured to detect an electron generated by a track of a recoil electron generated by Compton scattering and configured to output a detection signal configured to specify a position of a pixel that has detected the electron and a time when the pixel has detected the electron, and wherein the detection modules are configured to detect an incident position of scattered Y rays generated by the Compton scattering, wherein the plurality of radiation detection devices each comprises a first radiation detection device and a second radiation detection device , wherein the first radiation detection device includes a first pixel electrode portion including a plurality of first pixel electrodes, and the second radiation detection device includes a second pixel electrode portion including a plurality of second pixel electrodes, wherein the first radiation detection device and the second radiation detection device share a chamber into which a gas is introduced, and a first insulating member provided in the chamber, wherein the first radiation detection device is provided on a first surface of the first insulating member , and the second radiation detection device is provided on a second surface located on an opposite side to the first surface of the first insulating member , and the second radiation detection device annularly surrounding the outer circumference of the first radiation detection device and the first radiation detection device being arranged to annularly surround a region in which a specimen is placed. <EOS>