The out-of-core neutron irradiation facility of HANARO for measurement of neutron cross-section

Neutron cross-section measurements using the out-of-core thermal neutron irradiation facility of HANARO are introduced. This facility was designed and constructed using a long beam tube and a fast neutron and gamma-ray filter composed of thick silicon and bismuth single crystals. It gives a thermal neutron field with a flux over 1x109 n/cm2 s and a Cd ratio over 150 and has a spacious irradiation room. It can be used for various experiments, and one of the applications is the measurement of the thermal neutron cross-sections for several nuclides. In the design stage of the facility, the total neutron cross-sections of the silicon and bismuth crystals were measured by using the neutron transmission and time-of-flight method. For the silicon single crystal, the measured cross-section showed good agreement with the calculated value based on the semiempirical formula. The measurements for the bismuth crystal were much larger than those anticipated by calculation. From the result, it was confirmed that several parameters related to the Bragg scattering should be considered in the neutron transmission through the bismuth single crystal. On the other hand, the thermal neutron capture cross section for the W-180 nucleus was measured by using the activation and gamma-ray detection method. A natural tungsten foil was irradiated in this facility for 5 hours, and finally the capture cross-section was obtained at a value of 22.6±1.7 b. Measured data for the cross-section for W-180 is very rare, but the production of W-181 is very important since it is a useful radioisotope neutrino source for the various basic neutrino experiments. From the results, it was confirmed that the out-of-core neutron irradiation facility of HANARO would be useful for measuring neutron cross-sections. (author)