Photons originating from one of the major backgrounds, $K^+ \rightarrow \pi^+ \pi^0$ ($B.R. = 20.7 \%$), propagate at angles greater than 50 \si{\milli\radian} only in a 0.2\% fraction of $\pi^+ \pi^0$ events. A photon veto system was therefore developed, that ensures a rejection inefficiency lower than $10^{-8}$ in the fiducial energy range of the signal. Photon veto detectors cover a 50 \si{\milli\radian} angular region around the beam.\\

The photon veto system is partitioned in four sub-detectors that cover different angular regions and employ three different technologies:
\begin{itemize}
\item Twelve LAV stations cover the angular region between 8.5 and 50 \si{\milli\radian}.
\item SAC and IRC cover the inner angular region, from about 0 to 1 \si{\milli\radian}.
\item The LKr calorimeter covers angles between 1 and 8.5 \si{\milli\radian}.
\end{itemize}

The \textbf{Large Angle Veto} (LAV) detector reuses the 10$\times$10$\times$37 \si{\centi\meter^3} lead-glass blocks from the OPAL electromagnetic calorimeter \cite{opal}, arranged in 12 annular stations (\figurename~\ref{fig:det:lav}).\\

Of the 12 LAV counters, 11 are placed inside the $3 \times 10^{-7}$ \si{\milli\bar} vacuum tank hosting the decay region, and one is placed between the RICH and the CHOD sub-detectors.\\

Each ring-shaped station of the LAV has an increasing diameter as the distance from the target grows, and consists of four to five rings of lead-glass blocks read out at the outer side by 76 \si{\milli\meter} diameter Hamamatsu photomultipliers.\\

Photons incident onto the LAV blocks start electromagnetic avalanches, detected through the collection of \v{C}erenkov light emitted by $e^+ e^-$ pairs.\\

Thanks to its low threshold, the LAV system will be also able to detect muons and pions in the beam halo \cite{NA62:1529961}.\\

The \textbf{Liquid Krypton Calorimeter} (LKr), placed between the RICH and the MUV detectors, is the same used in the NA48 experiment. Its main purpose is to reject photons that fly at an angle between 1 and 8.5 \si{\milli\radian} to a level of $10^{-5}$. However, the LKr can also provide accurate measurements of the energy of electrons and positrons, useful for the rejection of the $K^+ \rightarrow \pi^0 e^+ \nu_e$ background.\\

The calorimeter volume is filled with 9 \si{\meter^3} of liquid krypton, whose characteristics are summarised in \tablename~\ref{tab:det:lkr}.\\

In this quasi-homogeneous calorimeter, the active material is 127 \si{\centi\meter} (approximately 27 $X_0$) thick, and can fully contain 50 \si{\giga\electronvolt} showers.
The active volume is divided in 13248 2$\times$2$\times$127~\si{\centi\meter^3} double ionization cells with Cu-Be ribbons as central anodes (\figurename~\ref{img:det:lkrcells}).
Despite being originally built for the NA48 experiment, the LKr calorimeter is still a state-of-the-art piece of hardware, featuring excellent resolution in energy, space and time:
\begin{align}
&\frac{\sigma_E}{E} = \frac{0.032}{\sqrt{E}} \oplus \frac{0.09}{E} \oplus 0.0042\; \si{\giga\electronvolt}\\
&\sigma_x = \sigma_y = \frac{0.42}{\sqrt{E}} \oplus 0.06\; \si{\centi\meter}\\
&\sigma_t = \frac{2.5}{\sqrt{E}}\; \si{\nano\second}
\end{align}
In the above formulas all energies are expressed in \si{\giga\electronvolt}.\\

%\begin{figure}
%\centering
%\includegraphics[width=\textwidth]{Images/pdf/na62/detector/lav-full}
%\caption{A view of the LAV-12 station.}\label{fig:det:lav}
%\end{figure}
%
%\begin{figure}
%\centering
%\def\svgwidth{\textwidth}
%\input{Images/pdf/na62/detector/lkr-tex.pdf_tex}
%\caption{Liquid krypton calorimeter electrode structure \cite{lkr}.}
%\end{figure}

\begin{figure}
\centering
\subfigure[A view of the LAV-12 station.]{\includegraphics[width=\textwidth]{Images/pdf/na62/detector/lav-full}\label{fig:det:lav}}
\subfigure[Liquid krypton calorimeter electrode structure \cite{lkr}.]{\def\svgwidth{\textwidth}\input{Images/pdf/na62/detector/lkr-tex.pdf_tex}\label{img:det:lkrcells}}
\caption{Technical designs of a LAV station (top panel) and of the LKr calorimeter (bottom panel).}
\end{figure}

\begin{table}
\centering
\begin{tabular}{cc}
\hline
\hline
$X_0$ & 4.7 \si{\centi\meter}\\
Moliere radius & 6.1 \si{\centi\meter}\\
Bath temperature & 119.8 \si{\kelvin}\\
\hline
\hline
\end{tabular}
\caption{Characteristics of the liquid krypton filling the calorimeter \cite{lkr}.}\label{tab:det:lkr}
\end{table}

Both the small-angle veto calorimeters, \textbf{IRC} and \textbf{SAC}, are ``\textit{shashlyk}'' type calorimeters, i.e. detectors made of lead absorber layers interspaced with plastic scintillator plates used as active material \cite{atoian}.\\

Due to the geometry of the experiment, photons hitting IRC or SAC will have an energy greater than 5 \si{\giga\electronvolt}.
Interacting with the lead plates, they will then start electromagnetic showers. A wavelength-shifting dopage is added to the scintillator tiles in order to gather the light emerging from the showers into fibers read out by photomultipliers.\\

The \textbf{Ring-shaped Calorimeter} IRC is placed around the beam line in front of the LKr, and covers the angular region between such detector and the SAC.\\
% not occupied by the LKr.\\

A dipole magnet bends the beam so that charged particles cannot hit the SAC (\textbf{Small Angle Calorimeter}), the most forward detector in the NA62 setup.\\

The combined detection inefficiency of the small angle vetoes is requested to be lower than $10^{-8}$.