## 2 Overview
### Chopper  
Details regarding the chopper setup of SKADI have already been published, including the analytical calculations used to develop a balanced chopper setup [11]. SKADI will provide the full infrastructure for four choppers, only equipping the first two positions (bandwidth and frame-overlap). High resolution choppers thus can be added at a later time. This results in chopper positions of 15.5 and 22.85 m (for 7 and 14 Hz) with additional chopper pits at 26.3 and 26.6 m. The choppers are optimized for a transmission of neutron wavelengths between 3 and 8 A  and offer the option for pulse skipping mode to achieve a wider wavelength band of 3 to 13 A.  
It should be noted that the simulation always assumes perfectly absorbing chopper disks. Our workshop can reliably produce chopper disks absorbing better than \(10^{-7}\) that are averaged over the full shaded area.  
In order to elucidate the impact of high resolution choppers, the different achievable resolutions for simulated \(\delta\)-peaks are shown in Figure 5. Here, we compare the resolution improvement by either the additional choppers or a reduction of the slit size while only using two choppers. While the wavelength resolution \(\Delta\lambda/\lambda\) can be considerably improved by using four instead of two choppers, this does not fully feed into a much improved Q-resolution \(\sigma Q/Q\), as in the pertinent Q-range the aperture at sample position and the pixel size of the detector play a dominant role. However, looking at the simulated scattering from a sample creating exact \(\delta\)-peaks, it can be seen that the background is lowered by approximately 10% when comparing the smaller apertures with the 1 cm apertures combined with high resolution choppers. Meanwhile, the peak width is nearly unchanged. This means a better signal-to-noise ratio can be achieved. Here, it also should be noted that the absolute count rate of contributing neutrons (neutrons that are scattered within a peak) is bigger by a factor of 2 in the case of the four chopper solution.  
Figure 5: A set of simulated \(\delta\)-peaks for different instrument configurations. While there is no major impact on the resolution for the additional choppers, the background is considerably lowered. Additionally, the absolute number of neutrons transmitted in the useful Q-band is higher by a factor of 2 for the chopper setup. Simulations where performed for 20 m collimation and sample apertures of 1 cm (\(\blacktriangle\)), 0.875 cm (\(\bullet\)), and with 1 cm sample aperture using the high-resolution choppers (\(\blacktriangledown\)). For better visibility, the lower cutoff is at a normalized intensity of \(~{}1\times 10^{-4}\). In order to ascertain the respective background intensities, the intensities have been summed over the areas that are shaded blue, which results in 0.074 for the 1 cm slit opening, 0.062 for the 0.875 cm slit opening, and 0.056 for the high resolution chopper setup.  
Because the choppers also should inhibit frame overlap between the different pulses, it needs to be considered that this function is still adequately performed by a set of two choppers. Simulations showed that at 40 A  an additional signal is visible with a relative intensity of \(10^{-6}\) compared to the 3-8 A  wavelength band. For initial operation, this seems to be appropriate, at a later time a frame overlap mirror may improve this signal ratio, even without the use of additional choppers. Here, it also should be noted that, in addition the 40 A  neutrons will be attenuated far stronger by any window material. Additionally, in the case of a sufficiently long collimation, they will drop out of the collimation and be absorbed (\(\sim\)0.4 mm drop per meter flight-path for a 40 A  neutron).