% CORRETTO BABBO
We worked on some time-performance scans as well, to check how fast the prototype response was, and what could be done to improve it. First we estimated the latency via the LabVIEW software that we used to interact with the VFAT2 chips, and we found it to be around $17$ \textit{clock cycles}. Then we investigated all the latencies in the $[10clk,19clk]$ interval by means of a set of acquisitions at different thresholds and MSP lengths.\\

\begin{sidewaysfigure}
	\centering
	\includegraphics[width=21cm]{Images/mspl-legend2}%{Images/mspl}
	\caption{Latency scans performed at four different $MSPL$ values. Data sets were taken at various threshold values between $-100ds$ and $-40ds$, during a $\mu^-$ beam.}
	\label{lat-1}
\end{sidewaysfigure}
\figurename~\vref{lat-1} shows all the data sets. Efficiency is plotted versus latency; indeed, the signal starts at $17clk$, and the efficiency reaches 90\% only at low thresholds or at $MSPL \geq 3 clk$. This is due to the threshold-crossing time spread, that is longer than a single clock cycle when the detector is filled with Ar/CO\subscript{2} (it can reach $\sim 60 ns$, as pointed out in \chaptername~\vref{jitter}, while a clock cycle lasts $25ns$).\\

We can select two efficient working points:
\begin{enumerate}
	\item $MSPL = 4clk$, with any threshold value such that $40ds < th < 100ds$;
	\item $MSPL = 3clk$, with threshold $40ds \leq th \leq 60ds$.
\end{enumerate}
During these tests, the rate of incoming particles was of the order of magnitude of $kHz$. In LHC, where the rate is much higher, a monostable pulse lenght $ >1clk $ may cause superposition of events and loss of time resolution. In that case, we would want to stretch the $MSPL$ as little as possible.\\

At the test-beam we made large use of the first working point listed above. We could have used $MSPL=3clk$, setting $th = 40ds$ in order to reach the same detecting efficiency, which however might have not been enough to cut all of the noise off.\\

Indeed, we clearly got some noise at $th = -40 ds$. The dotted lines in \figurename~\ref{lat-1} show that the chamber was detecting charge even out of the right latency range: that signal was not related to the beam and it was caused by noise. This is confirmed by the fact that the hit count increased as the MSPL lenghtened: by setting a longer MSPL we just integrated the signal (both the ``real'' and the noisy hits) over a longer time interval.\\
\begin{figure}
	\centering
	\includegraphics[width=12cm]{Images/latencyarco2-th-nocolors}
	\caption{Noise counts become visible at $th \leq 40ds$}
	\label{mspl-noise}
\end{figure}

In \figurename~\vref{mspl-noise} I explain this behaviour, showing $MSPL=2clk$ and $MSPL=4clk$ data sets. For both of them, thicker lines represent high thresholds and thinner lines low thresholds. The number of hits does not fall to $0$ outside the latency boundaries; indeed, where for thresholds $th > 40ds$ the efficiencies are null, we see that:
$$
\mathcal{E}_{MSPL=4clk}^{th=40ds} \simeq 2 * \mathcal{E}_{MSPL=2clk}^{th=40ds} \qquad \left(lat \leq 11clk \; \vee \; lat \geq 18clk \right)
$$
With $MSPL=4clk$, noise hits are indeed counted for twice as much time as with $MSPL=2clk$.\\

% TDC %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\input{Parts/TDC}
% END TDC %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

Increasing detectors time performance is a priority. The LHC machine is expected to start working at very high frequency and intensity by the end of 2011, with a bunch crossing every $25ns$ and a luminosity $ \mathcal{L} \geq 10^{31} \dfrac{1}{cm^2 \cdot s} $. TOTEM GEM-based detectors (\textbf{T2}) cover a region of very high pseudorapidity\footnote{Being $ \theta $ the angle between the momentum~$ \overrightarrow{p} $ of the incoming particle and the beam direction, we define \textbf{pseudorapidity} $ \eta = - \ln \left( \tan \dfrac{\theta}{2} \right) $. Therefore, as the angle decreases, $\eta \longrightarrow \infty$.}, and therefore they run through extreme radiation conditions. We ran a test trying to find a new gas mixture for those TOTEM detectors, which would allow for a faster detector response without modifying the divider.\\

A percentage of CF\subscript{4} was added to the gas mixture inside the LG, getting to an Ar/CO\subscript{2}/CF\subscript{4} $60/20/20$ configuration. After the High-Voltage scan described in \chaptername~\vref{ch-hvscan} we repeated some of the latency scans, whose results are compared to those we got with the previous gas mixture in \figurename~\vref{lat-2}.\\
\begin{figure}%[htbp]
	\centering
	\subfigure[Standard gas mixture results]
%	{\includegraphics[width=6.6cm]{Images/latencyarco2}\label{standardgas}}		% WIDTH=6.7cm TO HAVE THEM ON THE SAME LINE
	{\includegraphics[width=6.6cm]{Images/latencyarco2-nocolors}\label{standardgas}}		% WIDTH=6.7cm TO HAVE THEM ON THE SAME LINE
	% B/W VERSIONS ARE THERE, JUST USE THEM....
	\hspace{2mm}
	\subfigure[Gas mixtures comparison at $MSPL=2clk$]
%	{\includegraphics[width=6.6cm]{Images/latencygases}\label{gascomp}}
	{\includegraphics[width=6.6cm]{Images/latencygases-nocolors}\label{gascomp}}
	\caption{Data taken adding CF\subscript{4} to the standard Ar/CO\subscript{2} $70/30$ gas mixture (same internal voltages and fields as for Ar/CO\subscript{2})}
	\label{lat-2}
\end{figure}

We focussed on $MSPL=2clk$ tests, since a lenght of $2clk$ would be suitable for the future TOTEM runs. \figurename~\vref{gascomp} clearly shows that CF\subscript{4} improves the response speed of this kind of detectors: the new gas mixture allows to approach full efficiency at $th=-60ds$ and $MSPL=2clk$. On the other hand, we needed to supply a little more current on the divider. We did not observe any discharge, but we could run only a few tests with this gas mixture. It must be pointed out that we do not know what the exact gain value of the chamber was when we worked with CF\subscript{4}. However, previous tests performed by the authors of \cite{3gem-fast} suggest that at our working point the gain of Ar/CO\subscript{2}/CF\subscript{4} $60/20/20$ mixture was even lower, by a factor of five, than that of Ar/CO\subscript{2} $70/30$.\\

The detector was designed to work with Ar/CO\subscript{2} $70/30$. It is now clear that redesigning its divider to work with this new gas mixture should improve the detector time resolution. In addition, a high efficiency should be reached with a lower gain, which would result in a lower probability of discharge.