Construction of neutron guide tubes in upgraded JRR-3.

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Abstract
Five neutron guide tubes have been installed in the upgraded JRR-3 (Japan Research Reactor No. 3). Two of them are for thermal neutrons and the other three are for cold ones. The characteristic wavelength of the thermal neutron guide tubes is 2 Å, and those of the cold neutron guide tubes are 4 and 6 Å. The longest guide tube is 59.9 m long and the total length of guide tubes is 232.1 m. The beam sizes are 2 cm × 20 cm for the thermal neutron beams and 2 cm × 12 cm for the cold neutron beams. A curved part of the neutron guide is assembled by a polygonal approximation with use of 85 cm long straight units. The neutron mirrors of these units are made of natural Ni deposited borosilicate glasses. The Ni layer is about 2,000 Å in thickness. The mean fabrication error of guide tube units is 4 μm. The mean installation errors are 8 μm for the positional abutment error and 5 × 10−6 rad for the angular error. The neutron losses by these errors will be about 5%, and the neutron fluxes at the exits of the neutron guides are estimated to be about 2 × 108 n/cm2·s.

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