A new design for the standard pinhole small-angle neutron scattering instrument D11

Monte Carlo simulations have been performed to check ideas for improvements of the collimating guide system of the standard pinhole small-angle neutron scattering ( SANS) instrument D11 at Institut Laue-Langevin, which reaches very low Q values by using long detector and collimation distances. According to the simulations, improved guide characteristics can increase the flux at the sample by about 10-15% for short collimation distances. Widening the guide width over the first few metres by 50% will yield a flux gain of about 40% for long and intermediate collimation distances, but a loss for short distances. A gain for all collimation distances can be achieved by a reduction of the guide cross section over the last few metres ( in addition to the widening). The best performance was found for a quasi-elliptical exit that reduces width and height to 60% of its nominal size. The effect on divergence of the neutron arriving at the sample has been studied. The changes will be realized in the framework of the ongoing D11 renewal project.