The momentum range and the position of the \v{C}erenkov threshold determined the choice of the gas and gas pressure: the vessel, a 18~\si{\meter} long and 2.8~\si{\meter} wide cylinder, will be filled with neon at atmospheric pressure. This way, the \v{C}erenkov threshold momentum for a pion will be $P_{th} \simeq 12.5$~\si{\giga\electronvolt/\textit{c}}, 20\% smaller than the lower limit of the accepted momentum range. The refraction index $n$ will be such that $(n-1) \simeq 60 \times 10^{-6}$ \cite{na62-tdr}: according to Eqn.~\ref{eqn:cher_pth}, this value almost perfectly corresponds to the emission threshold for the mass of the charged pion. The Ne gas also guarantees small dispersion \cite{2008NIM-rich}. \figurename~\ref{img:det:rich-vessel} shows the technical design of the vessel that will host the RICH detector.\\%, i.e. a ferro-pearlitic steel cylinder in the current design \cite{na62-tdr}.\\

The vessel will be placed between the last STRAW chamber and the LKr calorimeter. It will be rotated by 2.4~\si{\milli\radian} with respect to the $z-$axis: this way, its central hole around the beam pipe will gather the charged beam component (and the beam halo) bent by the spectrometer magnet, while most of the kaon decay products will cross the active volume.\\

\begin{figure}
\centering
\subfigure[]{\includegraphics[width=0.75\textwidth]{Images/pdf/na62/detector/interfaces}\label{img:det:rich-vessel}}\hfill
\subfigure[]{\includegraphics[width=0.24\textwidth]{Images/pdf/na62/detector/rich_mirrors}\label{img:det:rich-mirrors}}
\caption{Panel~\ref{img:det:rich-vessel}: technical drawing of the RICH vessel (the beam goes from right to left). Panel~\ref{img:det:rich-mirrors}: front view of the system of 18 hexagonal and 2 semi-hexagonal mirrors.}\label{img:det:rich-setup}
\end{figure}

A mosaic of 18 hexagonal and 2 semi-hexagonal mirrors made of alu\-mi\-nium-coated 25~\si{\milli\meter} thick glass covered with a thin dielectric film, with sides 35~\si{\centi\meter} long, reflects the \v{C}erenkov cone onto the RICH focal plane. In order to avoid absorption of light by the beam pipe, the mirrors actually form two independent spherical surfaces (shown in \figurename~\ref{img:det:rich-mirrors}), with the foci corresponding respectively to the two PMT flanges\footnote{In the following, the left and right sides will be often referred to as Jura and Sal\`{e}ve flanges respectively, after the two mountains overlooking the Geneva area.}. The collective 34~\si{\meter} curvature radius of the mirrors layout results in a nominal focal length $f = 17$~\si{\meter}.\\

Detector simulations showed that the best compromise between the requirements of photon acceptance and angular resolution, and the need to maintain the cost of the apparatus at an affordable level, can be achieved by arranging 1952 photomultipliers on the vertices of the cells of a compact hexagonal lattice. The layout will consist of two support flanges, 70~\si{\centi\meter} in diameter, each hosting 976 Hamamatsu photomultipliers \cite{na62-tdr}.\\

Since the RICH will also participate to the level 0 trigger, producing a primitive each time a charged particle crosses its volume, a time resolution of about 100~\si{\pico\second} is required, leading to the choice of fast single-anode photomultipliers.\\

The RICH performance has been tested and verified with particle beams using a full length prototype \cite{2008NIM-rich}.