Upgrade of the primary spectrometer of the cold triple-axis spectrometer FLEX at the BER II reactor

Monte Carlo simulation results for the upgrade of the FLEX/V2 cold neutron triple-axis spectrometer are presented. The main goals are increased flux and energy resolution, while keeping a low background. In the same time the energy transfer range is significantly increased, bridging the gap between cold and thermal neutrons. We aim towards a flexible instrument, by employing the virtual source concept defining a beam before a double-focusing monochromator. This enables fine-tuning according to energy resolution, intensity and sample size requirements. A velocity selector adds the extra flexibility of tuning the spectrometer to any desired fixed wavevector. Neutron ray-tracing programs McStas and VITESS were extensively used and tested in this process.