One disadvantage of high-energy beams is that kaons cannot be efficiently separated from the $p / \pi$ content of the beam by means of the beam optics. As a consequence, upstream detectors are exposed to about 17 times the ``useful'' rate of particles. The identification of kaons before they decay is indeed a critical aspect in such a high-rate environment.\\

Positive kaon tagging is achieved by letting the beam traverse a differential \v{C}erenkov counter (CEDAR). The detector is filled with hydrogen at a pressure of 3.6 \si{\bar}, and it has a total thickness of 6.6 $X_0$.\\

A particle crossing a radiator with refractive index $n$ at a speed $\beta$ emits a cone of \v{C}erenkov light at an angle $\theta_c \left(\beta, n\right)$ (see \chaptername~\ref{sect:rich}). Since the momentum of the beam is known, the \v{C}erenkov angle, at a fixed gas pressure and therefore fixed $n$, is a function of the mass of the particle. The gas pressure is therefore adjusted so that only the wanted particle type can emit \v{C}erenkov radiation at the chosen light detection angle.\\

The \v{C}erenkov light is reflected by a spherical mirror onto a ring-shaped diaphragm that vehicles light into 8 clusters of 32 photomultipliers each \cite{na62-tdr}. The number of photomultipliers was increased, compared to the original CEDAR on the beam line, in order to decrease the photon rates on each readout device, reducing dead time and accidental noise as a consequence.\\

\begin{figure}
\centering
\subfigure{\includegraphics[width=0.59\textwidth]{Images/pdf/na62/detector/cedar-scan2-full}\label{img:det:cedar-scan}}
\hfill
\subfigure{\includegraphics[width=0.39\textwidth, trim = 0cm -0.25cm 0cm 0cm]{Images/pdf/na62/detector/cedar-photons}\label{img:det:cedar-photons}}
\caption{On the left, a pressure scan on a 75 \si{\giga\electronvolt} beam shows three peaks, corresponding to pions, kaons and protons respectively. The plot shows the counting rate of the CEDAR detector, normalized to the total beam rate, as a function of the pressure of the N\subscript{2} gas filling the chamber \cite{Ceccucci:1070584}. The panel on the right displays the photons for 100 simulated CEDAR events: ellipsoidal mirrors defocus the light onto the photodetector planes, in order to lower the counting rate of each photomultiplier \cite{na62-tdr}.}\label{img:det:cedar}
\end{figure}