The availability of such a good tracking system allowed us to highlight some spatial defects of the prototype. When we direct a flux of charged particles over a detector, for some reasons the response may vary according to which region of the chamber is irradiated. For instance, it happened that a pad was disconnected from the corresponding pin on its VFAT2 chip; or that we focused the beam over a region of the detector containing a piece of the spacer frame.\\
\begin{figure}[!htbp]
	\centering
	\subfigure[Zone A]
	{\includegraphics[width=6.6cm]{Images/radiography/A}\label{lg-point-a}}
	\hspace{2mm}
	\subfigure[Zone B]
	{\includegraphics[width=6.6cm]{Images/radiography/B}\label{lg-point-b}}
%	\vspace{2mm}
	\subfigure[Zone C]
	{\includegraphics[width=6.6cm]{Images/radiography/C}\label{lg-point-c}}
	\hspace{2mm}
	\subfigure[Zone D]
	{\includegraphics[width=6.6cm]{Images/radiography/D}\label{lg-point-d}}
%	\vspace{2mm}
	\subfigure[Zone E]
	{\includegraphics[width=6.6cm]{Images/radiography/E}\label{lg-point-e}}
	\hspace{2mm}
	\subfigure[Zone F]
	{\includegraphics[width=6.6cm]{Images/radiography/F}\label{lg-point-f}}
%	\vspace{2mm}
	\subfigure[Zone G]
	{\includegraphics[width=6.6cm]{Images/radiography/G}\label{lg-point-g}}
	\hspace{2mm}
	\subfigure[Zone H]
	{\includegraphics[width=6.6cm]{Images/radiography/H}\label{lg-point-h}}
%	\vspace{2mm}
	\caption{A radiography of the prototype triple GEM detector}
	\label{lg-imaging-1}
\end{figure}

\begin{figure}[!htbp]
	\centering
	\subfigure[Zone I]
	{\includegraphics[width=6.6cm]{Images/radiography/I}\label{lg-point-i}}
	\hspace{2mm}
	\subfigure[Zone L]
	{\includegraphics[width=6.6cm]{Images/radiography/L}\label{lg-point-l}}
%	\vspace{2mm}
	\subfigure[Zone M]
	{\includegraphics[width=6.6cm]{Images/radiography/M}\label{lg-point-m}}
	\hspace{2mm}
	\subfigure[Zone N]
	{\includegraphics[width=6.6cm]{Images/radiography/N}\label{lg-point-n}}
%	\vspace{2mm}
	\subfigure[Zone O]
	{\includegraphics[width=6.6cm]{Images/radiography/O}\label{lg-point-o}}
	\hspace{2mm}
	\subfigure[Zone P]
	{\includegraphics[width=6.6cm]{Images/radiography/P}\label{lg-point-p}}
	\caption{A radiography of the prototype triple GEM detector}
	\label{lg-imaging-2}
\end{figure}
Figures~\vref{lg-imaging-1} and~\vref{lg-imaging-2} display a ``radiography'' of the LG prototype, made by moving the detector around in order to check the response of several areas (see \figurename~\vref{lgpads}). The graphs plot the two-dimensional beam profile as it was detected by the tracker, with the condition that for every track there were corresponding hits on the LG channels.\\

We spotted:
\begin{itemize}
	\item \textit{dead} (disconnected) pads: see Figures~\ref{lg-point-d}, \ref{lg-point-e} and \ref{lg-point-m};
	\item thin spacers: see Figures~\ref{lg-point-b}, \ref{lg-point-h}, \ref{lg-point-l}, \ref{lg-point-m}, \ref{lg-point-n} and \ref{lg-point-p}. In particular, \figurename~\ref{lg-point-m} shows a misalignment between a spacer and the edge of a cathode sector;
	\item segments of the thick central spacer covering the seam between the GEM foils: see Figures~\ref{lg-point-b}, \ref{lg-point-c} and \ref{lg-point-g};
	\item the edge of the chamber: see Figures~\ref{lg-point-f} and \ref{lg-point-n}.
\end{itemize}

\figurename~\vref{eff-around} and \tablename~\vref{noisecontrib} show the computed average efficiency for all these regions, and some more information that will be discussed below.\\
\begin{figure}
	\centering
	\subfigure[LG efficiency]
	{\includegraphics[width=8cm]{Images/homogeneity}\label{eff-around}}
	\hspace{2mm}
	\subfigure[Spacer frame view]
	{\includegraphics[width=4cm]{Images/spacers}\label{spacers}}
	\caption{Efficiency scan over various critical chamber regions. On the right, a broken GEM layer and its spacer frame (same model as those inserted in the prototype under test)}
	\label{eff-around-tot}
\end{figure}

A deeper study of the response of the detector in the junction area (\figurename~\vref{eff-junct}) shows the steepness of the slope in the curve of efficiency as a function of $y$, which in that plot varies perpendicularly to the spacer that covers the seam. We can see that the low-efficiency area around the spacer is narrow, as required. No collateral malfunctionings were observed in spacer, border and junction areas.\\
\begin{figure}%[!htbp]
	\centering
	\subfigure[Efficiency versus $y$]
	{\includegraphics[width=8cm]{Images/radiography/C-effoverjunction}\label{eff-junct}}
	\hspace{2mm}
	\subfigure[Spacer and inactive area width]
	{\includegraphics[width=4cm]{Images/spacer-center}\label{sp-centr}}
	\caption{Loss of efficiency at the GEM foils junction}
	\label{eff-junct-tot}
\end{figure}
% ADD SEAM IMAGE AND WIDTH DATA

However, \figurename~\vref{eff-junct} also shows an unexpected problem, which we became aware of only long after the end of the test beam: the side of the detector which was not directly involved in the tests appears to be less efficient. This effect may be too high to be explained in terms of lack of \textit{charging-up}, and it is visible in all the three junction zones we analized. It is not due to visible asymmetries between the two voltage distribution boards either, whose input impedances were both measured to be equal to $5.40 M\Omega$. The set of data we gathered so far is not enough to allow a proper explanation of this phenomenon.\\

As already said, we came across this problem after the test beam period finished, and thus we were not able to set up another irradiation test to investigate it. A new gain curve is needed and will be worked out as soon as possible with Cu X-Rays, to compare it with the absolute gain calibration made by S. D. Pinto \cite{pinto-lg}. 					% CITARE ALTRI DOCUMENTI?