% CORRETTO BABBO
Before the scheduled test beam period, Serge Duarte Pinto\footnote{CERN researcher, on behalf of RD51 group} had performed some tests to study the gain of the prototype, using Ar/CO\subscript{2} 70/30 gas mixture and Cu X-Rays.\\

The gain of the detector represents the ratio between its output and input currents, that is:
$$
G = \dfrac{I_{OUT}}{q*f}
$$
where $q$ is the charge gathered by the first layer of the detector. In this setup $q$ can be computed as:
$$
q = n * e
$$
where $e$ is the electron charge and $n$ is the average number of electrons produced in the drift region by the incident photon or charged particle; $f$ is the interaction rate of the incident particles in the gas. Since we know the energies of the characteristic X-ray lines of the source, and the ionization potential of the gas mixture in use, $n$ is well determined. In this setup (Ar/CO\subscript{2}~70/30) we have $ n \simeq 290 $.\\

After $q$ is computed, we need to measure $I_{OUT}$ and $f$ in order to obtain $G$. The interaction rate $f$ is measured by sending the LG output signals first to an amplifier, then to a comparator, and finally to a counter. Connecting all the 128 pads of each readout line together, $I_{OUT}$ can be measured with an amperometer.\\

\begin{figure}
	\centering
	\includegraphics[width=12cm]{Images/gaincurve-nocolors}%{Images/gaincurve}
	\caption{Cu X-Ray gain curve for the large prototype GEM detector (S. D. Pinto)}
	\label{gaincurve}
\end{figure}

\figurename~\vref{gaincurve} shows the results of S. D. Pinto's measurements. There are two different plots for the two sides of the chamber; the graphs are not symmetric due to a wrong offset of one of the amperometers, which becomes relevant at low current levels. However, the high-gain regions of the two graphs coincide, and they can be fitted fairly well with an exponential function, as expected for a GEM detector.\\

At the test beam we raised the current over $830 \mu A$, where Pinto's data end, losing the possibility to check the exact gain our detector had. We got to such high voltage values in order to test the prototype with a different gas mixture (see \chaptername~\vref{ch-hvscan} and following).\\

For this and other reasons, a new gain curve is required, which will touch high divider current values and will check the uniformity of the detector within its left and right sides.