Bi-spectral moderator for spallation sources optimized for instrument requirements

At a spallation neutron source, significant increase in the performance of neutron scattering instruments can be achieved if the target-moderator-reflector (TMR) and the following neutron guides are specifically tailored to the needs of different instrument classes. In order to define optimal quantities (e.g. positions of peak neutron flux, used wavelength band) for the Figure of Merit (FoM), a survey has been conducted at PSI covering the experience of the local instrument scientists. Based on this survey we introduce several FoMs which show the potential of tailoring the neutron spectrum to specific instrument needs. The developed methodology is adopted to optimize the top moderator in the conceptual design of the TMR of the European Spallation Source (ESS) Project. A parametrized geometry model of the ESS TMR is built in MCNPX and used within an optimization framework to study and optimize the moderator performance in the thermal and cold regions of the neutron spectrum. The results obtained with the optimized setup are compared to the ESS 2003 Project. Furthermore, experience is gathered while performing these simulations, e.g. the examination of the pulse shapes obtained with time-dependent calculations shows that for long-pulse target stations the peak brightness can with sufficient precision be obtained from time-independent calculations by scaling with the proton pulse duty factor.