30 MeV X-band Electron Linac Neutron Source for Nuclear Data Study for Fukushima Accident Analysis

Abstract

X-band (11.424GHz) electron linac as a neutron source for nuclear data study for the Fukushima nuclear plant accident is under development. Originally we developed the linac for Compton scattering X-ray source. Quantitative material analysis and forensics for nuclear security will start several years later after the safe settlement of the accident is established. For the purpose, we should now accumulate more precise nuclear data of U, Pu, TRU and MA (Cm, Am, etc.) especially in epithermal (0.1-10eV) neutrons. Therefore, we have decided to suspend the Compton scattering X-ray experiment and move the linac in the core space of the experimental nuclear reactor “Yayoi” which is now under the decommission procedure. Due to the compactness of the X-band linac, the electron gun and accelerating tube and other components can be installed in a small space in the core. First we plan to perform the TOF (Time Of Flight) transmission measurement for study of the total cross sections of the nuclei for 0.1-10eV neutrons. Therefore, if we adopt a TOF line of less than 10 m, the macro-pulse length of generated neutrons should be shorter than 100ns. Electron energy, macro-pulse length, power and neutron yield are ∼30 MeV, 100ns - 1μs, ∼0.4kW and ∼1012 n/s, ∼103 n/cm2/s at samples, respectively. Optimization of the design of a neutron target (Ta, W, 238U), TOF line and neutron detector (Ce:LiCAF) of high sensitivity and fast response is underway. For the purpose, we are upgrading the electron gun and buncher to realize higher current and beam power with a reasonable beam size in order to avoid damage of the neutron target. Although the neutron flux is limited in case of X-band electron linac based source, we take advantage of its short pulse aspect and availability for nuclear data measurement with a short TOF system. We plan to perform the two phased development and measurement. First, we form a tentative configuration in the current experimental room for Compton scattering in 2014. Then, after the decommissioning has been finished, we move it the “Yayoi” room and perform the operation and measurement.