Modeling fast neutron shielding for the direct nn-scattering experiment
Modern high power accelerators and reactors provide the possibility of a direct measurement of neutron-neutron scattering. This experiment addresses the fundamental problem of charge symmetry in the nuclear force by comparing the proton-proton, pp, and neutron-neutron, nn, scattering lengths. The high flux pulsed reactor YAGUAR will be used to perform the experiment, with a neutron detector placed in an evacuated underground channel. Of paramount importance is the detector background. Here we address the fission neutron background component due to the delayed neutrons multiplied in the subcritical reactor. The Monte Carlo MCNP4-C code with the weight window variance reduction technique was chosen to calculate the neutron transport for a 6-m model of the shield consisting of water, concrete, and earth, with a 7-cm diameter channel lined with boron carbide. The present calculations are important for refining the background calculations for the real nn-experiment. These results can also be extended to design studies of biological shields at accelerators.
