Monte Carlo simulations of a powder diffractometer installed on a long pulse target station using wavelength frame multiplication

Abstract

MC simulations of neutron diffractometer have been performed using the VITESS software package to check the feasibility of the frame multiplication concept and to compare a powder instrument installed on a long pulse target station (LPTS) with an equivalent instrument installed on a decoupled poisoned moderator of a short pulse target station (SPTS) of the same power. The simulations show that frame multiplication can be realised, if five choppers are used to determine the frame. For the same FWHM peak width, the LPTS yields comparable peak intensities for wavelengths of about 1 Å, much higher intensities than the SPTS for long wavelengths, and a quite symmetric peak shape for all wavelengths. Therefore, a combination of high-energy LPTS and pulse shaping choppers can be used instead of a decoupled poisoned moderator of a SPTS to run a TOF powder diffractometer. It is the better choice (than the SPTS solution), because it matches the performance of SPTS in the case most favourable for SPTS: highest resolution offered by the poisoned moderator, has the capability of easily increasing the intensity by its variable resolution and gives superior intensity for longer neutron wavelengths.

Keywords:

• MC simulations
• Pulsed neutron source
• Diffractometer
• Frame multiplication
• Beam extraction system
• Ballistic guide

Acknowledgement

We like to thank Sergey Manoshin for his suggestions to improve the ballistic guide. This work has been supported by the SCANS network within the “Improving human potential programme” of the European Commission under contract HPRI-CT-1999-500013.