Film coated Si-wafer neutron collimators characterized by transmission experiments and simulations

Abstract

Two types of film coated collimators were fabricated for neutron experiments: one was a Si wafer collimator coated with Ni–B4C layers (SN), and the other was a Si wafer collimator coated with B4C layers (SB). The neutron transmission was measured by either rocking the sample collimators (ω scan) or scanning the detector. Under a high-resolution ω scan, we confirmed enhancement in the peak intensity and narrowing in the peak width for both the SN and SB, whereas a parasitic transmission was observable only for the SN. Such characteristics were fairly reproduced by the statistical Monte Carlo simulations of the McStas program by assuming probable reflectivity (Rmod). Further, we prepared a mathematical mapping method for tracing the N-times reflected neutrons in the (X,ϕ) plane and succeeded in specifying the transmission process in the two sample collimators; the increased transmission and peak narrowing were attributed to the ω-sensitive single reflection process, whereas the parasitic transmission arose from the Rmod-sensitive double reflection process.