Simulations of Chopper Jitter at the LET Neutron Spectrometer at the ...

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Simulations of Chopper Jitter at the LET Neutron Spectrometer at the ISIS TS2 6monitors interspersed at suitable locations. Note that the monitors used in the simulationsdoes not emulate physical detectors, in as much as they do not alter or absorb thedetected neutron rays.The simulations used to generate the figures in this paper were typically run with 5∗10 8rays, which corresponds to approximately 20 min. CPU-time on an ordinary dual-corelaptop.Fig. 5 shows the energy distribution of the beam at various positions in the instrument,with perfect beam choppers. Notice the multiple peaks after the first resolution choppers,reduced to the single 5 meV peak by the PR chopper, as would be expected in thephysical instrument.To illustrate the performance of the spectrometer, we have performed virtualexperiments using an incoherent elastic scatterer[8]. Typical outcomes of theseexperiments are shown in fig. 6, while the deduced line width is shown in fig. 9.With highly imperfect beam choppers (large jitter), the detected energy distributioncan be seen to noticeably widen. This is illustrated in fig. 6, and clear effects are visiblefor jitter values of 10 µs. The line shape is almost perfectly Gaussian at zero jitter, thenbecomes increasingly Lorentzian with added jitter.As expected from the ToF equation (2), the effect of the jitter depends significantlyon the speed of the choppers, as can be seen in fig. 7, where the intensity drops off muchmore rapidly for increasing jitter, with the chopper set to the higher speed, as can beseen in fig. 7.Simple analytical calculations of the time resolution at the detector positionsupports the simulated resolution increase with jitter, shown in figs. 6 and 9. Weuse the ToF equation to calculate the allowed arrival times of neutrons at the detector,for different values of jitter.4.1. Side peaks in simulationsWhen building non-standard components, care and attention is needed to avoidsimulation artefacts. As the chopper slits of the Res2 double choppers in the simulationsare triangular, and the guide openings of the double funnel system are rectangular, tryingto fit them onto the rectangular guide opening of the double funnel system, resulted inthe side peaks shown in the (x,t) diagram in fig. 8. What happens is that a short timebefore and after the double chopper turns to the open alignment, small slices of the10 mm wide guide openings are not covered by the absorbing section between the dualchopper slits (see fig. 2), which is 11 mm at its widest.In the actual instrument, the chopper slits have been shaped so as to avoid this effect.See fig. 3
Simulations of Chopper Jitter at the LET Neutron Spectrometer at the ISIS TS2 7(a) After Res1 choppers(b) After PR chopper(c) After CR chopper(d) After Res2 choppers(e) Sample position(f) TOF at sample positionFigure 5. a-e: Energy distribution of the beam at the chopper and sample positions,at nominal energy E = 5 meV. f: Time of flight (TOF) distribution at sample position.The slight tail seen on the left side of panels d,e can be traced back to the long time-tailof the source pulse. Note also the very small side peaks at 2.566 and 2.570 in panel f,explained in section 4.1
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