The Raison d’Être of Long Pulse Spallation Sources

Abstract

Current accelerator technology allows us today to envisage neutron sources with well above an order of magnitude increased neutron flux for neutron scattering experiments compared to the most intense existing neutron sources, i.e. the high flux reactor at ILL and the spallation sources ISIS, two facilities of complementary use in neutron scattering, with a substantial overlap though. The optimal design of a future, enhanced power source has to tackle the complex relations within the global system accelerator – target-station – neutron scattering instruments. An analysis of the fundamental aspects of these three very different ingredients shows that, depending on the neutron energy, two different technical solutions offer the best performance at lowest costs: (a) conventional spallation sources with short proton pulses in the µs range (SPSS) for hot and epithermal neutrons and (b) long pulse sources with proton beam pulses in the ms range (LPSS) for thermal and cold neutrons. This actually means that LPSS provides the technically most favorable solution for the overwhelming majority of thermal and cold neutron scattering experiments currently best performed on either continuous reactor (CW) sources or on existing SPSS. The two key ingredients for optimal instrument design on LPSS are disc chopper systems and long neutron guides. A new approach (called “ballistic”) allows us to build very long guides with low beam losses. Pairs of disc choppers provide both satisfactory resolution and great flexibility of adjustment. The discussion is completed by considering three examples of comparative instrument performance evaluations based on results of detailed Monte Carlo simulations: small angle scattering, powder diffraction and inelastic time-of-flight spectroscopy using repetition rate multiplication.