Using Fermi choppers to shape the neutron pulse

This work is part of a series of papers [H.N. Bordallo, G. Zsigmond, J.D.M. Champion, Physica B 350 (2004) e717; J. Peters, Nucl. Instr. and Meth. A 540 (2005) 419] about benchmarking studies related to Fermi choppers (FC) used for pulse shaping of neutron scattering instruments. Here, we discuss the comparison of simulated and analytical results to experimental data of the pulse shape observed immediately after the FC in the High Energy Transfer (HET) instrument at ISIS. From the simulation side, there is a combination of a better moderator description, new absorbing B4C apertures and a new geometrical description of the FC that enhanced the original results [H.N. Bordallo, G. Zsigmond, J.D.M. Champion, Physica B 350 (2004) e717]. Besides, the analytical formalism, recently derived in [J. Peters, Nucl. Instr. and Meth. A 540 (2005) 419], is applied to the FC module of the Monte Carlo (MC) software package VITESS used in versions 2.5 and 2.5.1. The overall agreement is now very good.