Primary spectrometer neutron optics simulations for a new cold neutron backscattering spectrometer

The future new cold neutron backscattering spectrometer IN16B at the Institut Laue-Langevin is being designed to maintain the extremely high energy resolution of the existing backscattering spectrometer IN16 (approximately 0.4 μeV full width at half maximum in a standard configuration). Simultaneously, a phase space transformation (PST) device will significantly increase the flux at the sample position at the expense of an acceptably more divergent incoming beam. A wide wavelength band (Δλ/λ ≈ 10%) has to be offered to the PST device in order to achieve a significant flux gain by a factor of at least 4. Thus, IN16B will have to be located at the end of a cold neutron guide, whilst the present IN16 is located at a side position along a guide. In order to optimize the layout of individual components and to estimate the instrument performance, the Monte Carlo simulation programs McStas and VITESS have been used. McStas and VITESS offer a general framework to compose virtual neutron scattering instruments and...