Measurements of neutron distribution in neutrons–γ-rays mixed field using imaging plate for neutron capture therapy

The imaging plate (IP) technique is tried to be used as a handy method to measure the spatial neutron distribution via the 157Gd(n,γ)158Gd reaction for neutron capture therapy (NCT). For this purpose, IP is set in a water phantom and irradiated in a mixed field of neutrons and γ-rays. The Hiroshima University Radiobiological Research Accelerator is utilized for this experiment. The neutrons are moderated with 20-cm-thick D2O to obtain suitable neutron field for NCT. The signal for IP doped with Gd as a neutron-response enhancer is subtracted with its contribution by γ-rays, which was estimated using IP without Gd. The γ-ray response of Gd-doped IP to non-Gd IP is set at 1.34, the value measured for 60Co γ-rays, in estimating the γ-ray contribution to Gd-doped IP signal. Then measured distribution of the 157Gd(n,γ)158Gd reaction rate agrees within 10% with the calculated value based on the method that has already been validated for its reproducibility of Au activation. However, the evaluated distribution of the 157Gd(n,γ)158Gd reaction rate is so sensitive to γ-ray energy, e.g. the discrepancy of the 157Gd(n,γ)158Gd reaction rate between measurement and calculation becomes 30% for the photon energy change from 33 keV to 1.253 MeV.