Development of an omnidirectional Compton camera using CaF2(Eu)  scintillators to visualize gamma rays with energy below 250 keV for radioactive environmental monitoring in nuclear medicine facilities

We developed an omnidirectional Compton camera for radioactive environmental monitoring which can visualize gamma rays with energy below 250 keV emitted from various radiopharmaceuticals used in nuclear medicine facilities to prevent occupational radiation exposure. An omnidirectional Compton camera based on high light yield scintillators CsI(Tl) or NaI(Tl) developed in our previous studies is a promising system for environmental radiation monitoring because it has a wide field of view and high sensitivity for sub-MeV gamma rays. However, its sensitivity rapidly decreases below 250 keV because photoelectric effect becomes more dominant than the Compton scattering process due to their large effective atomic numbers (approximately 50). Thus, CaF2(Eu)  was adopted, which has both low effective atomic number (approximately 15) and high light yield. Four CaF2(Eu)  crystals were arranged symmetrically to achieve a relatively uniform acceptance in all directions. Similarly, the detector rotation technique was adopted to suppress artificial patterns in a reconstructed gamma-ray image because of the small number of crystals. Through experiments in a laboratory and at a nuclear medicine facility, the capability of the camera to visualize gamma rays in energies from 250 keV to 60 keV with reasonable observation time for practical clinical use was confirmed.