Is operated to match the on-off of the beam irradiation (referred to as shields) structure for blocking nuclear debris, the beam during irradiation, to reduce the incidence to the detector of nuclear debris. This makes it possible to measure the annihilation radiation even during the irradiation or immediately after irradiation, can be imaged in three dimensions the irradiation field.核破砕片を遮蔽する構造物(遮蔽体と称する)を、ビーム照射のオンオフに合わせて動作させ、ビーム照射中は、核破砕片の検出器への入射を低減する。これにより、照射直後あるいは照射中にも消滅放射線を計測し、照射野を3次元的に画像化できる。