Investigation of timing and positioning of T₀ choppers at long pulse neutron sources

We examine the prerequisites for the operation of T₀ choppers at a long pulse spallation source using the parameters of the European Spallation Source (ESS). We discuss the constraints imposed to the chopper position and the operation parameters by the long pulse nature and the low repetition rate of the ESS. For an instrument having a moderator-to-detector distance L_D=155 m with a double elliptic neutron guide shape we analyze possible solutions for chopper rotational frequencies of 7 and 28 Hz, that are acceptable for a cold instrument, while for a bi-spectral or a thermal instrument the lower frequency requires the chopper to be placed at a large distance from the moderator. (author)