POLDI: Materials Science and Engineering Instrument at SINQ

Abstract

The time-of-flight (TOF) diffractometer POLDI (Pulse Overlap DIffractometer) operational at the continuous spallation source SINQ was specifically designed and optimised for the study of residual stresses and mechanical behaviour in engineering materials. The novel feature of POLDI compared to typical TOF instruments is the use of pulse overlap, whereby the faster neutrons of a pulse emitted from the slits of a chopper can catch up the slower neutrons from the previous slit. In this case, the flight time cannot be simply calculated by the arrival time of the neutron, but by recording the angular dependence of the neutron arrival time and the flight time of the neutron, the lattice spacing d can be determined [1]. In a plot of intensity vs. arrival time and scattering angle, each Bragg reflection is represented by a Bragg line (see Fig. 1). The slopes of these Bragg lines are given by Eq. (1):