## 2 Overview
### Neutron Extraction  
The neutron extraction is responsible for avoiding a direct line of sight between the moderator and the sample while transporting the highest brilliance possible. The two main considerations making this out of line of sight requirement necessary are radiation protection and signal to noise ratio. In the moderator area there is a high flux of particles other than neutrons, from the spallation process, and virtually any particle possible created by bremsstrahlung for which the accelerator provides enough energy. In addition to that, there is approximately one \(\gamma\) per neutron and high energy \(\gamma\) radiation may produce photo neutrons. Indeed, the signal to noise ratio with direct view of the moderator would be poor due to those additional particles. Existing detector technologies can hardly discriminate between that many different particles at such high flux rates. Even neglecting the necessity of having a clean spectrum at the sample and detector position, most of these particles would require higher shielding efforts when compared to cold or thermal neutrons. This is even more the case for an instrument like SKADI, which only requires the cold flux from \begin{table}
\begin{tabular}{l c} \hline \hline \multicolumn{2}{c}{**Quick Facts**} \\ \hline Spectrum & Cold \\ Overall Length of Instrument & 58 m \\ Q-range & a few \(10^{-4}-1\)Å\({}^{-1}\) \\ Spin-Polarization/-Analysis & yes / planned \\ Wavelength band & 5 (standard mode) or 10 Å(pulse skipping mode) \\ Wavelength range & 3–21 Å \\ Momentum resolution & 2–7\% (standard mode) or 1–7\% (pulse skipping mode) \\ Flux at sample & 8.8 \(\times\) 10\({}^{8}\) neutrons s\({}^{-1}\) cm\({}^{-2}\) @ 5 MW accelerator power \\  & and 4 m collimation \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Tabulated instrumental parameters for Small-K Advanced DIffractometer (SKADI). Figure 1: SKADI overview image. The scale bar is in sections of 10 m. From the moderator to the extraction there is a 2 m free flight path up to the neutron extraction. Inside the bunker, there is the neutron extraction of a double deflector type (see Section 2.1). This section also includes the heavy shutter, which vertically replaces the last section of neutron guide with an absorption block. The bunker wall is located between 11 and 15.5 m. Directly outside the bunker wall is the polarizer housing with an integrated chopper. Inside the collimation (from the bunker wall to the sample area), there is a other chopper disk at 22.85 m and the infrastructure for including two high resolution choppers further downstream. The collimation feeds into the sample area with the complete infrastructure for mounting the different setups for sample environments. The detector tube is directly attached to the sample area. The shielding is removed for better visibility. the moderator at very low divergencies. Furthermore, reducing the total flux of particles, the requirements on shielding are considerably more moderate. For cold neutrons, there are, in general, three approaches to that problem: (1) curved neutron guides (benders), (2) doglegs, and (3) deflectors. In the case of the curved neutron guides, channeled benders should also be considered. In order to transport the highest brilliance, and not merely the highest flux with highly diverging neutrons, it is a good approach to transport the ideal image of a fraction of the moderator equal to the cross section of the last defining aperture before the sample aperture. Higher divergent neutrons could possibly be transported, however would only be collimated away in the later parts of the instrument and increase shielding effort, while all of the neutrons as described before, can be used for the scattering experiment. In addition to that function, fast neutrons should be removed in the neutron extraction, which is usually happening as a by-product, since fast neutrons will not be affected by most of the optical components and, thus, propagate on a straight path.