## 2 Overview
### Detector  
The detector used for SKADI will be a SoNDe-type pixelated scintillation detector [7; 18; 19; 20]. Apart from the high-flux capability necessary [21], especially in the area of the primary beam, this detector also provides an excellent scalability and geometric freedom arranges the available active surface area as needed. In our previous publications, we also tested the sensitivity of our detector system, which is on the order of 90% for neutrons with a wavelength of \(\lambda=3\,\mathrm{\AA}\).  
In order to achieve a high simultaneous Q-range, SKADI will be equipped with three SoNDe type detectors, two of which are mounted on a movable carriage and have an aperture for low-angle scattering. Figure 9 shows the proposed arrangement of detector area. Here, it should be noted that the center hole is always \(20\times 20\) cm\({}^{2}\) and the distance ratio between the forward and middle detector, as measured from the sample, is always \(1/5\). The central detector is always at the same distance as the collimation setting to ensure an ideal mapping of the sample aperture on the detector, while the rear detector is fixed in the back, outside of the detector tank at 20 m. While, for some settings, there may be geometrical gaps, there is never a gap in Q-space due to the wavelength band used for the scattering experiment. In addition, the rear detector also serves as a beam monitor. A semitransparent beamstop will allow for investigating the full scattering down to the lowest angle, as the primary beam is only attenuated and never blocked.  
The pixel size throughout the detector is \(6\times 6\) mm\({}^{2}\) everywhere, apart for the 20 m position, where the resolution is \(3\times 3\) mm\({}^{2}\), in order to allow for an optimal resolution. There are several considerations going affecting the choice of pixel size. In fundamental terms, the smaller pixels allow for a higher count rate, since the used detector technology has a roughly constant dead time per pixel. Thus, decreasing pixel size and increasing pixel number linearly increases the achievable count rate. Furthermore, on most current day instruments, slit sizes of roughly 1 cm in both directions are chosen as a sample blind, while the final collimation blind before that is on the order of a few centimeters in both directions. Using those numbers, the SKADI detector would not increase the achievable resolution, since the beam size gives the limit. However, for SKADI with the higher flux, smaller apertures will probably be more commonly used. Additionally, when considering the VSANS data that were recorded with the VSANS prototype that will be used by SKADI (Figure 6), it is already now apparent that the detector needs a higher resolution, in order to take full advantage of the better flux and optics.  
The front and middle detector will be in a box at atmospheric pressure and located inside the vacuum tank, which is 20 m long and 2.2 m in diameter and made from aluminum [22]. Vacuum hoses in two cable chains will be required to bring the air from outside the vacuum tank to those air boxes. The cooling of the electronics and the signal out will also be routed through those hoses. Because of the \(1/5\) ratio of their relative distances from the sample, both of the detectors will have their own motorized support and share a common guiding rail.