Technical note: Optical imaging of lithium‐containing zinc sulfate plate in water during irradiation of neutrons from boron neutron capture therapy (BNCT) system

Abstract

Optical imaging of ionizing radiation is a possible method for dose distribution measurements. However, it is not clear whether the imaging method is also applicable to neutrons. To clarify this, we performed the imaging of neutrons in water from boron neutron capture therapy (BNCT) systems. Such systems require efficient distribution measurements of neutrons for quality assessment (QA) of the beams. A water-filled phantom was irradiated from the side with an epithermal neutron beam, in which a lithium-containing zinc sulfate (Li-ZnS(Ag)) plate was set in the beam direction, and during this irradiation the scintillation of the plate was imaged using a cooled charge-coupled device (CCD) camera. In the imaging, Li-6 in the Li-ZnS(Ag) plate captures neutrons and converts them to alpha particles (He-4) and tritium (H-3), while ZnS(Ag) in the Li-ZnS(Ag) plate produces scintillation light in the plate. We also conducted Monte Carlo simulation and compared its results with the experimental results. The image of the emitted light from the Li-ZnS(Ag) plate was clearly obtained with an imaging time of 0.5s. The depth and lateral profiles of the measured image using the Li-ZnS(Ag) plate showed the same shapes as the neutron distributions measured with gold foil, within a difference of 8%. The destructive effect of neutrons on the CCD camera increased approximately three times, but the unit was still working after the measurement. The optical imaging of neutrons in water is possible, and it has the potential to be a new method for efficient QA as well as for research on neutrons.